// Copyright 2018 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

library fuchsia.mediacodec;

// See codec.md for detailed interface documentation.  The comments here are a
// summary only.  Client implementers should see codec.md for more detail on any
// message that doesn't seem sufficiently-described here.  Codec server
// implementers should probably read codec.md before implementing.

// Overview of operation:
//
// 1. Create
//   * create via CodecFactory - see CodecFactory
// 2. Get input constraints
//   * OnInputConstraints() - sent unsolicited by codec shortly after codec
//     creation.
// 3. Provide input buffers
//   * SetInputBufferSettings() / AddInputBuffer()
// 4. Deliver input data
//   * QueueInputPacket() + OnFreeInputPacket(), for as long as it takes,
//     possibly working through all input packets repeatedly before...
// 5. Get output constraints and format
//   * OnOutputConfig() - may be delivered as early as before
//     OnInputConstraints() by some codecs, but a client must tolerate as late
//     as after substantial input data has been delivered including lots of
//     input packet recycling via OnFreeInputPacket().
//   * This message can arrive more than once before the first output data.
// 6. Provide output buffers
//   * SetOutputBufferSettings() / AddOutputBuffer()
// 7. Data flows, with optional EndOfStream
//   * OnOutputPacket() / RecycleOutputPacket() / QueueInputPacket() /
//     OnFreeInputPacket() / QueueInputEndOfStream() / OnOutputEndOfStream()
//
// It's possible to re-use a Codec instance for another stream, and doing so
// can sometimes skip over re-allocation of buffers.  This can be a useful thing
// to do for cases like seeking to a new location - at the Codec interface that
// can look like switching to a new stream.

// CodecBufferConstraints
//
// This struct helps ensure that packet count and buffer space are sufficient
// to avoid major problems.  For example, a video decoder needs sufficient
// video frame buffers to hold all potential reference frames concurrently +
// one more video buffer to decode into.  Else, the whole video decode pipe can
// easily deadlock.
//
// The secondary purpose of this struct is to help ensure that packet count and
// buffer space are sufficient to achieve reasonably performant operation.
//
// There are separate instances of this struct for codec input and codec output.
struct CodecBufferConstraints {
    // This is a version number the server sets on the constraints to allow the
    // server to determine when the client has caught up with the latest
    // constraints sent by the server.  The server won't emit output data until
    // the client has configured output settings and buffers with a
    // buffer_constraints_version_ordinal >= the latest
    // buffer_constraints_version_ordinal that had
    // buffer_constraints_action_required true.  See
    // buffer_constraints_action_required comments for more.
    //
    // A buffer_constraints_version_ordinal of 0 is not permitted, to simplify
    // initial state handling.  Other than 0, both odd and even version ordinals
    // are allowed (in constrast to the stream_lifetime_ordinal, neither the
    // client nor server ever has a reason to consider the latest version to be
    // stale, so there would be no benefit to disallowing even values).
    uint64 buffer_constraints_version_ordinal;

    // default_settings
    //
    // These settings are "default" settings, not "recommended" settings.
    //
    // These "default" settings can be passed to SetInputBufferSettings() /
    // SetOutputBufferSettings() as-is without modification, but a client doing
    // that must still obey the semantics of packet_count_for_client, despite the
    // codec server not having any way to really know the proper setting for
    // that field.
    //
    // For CodecBufferConstraints fields whose names end in "recommended", the
    // default_settings will have the corresponding setting field set to that
    // recommended value.
    //
    // The codec promises that these default settings as-is (except for
    // buffer_lifetime_ordinal) are guaranteed to
    // satisfy the constraints indicated by the other fields of
    // CodecBufferConstraints.  While client-side checking that these
    // settings are within the constraints is likely unnecessary in the client,
    // the client should still check that these values are within client-side
    // reasonable-ness bounds before using these values, to avoid letting a codec
    // server cause problems for the client.
    //
    // This structure will always have single_buffer_mode false.  See
    // single_buffer_mode_allowed for whether single_buffer_mode true is allowed.
    //
    // The client must set the buffer_lifetime_ordinal field to a proper value
    // before sending back to the server.  The 0 initially in this field will be
    // rejected by the server if sent back as-is.  See comments on
    // CodecPortBufferSettings.buffer_lifetime_ordinal.
    CodecPortBufferSettings default_settings;

    // For uncompresesd video, separate and complete frames, each in its own
    // separate buffer (buffer-per-packet mode), is always a requirement.

    // per_packet_buffer_bytes.*:
    //
    // These per-packet buffer bytes constraints apply to both buffer-per-packet
    // mode and single-buffer mode (see single_buffer_mode).  If
    // buffer-per-packet mode, the constraints apply to each buffer separately.
    // If single-buffer mode, the constraints need to be multiplied by the number
    // of packets to determine the constraints on the single buffer.

    // per_packet_buffer_bytes_min:
    //
    // If a client is using buffer per packet mode, each buffer must be at least
    // this large.  If a client is using single-buffer mode, the one buffer must
    // be at least per_packet_buffer_bytes_min * packet_count_for_codec_min in
    // size.
    uint32 per_packet_buffer_bytes_min;
    // Must be >= per_packet_buffer_bytes_min.  Delivering more than
    // this per input packet might not perform any better, and in fact might
    // perform worse.
    uint32 per_packet_buffer_bytes_recommended;
    // Must be >= per_packet_buffer_bytes_recommended.  Can be 0xFFFFFFFF if there
    // is no explicitly-enforced limit.
    uint32 per_packet_buffer_bytes_max;

    // Minimum number of packet_count_for_codec.
    //
    // Re. input and output:
    //
    // This is a strict min for packet_count_for_codec, but a client can use more
    // packets overall if the client wants to, by using a larger value for
    // packet_count_for_codec and/or using a non-zero packets_for_client.  A good
    // reason to do the former would be if the client might tend to deliver a few
    // not-very-full buffers occasionally - or to have a few extra packets within
    // which to satisfy codec_input_bytes_min.  A good reason to do the latter
    // would be if a client needs to hold onto some packets for any "extra"
    // duration.
    //
    // If a client specifies a larger packet_count_for_codec value than
    // packet_count_for_codec_min, a server is permitted (but not encouraged) to
    // not make progress until packet_count_for_codec are with the server,
    // not merely packet_count_for_codec_min.
    //
    // For decoder input and audio encoder input: The packet_count_for_codec_min
    // may or may not contain enough data to allow the codec to make progress
    // without copying into an internal side buffer.  If there isn't enough data
    // delivered in packet_count_for_codec_min packets to permit progress, the
    // codec must copy into its own side buffer internally to make progress.
    //
    // If a client intends to use extra packets for client-side purposes, the
    // client should specify the extra packets in packets_for_client instead of
    // packet_count_for_codec, but packet_count_for_codec must still be >=
    // packet_count_for_codec_min.
    //
    // See codec.md for more on packet_count_for_codec_min.
    uint32 packet_count_for_codec_min;

    // This must be at least packet_count_for_codec_min and at most
    // packet_count_for_codec_recommended_max.
    //
    // This value is likely to be used as-is by most clients, so if having one
    // additional packet is a big performance win in a large percentage of
    // scenarios, it can be good for the server to include that additional packet
    // in this value.
    uint32 packet_count_for_codec_recommended;

    // This can be the same as packet_count_for_codec_max or can be lower.
    // Values above this value and <= packet_count_for_codec_max are not
    // recommended by the codec, but should still work given sufficient resoures
    // available to both the client and the codec.
    uint32 packet_count_for_codec_recommended_max;

    // This can be 0xFFFFFFFF if there's no codec-enforced max, but codecs are
    // encouraged to set a large but still plausibly-workable max, and clients
    // are encouraged to request a number of packets that isn't excessively large
    // for the client's scenario.
    uint32 packet_count_for_codec_max;

    // Normally this would be an implicit 0, but for now we have a min so we can
    // force the total number of packets to be a specific number that we know
    // works for the moment.
    uint32 packet_count_for_client_min;

    // packet_count_for_client_max
    //
    // The client must set packet_count_for_client to be <=
    // packet_count_for_client_max.
    //
    // This value must be at least 1.  This can be 0xFFFFFFFF if there's no
    // codec-enforced max.  Clients are encouraged to request a number of
    // packets that isn't excessively large for the client's scenario.
    uint32 packet_count_for_client_max;

    // single_buffer_mode_allowed false allows a codec that's not required to
    // support single-buffer mode for a given input or output the ability to
    // decline to support single-buffer mode on that input/output.
    //
    // All encoder output, regardless of audio or video: server support for
    // single-buffer mode is optional.  Please see codec.md for all the rules
    // regarding single-buffer mode on output before using single-buffer mode on
    // an output.
    //
    // Audio decoder output: server support for single-buffer mode is required.
    //
    // Video decoder output: There is little reason for a video decoder to
    // support single-buffer mode on output.  Nearly all video decoders will set
    // this to false for their output.
    //
    // All decoder inputs: Servers must support single-buffer mode on input.  The
    // client is responsible for managing the input buffer space such that
    // filling an input packet doesn't over-write any portion of an input packet
    // already in flight to the codec.
    //
    // Encoder inputs: Server support for single-buffer mode on encoder input is
    // optional.  This is more often useful for audio than for video.
    //
    // Support for buffer-per-packet mode is always required on both input and
    // output, regardless of codec type.
    bool single_buffer_mode_allowed;

    // If true, the buffers need to be physically contiguous pages, such as can be
    // allocated using zx_vmo_create_contiguous().
    bool is_physically_contiguous_required;
    // VERY TEMPORARY HACK / KLUDGE - we want the BufferAllocator (or one of the
    // participant drivers that needs physically contiguous buffers) to call
    // zx_vmo_create_contiguous(), definitely not the Codec client, but until the
    // BufferAllocator can be reached from a driver, this is to grant the client
    // special powers it really shouldn't have, very temporarily until
    // BufferAllocator is hooked up properly at which point this can be removed.
    // Strictly speaking we could reverse which end allocates buffers in the Codec
    // interface to avoid this hack even before BufferAllocator, but the overall
    // path seems shorter if we jump directly from this to using BufferAllocator.
    handle? very_temp_kludge_bti_handle;
};

// CodecOutputConfig
//
// The codec-controlled output configuration, including both
// CodecBufferConstraints for the output and CodecFormatDetails for the output.
//
// TODO(dustingreen): Need a better name for this struct, but still short
// hopefully.  It's stuff the codec gets to control, not the client.  It's
// different than output buffer settings, which the client does get to control
// to some extent.  It's different than any configurable output settings the
// client might specifiy for output of an encoder.
struct CodecOutputConfig {
    // A client which always immediately re-configures output buffers on reciept
    // of OnOutputConfig() with buffer_constraints_action_required true can safely
    // ignore this field.
    //
    // A client is permitted to ignore an OnOutputConfig() message even with
    // buffer_constraints_action_required true if the client knows the server has
    // already been told to discard the remainder of the stream with the same
    // stream_lifetime_ordinal or if this stream_lifetime_ordinal field is set to
    // 0.  The server is required to re-send needed output config via
    // OnOutputConfig() with new stream_lifetime_ordinal and
    // buffer_constraints_action_required true, if the the most recent completed
    // server-side output config isn't what the server wants/needs yet for the
    // new stream.
    uint64 stream_lifetime_ordinal;

    // buffer_constraints_action_required
    //
    // When the buffer constraints are delivered, they indicate whether action is
    // required.  A false value here permits delivery of constraints which are
    // fresher without forcing a buffer reconfiguration.  If this is false, a
    // client cannot assume that it's safe to immediately re-configure output
    // buffers.  If this is true, the client can assume it's safe to immediately
    // configure output buffers once.
    //
    // A client is permitted to ignore buffer constraint versions which have
    // buffer_constraints_action_required false.  The server is not permitted to
    // change buffer_constraints_action_required from false to true for the same
    // buffer_constraints_version_ordinal.
    //
    // For each configuration, a client must use new buffers, never buffers that
    // were prevoiusly used for anything else, and never buffers previously used
    // for any other Codec purposes.  This rule exists for multiple good reasons,
    // relevant to both mid-stream changes, and changes on stream boundaries.
    // A client should just use new buffers each time.
    //
    // When this is true, the server has already de-refed as many low-level
    // output buffers as the server can while still performing efficient
    // transition to the new buffers and will de-ref the rest asap.  A Sync() is
    // not necessary to achieve non-overlap of resource usage to the extent
    // efficiently permitted by the formats involved.
    //
    // If buffer_constraints_action_required is true, the server _must_ not
    // deliver more output data until after output buffers have been configured
    // (or re-configured) by the client.
    //
    // See codec.md for more on buffer_constraints_action_required.
    bool buffer_constraints_action_required;
    CodecBufferConstraints buffer_constraints;

    // format_details
    //
    // It's up to the client to determine if a change in
    // format_details.format_details_version_ordinal implies any client action is
    // required, based on particular fields in format_details vs. any old value.
    // The server guarantees that if the format has changed, then
    // format_details.format_details_version_ordinal will change, but a change to
    // format_details.format_details_version_ordinal does not guarantee that the
    // format details actually changed.  Servers are strongly encouraged to not
    // change format_details.format_details_version_ordinal other than before the
    // first output data of a stream unless there is a real mid-stream format
    // change in the stream.  Unnecessary mid-stream format changes can cause
    // simpler clients that have no need to handle mid-stream format changes to
    // just close the channel.  Format changes before the first output data of a
    // stream are not "mid-stream" in this context - those can be useful for
    // stream format detection / setup reasons.
    //
    // Note that in case output buffers don't really need to be re-configured
    // despite a format change, a server is encouraged, but not required, to
    // set buffer_constraints_action_required false on the message that conveys
    // the new format details.  Simpler servers may just treat the whole output
    // situation as one big thing and demand output buffer reconfiguration on any
    // change in the output situation.
    //
    // A client may or may not actually handle a new buffer_constraints with
    // buffer_constraints_action_required false, but the client should always
    // track the latest format_details.
    //
    // An updated format_details is ordered with respect to emitted output
    // packets, and applies to all subsequent packets until the next
    // format_details with larger version_ordinal.  A simple client that does not
    // intend to handle mid-stream format changes should still keep track of the
    // most recently received format_details until the first output packet
    // arrives, then lock down the format details, handle those format details,
    // and verify that any format_details.format_details_version_ordinal received
    // from the server is the same as the locked-down format_details, until the
    // client is done with the stream.  Even such a simple client must tolerate
    // format_details.format_details_version_ordinal changing multiple times
    // before the start of data output from a stream (any stream - the first
    // stream or a subsequent stream).  This allows a codec to request that
    // output buffers and output format be configured speculatively, and for the
    // output config to be optionally adjusted by the server before the first
    // data output from a stream after the server knows everything it needs to
    // know to fully establish the initial output format details.  This
    // simplifies codec server implementation, and allows a clever codec server
    // to guess it's output config for lower latency before any input data, while
    // still being able to fix the output config (including format details) if
    // the guess turns out to be wrong.
    //
    // Whether the format_details.format_details_version_ordinal will actually
    // change mid-stream is a per-codec and per-stream detail that is not
    // specified in comments here, and in most cases also depends on whether the
    // format changes on the input to the codec.  Probably it'll be fairly common
    // for a client to use a format which technically supports mid-stream format
    // change, but the client happens to know that none of the streams the client
    // intends to process will ever have a mid-stream format change.
    CodecFormatDetails format_details;
};

// Default values for input and output
// CodecBufferConstraints.default_settings.packet_count_for_codec.
//
// These are defined as "const" in FIDL to avoid all server implementations
// needing to separately define their own values, and these should be
// reasonable as default values, but strictly speaking this is not intended to
// promise that this value won't change from build to build.  If a client cares
// about a specific number, the client should separately define what that
// number is and ensure that CodecPortBufferSettings.packet_count_for_client is
// at least large enough.
//
// In contrast to packet_count_for_client, the packet_count_for_codec is much
// more codec-specific, so this file has no numbers for that - each codec will
// set those as appropriate for the specific codec.
//
// These are not "recommended" values, only "default" values, in the sense that
// the codec doesn't really know what the correct setting for these values is
// for a given client, and if the default is not appropriate for a client,
// large problems could result such as deadlock.  See the comments on
// packet_count_for_client.
//
// Despite these defaults, every client should ideally care about the
// packet_count_for_client setting and should ensure that the setting is at
// least large enough to cover the number of packets the client might ever need
// to camp on for any non-transient duration concurrently.  The defaults are
// only intended to be plausible for some clients, not all clients.
//
// One for the client to be filling and one in transit.
const uint32 kDefaultInputPacketCountForClient = 2;
// One for the client to be rendering, and one in transit.
const uint32 kDefaultOutputPacketCountForClient = 2;

// For input, this is the default on a fairly arbitrary basis.
//
// TODO(dustingreen): Do we want the default for audio encoding to be
// single_buffer_mode true instead?  If so, we may split this up by audio/video
// encoder/decoder.
const bool kDefaultInputIsSingleBufferMode = false;
const bool kDefaultOutputIsSingleBufferMode = false;

// CodecPortBufferSettings
//
// See relevant corresponding constraints in CodecBufferConstraints.  The
// settings must satisfy the constraints.
//
// The client informs the codec of these settings and then separately informs
// the codec of each buffer.
//
// TODO(dustingreen): Rename from CodecPortBufferSettings to CodecBufferSettings
// in a separate CL (after some other stuff is out of the way first).
struct CodecPortBufferSettings {
    // buffer_lifetime_ordinal
    //
    // The containing message starts a new buffer_lifetime_ordinal.
    //
    // There is a separate buffer_lifetime_ordinal for input vs. output.
    //
    // Re-use of the same value is not allowed.  Values must be odd.  Values must
    // only increase (increasing by more than 2 is permitted).
    //
    // A buffer_lifetime_ordinal lifetime starts at SetInputBufferSettings() or
    // SetOutputBufferSettings(), and ends at the the earlier of
    // CloseCurrentStream() with release_input_buffers/release_output_buffers set
    // or SetOutputBufferSettings() with new buffer_lifetime_ordinal in the case
    // of mid-stream output config change.
    //
    // See codec.md for more on buffer_lifetime_ordinal.
    uint64 buffer_lifetime_ordinal;

    // buffer_constraints_version_ordinal
    //
    // This value indicates which version of constraints the client is/was aware
    // of so far.
    //
    // For input, this must always be 0 because constraints don't change for
    // input (settings can change, but there's no settings vs current constraints
    // synchronization issue on input).
    //
    // For output, this allows the server to know when the client is sufficiently
    // caught up before the server will generate any more output.
    //
    // When there is no active stream, a client is permitted to re-configure
    // buffers again using the same buffer_constraints_version_ordinal.
    //
    // See codec.md for more on buffer_constraints_version_ordinal.
    uint64 buffer_constraints_version_ordinal;

    // The total packet count is split into two pieces to disambiguate how many
    // packets are allocated for the client to hold onto for whatever reason, vs.
    // how many packets are allocated for the server to hold onto for whatever
    // reason.
    //
    // Extra packets to provide slack for peformance reasons can be in either
    // category, but typically packet_count_for_codec_recommended will already
    // include any performance-relevant slack for the server's benefit.

    // packet_count_for_codec
    //
    // How many packets the client is allocating for the codec server's use.
    // This must be >= CodecBufferConstraints.packet_count_for_codec_min.  If
    // constraints change such that this would no longer be true, the server will
    // send an OnOutputConfig() event.
    //
    // The codec server is allowed to demand that all of packet_count_for_codec
    // become free before making further progress, even if
    // packet_count_for_codec is > packet_count_for_codec_min.
    //
    // A reasonable value for this is
    // CodecBufferConstraints.packet_count_for_codec_recommended.
    //
    // See codec.md for more on packet_count_for_codec.
    uint32 packet_count_for_codec;

    // packet_count_for_client
    //
    // This must be at least 1.  The server will close the channel if this is 0.
    //
    // How many packets the client is allocating for the client's use.  The
    // client may hold onto this many packets for arbitrarily-long duration
    // without handing these packets to the codec, and despite doing so, the
    // codec will continue to make progress and function normally without getting
    // stuck.  The client holding onto additional packets transiently is ok, but
    // the client needs to hand those additional packets back to the codec
    // eventually if the client wants the codec to make further progress.
    //
    // In addition to this value needing to include at least as many packets as
    // the client ever intends to concurrently camp on indefinitely, any extra
    // slack to benefit client-side performance should also be included here.
    //
    // A typical value for this could be at least 2, but it depends strongly on
    // client implementation and overall client buffering goals.  It is up to the
    // client to determine how many packets are needed in this category by any
    // parts of the overall system that will be holding onto packets for any
    // reason.  Those parts of the system should have a documented and possibly
    // queryable defined value to help determine this number.  Setting this value
    // lower than it actaully needs to be can result in the codec not making
    // progress as it sits waiting for packets, with the client unable to recycle
    // any more packets to the codec.  That situation can be difficult to
    // diagnose, while excessively-large values here are wasteful, so care is
    // warranted to set this value properly.
    uint32 packet_count_for_client;

    // per_packet_buffer_bytes
    //
    // In buffer-per-packet mode, we require that each buffer have usable bytes
    // equal to per_packet_buffer_bytes.  Use of differently-sized low-level
    // buffers is possible, but the size of the portion used via the Codec
    // interface per CodecBuffer must be the same for all the buffers.
    //
    // In single-buffer mode, we require the portion of the low-level buffer used
    // via the Codec interface to be size (packet_count_for_codec +
    // packet_count_for_client) * per_packet_buffer_bytes.
    //
    // TODO(dustingreen): determine if we need to relax these restrictions a bit
    // for convenience when using gralloc video buffers.
    uint32 per_packet_buffer_bytes;

    // If true, there is only one buffer with index 0 which all packets
    // implicitly refer to.  If false, the packet_index and buffer_index are 1:1,
    // and each packet refers to its corresponding buffer.
    //
    // While it's possible to set up single_buffer_mode false with each buffer
    // referring to the same underlying VMO, single_buffer_mode true is more
    // efficient for that case since only one mapping is created.
    bool single_buffer_mode;
};

// CodecBuffer
//
// The CodecBuffer struct represents a pre-configured buffer.
//
// Both input and output uses CodecBuffer(s), but the two sets of buffers are
// separate.
//
// The client uses SetInputBufferSettings() + AddInputBuffer() * N to inform
// the codec about all the input buffers.
//
// The client uses SetOutputBufferSettings() + AddOutputBuffer() * N to inform
// the codec about all the output buffers.
//
// When single_buffer_mode is true, there is only buffer_index 0 shared by all
// CodecPacket(s) of the relevant input or output.  When single_buffer_mode is
// false, the buffer_index equals the packet_index.
struct CodecBuffer {
    // When using AddOutputBuffer()/AddInputBuffer(), this must match the
    // buffer_lifetime_ordinal of the most recent
    // SetOutputBufferSettings()/SetInputBufferSettings().
    uint64 buffer_lifetime_ordinal;

    // Buffers must be added via AddOutputBuffer() / AddInputBuffer() in order by
    // buffer_index, and the buffer_index is always equal to 0 or equal to the
    // packet_index (depending on single_buffer_mode true or false), but it's
    // still nice to have CodecBuffer include the buffer_index if only for easier
    // debugging.
    uint32 buffer_index;

    // For each new buffer_lifetime_ordinal, a client must use new low-level
    // buffers.  This rule exists for multiple very good reasons, and is relevant
    // to mid-stream changes, changes on stream boundaries, and both input and
    // output buffers.  A new buffer_lifetime_ordinal needs new low-level
    // buffers, not just new CodecBuffer(s).  If you find yourself copying
    // compressed input data into new low-level input buffers solely due to this
    // rule, consider asking the source of the data for the ability to directly
    // fill new VMOs.  The rule exists for good reasons, even for input buffers.
    //
    // The previous paragraph does not prohibit carving up VMOs into sub-pieces
    // and using different sub-pieces as different CodecBuffer(s), with some VMOs
    // used for more than one CodecBuffer and possibly others used for only one
    // CodecBuffer.  While this is permitted and enables some optimizations, it's
    // not expected to be particularly common.
    //
    // See codec.md for more on CodecBufferData, and more on why we never re-use
    // the same low-level buffers for different buffer_lifetime_ordinal(s).
    CodecBufferData data;
};

// CodecBufferData
//
// For the moment, a VMO per buffer is the only type of buffer.
//
// This is extremely likely to change significantly when adding gralloc stuff,
// but the idea with this union is to have a struct per logical way of storing
// the data.  Any multi-domain storage within a gralloc buffer will likely be
// only indirectly represented here.
union CodecBufferData {
    CodecBufferDataVmo vmo;

    // TODO(dustingreen): add the gralloc way
};

// CodecBufferDataVmo
//
// Details for a buffer backed by a VMO.
struct CodecBufferDataVmo {
    // The same VMO can be used by more than one CodecBuffer (only of the same
    // buffer_lifetime_ordinal), but each vmo_handle must be a separate handle.
    handle<vmo> vmo_handle;

    // Offset within the VMO of the first usable byte.  Must be < the VMO's size
    // in bytes.
    uint64 vmo_usable_start;

    // VMO-relative offset that's one past the last usable byte.  This can point
    // one byte beyond the end of the VMO if desired.  In other words, this can
    // be equal to the VMO's size, to indicate that the last byte of the VMO is
    // usable (and possibly many byte before that, depending on
    // vmo_usable_start).
    uint64 vmo_usable_size;
};

// CodecPacketHeader
//
// When referring to a free packet, we use CodecPacketHeader alone instead of
// CodecPacket, since while a packet is free it doesn't really have meaningful
// offset or length etc.
//
// A populated CodecPacket also has a CodecPacketHeader.
struct CodecPacketHeader {
    // This is which buffer configuration lifetime this header is referring to.
    //
    // A packet_index is only really meaningful with respect to a particular
    // buffer_lifetime_ordinal.
    //
    // See CodecPortBufferSettings.buffer_lifetime_ordinal.
    //
    // For QueueInputPacket(), a server receiving a buffer_lifetime_ordinal that
    // isn't the current input buffer_lifetime_ordinal will close the channel.
    //
    // For OnFreeInputPacket() and RecycleOutputPacket(), the reciever (client or
    // server) must ignore a message with stale buffer_lifetime_ordinal.
    uint64 buffer_lifetime_ordinal;

    // The overall set of packet_index values is densely packed from 0..count-1
    // for input and output separately.  They can be queued in any order.
    //
    // Both the client and server should validate the packet_index against the
    // known bound and disconnect if it's out of bounds.
    //
    // When running in buffer-per-packet mode, the packet_index is also the
    // buffer index.  When running in single-buffer mode, the buffer index is
    // always 0 referring to the single buffer.
    //
    // The packet_index values don't imply anything about order of use of
    // packets. The client should not expect the ordering to remain the same over
    // time - the codec is free to hold on to an input or output packet for a
    // while during which other packet_index values may be used multiple times.
    //
    // For a given properly-functioning Codec instance, packet_index values will
    // be unique among concurrently-outstanding packets.  Servers should validate
    // that a client isn't double-using a packet and clients should validate as
    // necessary to avoid undefined or unexpected client behavior.
    uint32 packet_index;
};

// CodecPacket
//
// A CodecPacket represents a chunk of input or output data to or from a codec.
//
// codec output:
// While the CodecPacket is outstanding with the client via OnOutputPacket(),
// the codec will avoid modifying the referenced output data.  After the client
// calls RecycleOutputPacket(packet_index), the codec is notified that the
// client is again ok with the referenced data changing.
//
// codec input:
// The client initially has all packet_index(es) available to fill, and later
// gets packet_index(s) that are again ready to fill via OnFreeInputPacket().
// The client must not modify the referenced data in between QueueInputPacket()
// and OnFreeInputPacket().
struct CodecPacket {
    CodecPacketHeader header;

    // Which buffer this packet refers to.  For single-buffer mode this will
    // always be 0, but for multi-buffer mode, a given in-flight interval of a
    // packet can refer to any buffer.  The packet has an associated buffer only
    // while the packet is in-flight, not while the packet is free.
    //
    // The default value makes accidental inappropriate use of index 0 less
    // likely (will tend to complain in an obvious way if not filled out instead
    // of a non-obvious data corruption when decoding buffer 0 repeatedly instead
    // of the correct buffers).
    uint32 buffer_index = 0x80000000;

    // stream_lifetime_ordinal
    //
    // The value 1 is the lowest permitted value after codec creation.  Values
    // sent by the client must be odd.  Values must only increase.
    //
    // A stream_lifetime_ordinal represents the lifetime of a stream.  All
    // messages that are specific to a stream have the stream_lifetime_ordinal
    // value and the value is the same for all messages relating to a given
    // stream.
    //
    // See codec.md for more on stream_lifetime_ordinal.
    uint64 stream_lifetime_ordinal;

    // start_offset and valid_length_bytes
    //
    // Which part of the relevant buffer is this packet using.  These are valid
    // for input data that's in-flight to the codec, and are valid for output data
    // from the codec.
    //
    // For compressed formats and uncompressed audio, the data in
    // [start_offset, start_offset + valid_length_bytes) is the contiguously valid
    // data refered to by this packet.
    //
    // For uncompressed video frames, CodecFormatDetails is the primary means of
    // determining which bytes are relevant.  The offsets in CodecFormatDetails
    // are relative to the start_offset here.  The valid_length_bytes must be
    // large enough to include the full last line of pixel data, including the
    // full line stride of the last line (not just the width in pixels of the
    // last line).
    //
    // Despite these being filled out, some uncompressed video buffers are of
    // types that are not readable by the CPU.  These fields being here don't
    // imply there's any way for the CPU to read an uncompressed frame.
    //
    // TODO(dustingreen): Do we have any reason to require that these be filled
    // out for opaque uncompressed video frames that the CPU can't read?  In that
    // case do we want to require them just so they can be potentially passed on
    // to a HW renderer in case the HW renderer has any use for them?  Or more
    // likely, it may just be that these tend to refer to the whole size of an
    // uncompressed buffer, with format_details taking care of specifying which
    // bytes are actually relevant.
    uint32 start_offset;

    // valid_length_bytes
    //
    // This must be > 0.
    //
    // The semantics for valid data per packet vary depending on data type as
    // follows.
    //
    // uncompressed video - A video frame can't be split across packets.  Each
    // packet is one video frame.
    //
    // uncompressed audio - Regardless of float or int, linear or uLaw, or number
    // of channels, a packet must contain an non-negative number of complete
    // audio frames, where a single audio frame consists of data for all the
    // channels for the same single point in time.  Any codec-specific internal
    // details re. lower rate sampling for LFE channel or the like should be
    // hidden by the Codec server implementation.
    //
    // compressed data input - A packet must contain at least one byte of data.
    // See also codec_input_bytes_min.  Splitting AUs at arbitrary byte
    // boundaries is permitted, including at boundaries that are in AU headers.
    //
    // compressed data output - The codec is not required to fully fill each
    // output packet's buffer.
    uint32 valid_length_bytes;

    // This value is not strictly speaking a timestamp.  It is an arbitrary
    // unsigned 64-bit number that, under some circumstances, will be passed by a
    // codec unmodified from an input packet to the exactly-corresponding output
    // packet.
    //
    // For timestamp_ish values to be propagated from input to output the
    // following conditions must be true:
    //  * promise_separate_access_units_on_input must be true
    //  * has_timestamp_ish must be true for a given input packet, to have that
    //    timestamp_ish value (potentially) propagate through to an output
    //  * the Codec instance itself decides (async) that the input packet
    //    generates an output packet - if a given input never generates an output
    //    packet then the timestamp_ish value on the input will never show up on
    //    any output packet - depending on the characteristics of the input and
    //    output formats, and whether a decoder is willing to join mid-stream, etc
    //    this can be more or less likely to occur, but clients should be written
    //    to accomodate timestamp_ish values that are fed on input but never show
    //    up on output, at least to a reasonable degree (not crashing, not
    //    treating as an error).
    //
    // See codec.md for more on timestamp_ish.
    bool has_timestamp_ish;
    uint64 timestamp_ish;

    // start_access_unit
    //
    // If promise_separate_access_units_on_input (TODO(dustingreen): or any
    // similar mode for output) is true, this bool must be set appropriately
    // depending on whether byte 0 _is_ or _is not_ the start of an access unit.
    // The client is required to know, and required to set this boolean properly.
    // The server is allowed to infer that when this boolean is false, byte 0 is
    // the first byte of a continuation of a previously-started AU.  (The byte at
    // start_offset is "byte 0".)
    //
    // If promise_separate_access_units_on_input is false, this boolean is
    // ignored.
    bool start_access_unit;

    // known_end_access_unit
    //
    // A client is never required to set this boolean to true.
    //
    // If promise_separate_access_units_on_input is true, for input data, this
    // boolean must be false if the last byte of this packet is not the last byte
    // of an AU, and this boolean _may_ be true if the last byte of this packet
    // is the last byte of an AU.  A client delivering one AU at a time that's
    // interested in the lowest possible latency via the decoder should set this
    // boolean to true when it can be set to true.
    //
    // If promise_separate_access_units_on_input is false, this boolean is
    // ignored.
    bool known_end_access_unit;
};

// Codec
//
// The Codec interface exists to anchor the configuration of input and output
// buffers, and depending on Codec server hosting strategy, the Codec inteface
// can, in some configurations, be 1:1 with a codec isolate (process),
// especially when using SW codecs.  The Codec can be used to process up to one
// stream at a time.
//
// Descriptions of actions taken by methods of this interface and the states of
// things are given as if the methods are synchronously executed by the codec
// server, but in reality, as is typical of FIDL interfaces, the message
// processing is async.  The states described are to be read as the state from
// the client's point of view unless otherwise stated.  Events coming back from
// the server are of course delivered async, and a client that processes more
// than one stream per Codec instance needs to care whether a given event is
// from the current stream vs. some older soon-to-be-gone stream.
//
// The Sync() method's main purpose is to enable the client to robustly prevent
// having both old and new buffers allocated in the system at the same time,
// since media buffers can be significantly large, depending. The Sync() method
// achieves this by only delivering it's response when all previous calls to
// the Codec interface have actually taken effect in the StreamControl ordering
// domain. Sync() can also be used to wait for the codec server to catch up if
// there's a possibility that a client might otherwise get too far ahead of the
// Codec server, by for example requesting creation of a large number of
// streams in a row.  It can also be used during debugging to ensure that a
// codec server hasn't gotten stuck.  Calling Sync() is entirely optional and
// never required for correctness - only potentially required to de-overlap
// resource usage.
//
// Semi-trusted Codec server - SW decoders run in an isolate (with very few
// capabilities) just in case the decoding SW has a vulnerability which could
// be used to take over the Codec server.  Clients of the codec interface using
// decoders and processing streams of separate security contexts, to a greater
// extent than some other interfaces, need to protect themselves against
// invalid server behavior, such as double-free of a packet_index and any other
// invalid server behavior.  Having fed in compressed data of one security
// context, don't place too much trust in a single Codec instance to not mix
// data among any buffers that Codec server has ever been told about.  Instead,
// create separate Codec instances for use by security-separate client-side
// contexts. While the picture for HW-based decoders looks somewhat different
// and is out of scope of this paragraph, the client should always use separate
// Codec instances for security-separate client-side contexts.
interface Codec {
    // EnableOnStreamFailed()
    //
    // Permit the server to use OnStreamFailed() instead of the server just
    // closing the whole Codec channel on stream failure.
    //
    // If the server hasn't seen this message by the time a stream fails, the
    // server will close the Codec channel instead of sending OnStreamFailed().
    EnableOnStreamFailed();

    // OnStreamFailed()
    //
    // The stream has failed, but the Codec instance is still usable for a new
    // stream.
    //
    // This message is only ever sent by the server if the client previously sent
    // EnableOnStreamFailed().  If the client didn't send EnableOnStreamFailed()
    // then the server closes the Codec channel instead.
    //
    // Codec server implementations are encouraged to handle stream errors (and
    // ideally to also report them via error_ bools of OnOutputPacket() and
    // OnOutputEndOfStream()) without failing the whole stream, but if a codec
    // server is unable to do that, but still can cleanly contain the failure to
    // the stream, the codec server can (assuming EnableOnStreamFailed() was
    // called) use OnStreamFailed() to indicate the stream failure to the client
    // without closing the Codec channel.
    //
    // An ideal Codec server handles problems with input data without sending
    // this message, but sending this message is preferred vs. closing the server
    // end of the Codec channel if the Codec server can 100% reliably contain the
    // stream failure to the stream, without any adverse impact to any later
    // stream.
    //
    // No further messages will arrive from the server regarding the failed
    // stream.  This includes any OnOutputEndOfStream() that the client would
    // have otherwise expected.
    //
    // TODO(dustingreen): Add at least an error_message string and _maybe_ a
    // zx_status_t, though that might tend to encourage mis-use of zx_status_t so
    // maybe just error_message for quicker debugging on the client side.  Also
    // plumb from CodecAdapterH264 and similar.
    -> OnStreamFailed(uint64 stream_lifetime_ordinal);

    // OnInputConstraints()
    //
    // The server sends this shortly after Codec creation to indicate input
    // buffer constraints.  The "min" and "max" input constraints don't change
    // for the life of the Codec.
    //
    // The "max" values for buffer size and count are large enough to support the
    // most demanding format the server supports on input.  The "recommended"
    // values should be workable for use with the input CodecFormatDetails
    // conveyed during Codec creation.  The "recommended" values are not
    // necessarily suitable if the client uses QueueInputFormatDetails() to
    // change the input format.  In that case it's up to the client to determine
    // suitable values, either by creating a new Codec instance instead, or
    // knowing suitable values outside the scope of this protocol.
    //
    // See comments on CodecBufferConstraints.
    //
    // This message is guaranteed to be sent unsolicited to the Codec client
    // during or shortly after Codec creation.  Clients should not depend on this
    // being the very first message to arrive at the client.  OnOutputConfig()
    // may be sent first by some codecs that already know their initial output
    // config without any input data, to encourage (but not stictly require) the
    // client to configure output buffers before feeding the first input, to
    // avoid a wasteful OnOutputConfig() being generated for that first stream if
    // the client has started configuring output but isn't done configuring
    // output before the client sends the first input data for the first stream.
    // A client is free to ignore OnOutputConfig() with a stale
    // stream_lifetime_ordinal, but handling OnOutputConfig() with
    // stream_lifetime_ordinal 0 (if any are sent) can help reduce latency to
    // first output.  See OnOutputConfig() for more details.
    //
    // The "min" and "max" input constraints are guaranteed not to change for a
    // given Codec instance.  The "recommended" values may effectively change
    // when the server processes QueueInputFormatDetails().  There is not any way
    // in the protocol short of creating a new Codec instance for the client to
    // get those new "recommended" values.
    //
    // TODO(dustingreen): Maybe provide a way for the client to get updated
    // "recommended" values for input, maybe only on request rather than via this
    // event, to keep things simpler for simpler clients.  Maybe separate the
    // recommendations from the constraints.
    -> OnInputConstraints(CodecBufferConstraints input_constraints);

    // SetInputBufferSettings() and AddInputBuffer()
    //
    // Configuring input buffers consists of calling SetInputBufferSettings()
    // followed by a number of calls to AddInputBuffer() equal to the number of
    // buffers set via SetInputBufferSettings().  In buffer-per-packet mode, this
    // is the same as the number of packets.  In single-buffer mode, this is 1.
    //
    // After OnInputConstraints(), the client uses these two methods to set up
    // input buffers and packets.
    //
    // Configuring input buffers is required before QueueInputPacket().
    //
    // The client can also re-set-up input buffers any time there is no current
    // stream.  The client need not wait until all previously-set-up input
    // buffers are with the client via OnFreeInputPacket().  The old
    // buffer_lifetime_ordinal just ends.  See codec.md for more info on "buffer
    // lifetime".
    //
    // The recommended way to de-overlap resource usage (when/if the client wants
    // to) is to send CloseCurrentStream() with release_input_buffers true then
    // send Sync() and wait for its response before allocating any new buffers.
    // How to cause other parts of the system to release their references on
    // low-level buffers is outside the scope of this interface.
    //
    // This call ends any previous buffer_lifetime_ordinal, and starts a new one.
    SetInputBufferSettings(CodecPortBufferSettings input_settings);

    // The client is required to add all the input buffers before sending any
    // message that starts a new stream else the codec will close the Codec
    // channel.
    //
    // When the last buffer is added with this message, all the input packets
    // effectively jump from non-existent to free with the client.  The Codec
    // will not generate an OnFreeInputPacket() for each new input packet.  The
    // client can immediately start sending QueueInputPacket() after sending the
    // last AddInputBuffer().
    AddInputBuffer(CodecBuffer buffer);

    // OnOutputConfig()
    //
    // This event informs the client of new output config.  The server will send
    // at least one of these messages before the first output packet of a stream,
    // but that message might not have buffer_constraints_action_required true.
    //
    // If buffer_constraints_action_required is true and the
    // stream_lifetime_ordinal matches the current stream, the client must react
    // by configuring or re-configuring output buffers.
    //
    // Some clients may prefer not to support mid-stream output config changes,
    // but even those clients are required to process OnOutputConfig() messages
    // up to the first output packet of each stream, as OnOutputConfig() is used
    // for stream format detection as well as for potential mid-stream output
    // config changes.
    //
    // For more on OnOutputConfig(), see cocec.md.
    -> OnOutputConfig(CodecOutputConfig output_config);

    // SetOutputBufferSettings() and AddOutputBuffer()
    //
    // These are not permitted until after the first OnOutputConfig().
    //
    // Roughly speaking, these messages are sent in response to OnOutputConfig()
    // with buffer_constraints_action_required true.
    //
    // Configuring output buffers consists of calling SetOutputBufferSettings()
    // followed by a number of calls to AddOutputBuffer() equal to the number of
    // buffers set via SetOutputBufferSettings().  In buffer-per-packet mode, this
    // is the same as the number of packets.  In single-buffer mode, this is 1.
    //
    // Configuring output buffers is _required_ after OnOutputConfig() is
    // received by the client with buffer_constraints_action_required true and
    // stream_lifetime_ordinal equal to the client's current
    // stream_lifetime_ordinal (even if there is an active stream), and is
    // _permitted_ any time there is no current stream.
    //
    // For more on SetOutputBufferSettings() and AddOutputBuffer(), see codec.md.
    SetOutputBufferSettings(CodecPortBufferSettings output_settings);
    AddOutputBuffer(CodecBuffer buffer);

    // FlushEndOfStreamAndCloseStream()
    //
    // This message is optional.
    //
    // This message is only valid after QueueInputEndOfStream() for this stream.
    // The stream_lifetime_ordinal input parameter must match the
    // stream_lifetime_ordinal of the QueueInputEndOfStream(), else the server
    // will close the channel.
    //
    // A client can use this message to flush through (not discard) the last
    // input data of a stream so that the codec server generates corresponding
    // output data for all the input data before the server moves on to the next
    // stream, without forcing the client to wait for OnOutputEndOfStream()
    // before queueing data of another stream.
    //
    // The difference between QueueInputEndOfStream() and
    // FlushEndOfStreamAndCloseStream():  QueueInputEndOfStream() is a promise
    // from the client that there will not be any more input data for the stream
    // (and this info is needed by some codecs for the codec to ever emit the very
    // last output data).  The QueueInputEndOfStream() having been sent doesn't
    // prevent the client from later completely discarding the rest of the
    // current stream by closing the current stream (with or without a stream
    // switch).  In contrast, FlushEndOfStreamAndCloseStream() is a request from
    // the client that all the previously-queued input data be processed
    // including the logical "EndOfStream" showing up as OnOutputEndOfStream()
    // (in success case) before moving on to any newer stream - this essentially
    // changes the close-stream handling from discard to flush-through for this
    // stream only.
    //
    // A client using this message can start providing input data for a new
    // stream without that causing discard of old stream data.  That's the purpose
    // of this message - to allow a client to flush through (not discard) the old
    // stream's last data (instead of the default when closing or switching
    // streams which is discard).
    //
    // Because the old stream is not done processing yet and the old stream's
    // data is not being discarded, the client must be prepared to continue to
    // process OnOutputConfig() messages until the stream_lifetime_ordinal is
    // done.  The client will know the stream_lifetime_ordinal is done when
    // OnOutputEndOfStream(), OnStreamFailed(), or the Codec channel closes.
    //
    // For more on FlushEndOfStreamAndCloseStream(), see codec.md.
    FlushEndOfStreamAndCloseStream(uint64 stream_lifetime_ordinal);

    // CloseCurrentStream()
    //
    // This "closes" the current stream, leaving no current stream.  In addition,
    // this message can optionally release input buffers or output buffers.
    //
    // If there has never been any active stream, the stream_lifetime_ordinal must
    // be zero or the server will close the channel.  If there has been an active
    // stream, the stream_lifetime_ordinal must be the most recent active stream
    // whether that stream is still active or not.  Else the server will close the
    // channel.
    //
    // Multiple of this message without any new active stream in between is not
    // to be considered an error, which allows a client to use this message to
    // close the current stream to stop wasting processing power on a stream the
    // user no longer cares about, then later decide that buffers should be
    // released and send this message again with release_input_buffers and/or
    // release_output_buffers true to get the buffers released, if the client is
    // interested in trying to avoid overlap in resource usage between old
    // buffers and new buffers (not all clients are).
    //
    // See also Sync().
    //
    // For more on CloseCurrentStream(), see codec.md.
    CloseCurrentStream(
        uint64 stream_lifetime_ordinal,
        bool release_input_buffers,
        bool release_output_buffers);

    // Sync() -> ()
    //
    // On completion, all previous Codec calls have done what they're going to do
    // server-side, _except_ for processing of data queued using
    // QueueInputPacket().
    //
    // The main purpose of this call is to enable the client to wait until
    // CloseCurrentStream() with release_input_buffers and/or
    // release_output_buffers set to true to take effect, before the client
    // allocates new buffers and re-sets-up input and/or output buffers.  This
    // de-overlapping of resource usage can be worthwhile for media buffers which
    // can consume resource types whose overall pools aren't necessarily vast in
    // comparsion to resources consumed.  Especially if a client is reconfiguring
    // buffers multiple times.
    //
    // Note that Sync() prior to allocating new media buffers is not alone
    // sufficient to achieve non-overlap of media buffer resource usage system
    // wide, but it can be a useful part of achieving that.
    //
    // The Sync() transits the Output ordering domain and the StreamControl
    // ordering domain, but not the InputData ordering domain.  For more on
    // ordering domains see codec.md.
    //
    // This request can be used to avoid hitting kMaxInFlightStreams which is
    // presently 10.  A client that stays <= 8 in-flight streams will comfortably
    // stay under the limit of 10.  While the protocol permits repeated
    // SetInputBufferSettings() and the like, a client that spams the channel
    // can expect that the channel will just close if the server or the channel
    // itself gets too far behind.
    Sync() -> ();

    // OnOutputPacket()
    //
    // This is how the codec emits an output packet to the codec client.
    //
    // Order is significant.
    //
    // The client should eventually call RecycleOutputPacket() (possibly after
    // switching streams multiple times), unless the buffer_lifetime_ordinal has
    // moved on.  A stream change doesn't change which packets are busy with
    // the client vs. free with the server.  See "packet lifetime" in codec.md for
    // more.
    //
    // The relevant buffer is buffer 0 if running in single-buffer mode, or the
    // buffer index is the same as packet_index if running in buffer-per-packet
    // mode.
    //
    // For low-level buffer types that support it, a Codec is free to emit an
    // output packet before the low-level buffer actually has any usable data in
    // the buffer, with the mechanism for signalling the presence of data
    // separate from the OnOutputPacket() message.  For such low-level buffer
    // types, downstream consumers of data from the emitted packet must
    // participate in the low-level buffer signalling mechanism to know when it's
    // safe to consume the data.  This is most likely to be relevant when using a
    // video decoder and gralloc-style buffers.
    //
    // The error_ bool(s) allow (but do not require) a Codec server to report
    // errors that happen during an AU or between AUs.
    //
    // The scope of error_detected_before starts at the end of the last delivered
    // output packet on this stream, or the start of stream if there were no
    // previous output packets on this stream.  The scope ends at the start of
    // the output_packet.
    //
    // The error_detected_before bool is separate so that discontinuities can be
    // indicated separately from whether the current packet is damaged.
    //
    // The scope of error_detected_during is from the start to the end of this
    // output_packet.
    -> OnOutputPacket(
        CodecPacket output_packet,
        bool error_detected_before,
        bool error_detected_during);

    // RecycleOutputPacket()
    //
    // After the client is done with an output packet, the client needs to tell
    // the codec that the output packet can be re-used for more output, via this
    // method.
    //
    // It's not permitted to recycle an output packet that's already free with the
    // codec server.  It's permitted but discouraged for a client to recycle an
    // output packet that has been deallocated by an explicit or implicit output
    // buffer de-configuration().  See buffer_lifetime_ordinal for more on that.
    // A server must ignore any such stale RecycleOutputPacket() calls.
    //
    // For more on RecycleOutputPacket(), see codec.md.
    RecycleOutputPacket(CodecPacketHeader available_output_packet);

    // OnOutputEndOfStream()
    //
    // After QueueInputEndOfStream() is sent by the Codec client, within a
    // reasonable duration the corresponding OnOutputEndOfStream() will be sent by
    // the Codec server.  Similar to QueueInputEndOfStream(),
    // OnOutputEndOfStream() is sent a maximum of once per stream.
    //
    // No more stream data for this stream will be sent after this message.  All
    // input data for this stream was processed.
    //
    // While a Codec client is not required to QueueInputEndOfStream() (unless
    // the client wants to use FlushEndOfStreamAndCloseStream()), if a Codec
    // server receives QueueInputEndOfStream(), and the client hasn't closed the
    // stream, the Codec server must generate a corresponding
    // OnOutputEndOfStream() if nothing went wrong, or must send
    // OnStreamFailed(), or must close the server end of the Codec channel.  An
    // ideal Codec server would handle and report stream errors via the error_
    // flags and complete stream processing without sending OnStreamFailed(), but
    // in any case, the above-listed options are the only ways that an
    // OnOutputEndOfStream() won't happen after QueueInputEndOfStream().
    //
    // There will be no more OnOutputPacket() or OnOutputConfig() messages for
    // this stream_lifetime_ordinal after this message - if a server doesn't
    // follow this rule, a client should close the Codec channel.
    //
    // The error_detected_before bool has the same semantics as the
    // error_detected_before bool in OnOutputPacket().
    -> OnOutputEndOfStream(
        uint64 stream_lifetime_ordinal,
        bool error_detected_before);

    //
    // Stream specific messages:
    //

    // QueueInputFormatDetails()
    //
    // If the input format details are still the same as specified during Codec
    // creation, this message is unnecessary and does not need to be sent.
    //
    // If the stream doesn't exist yet, this message creates the stream.
    //
    // All servers must permit QueueInputFormatDetails() at the start of a stream
    // without failing, as long as the new format is supported by the Codec
    // instance.  Technically this allows for a server to only support the exact
    // input format set during Codec creation, and that is by design.  A client
    // that tries to switch formats and gets a Codec channel failure should try
    // again one more time with a fresh Codec instance created with CodecFactory
    // using the new input format during creation, before giving up.
    //
    // These format details override the format details
    // specified during codec creation for this stream only.  The next stream will
    // default back to the format details set during codec creation.
    //
    // For now, QueueInputFormatDetails() sent mid-stream will fail the Codec
    // channel.  Clients shouldn't do this for now.
    //
    // This message is permitted at the start of the first stream (just like at
    // the start of any stream).  The format specified need not match what was
    // specified during codec creation, but if it doesn't match, the Codec channel
    // might close as described above.
    QueueInputFormatDetails(
        uint64 stream_lifetime_ordinal, CodecFormatDetails format_details);

    // QueueInputPacket()
    //
    // This message queues input data to the codec for processing.
    //
    // If the stream doesn't exist yet, this message creates the new stream.
    //
    // The client is required to be willing to send QueueInputPacket() prior to
    // the server's first OnOutputConfig(), and is permitted to start a new stream
    // without output buffers configured yet.
    //
    // The client must continue to deliver input data via this message even if the
    // codec has not yet generated the first OnOutputConfig(), and even if the
    // Codec is generating OnFreeInputPacket() for previously-queued input
    // packets.  The input data must continue as long as there are free packets
    // to be assured that the server will ever generate the first
    // OnOutputConfig().
    //
    // For more on QueueInputPacket(), see codec.md.
    QueueInputPacket(CodecPacket packet);

    // OnFreeInputPacket()
    //
    // The server sends this message when the codec is done consuming this packet
    // and the packet can be re-filled by the client.
    //
    // This is not sent for all packets when a new buffer_lifetime_ordinal starts
    // as in that case all the packets are initially free with the client.
    //
    // See comments on QueueInputBuffer() and "packet lifetime" in codec.md for
    // for description of buffer lifetime and packet lifetime.
    //
    // After receiving the available input buffer via this event, the codec
    // client can call later call QueueInputBuffer with appropriate offset and
    // length set.
    //
    // TODO(dustingreen): At the moment, there is no guarantee re. the order of
    // these messages with respect to the order of QueueInputPacket(), but at
    // least for decoders, it might be worthwhile to require that servers preserve
    // the order vs. QueueInputPacket(), to make it easier to feed input from a
    // ring buffer or similar.  For audio encoders it might still make sense.  For
    // video encoders probably not.
    -> OnFreeInputPacket(CodecPacketHeader free_input_packet);

    // Inform the server that all QueueInputPacket() messages for this stream
    // have been sent.
    //
    // If the stream isn't closed first (by the client, or by OnStreamFailed(), or
    // Codec channel closing), there will later be a corresponding
    // OnOutputEndOfStream().
    //
    // The corresponding OnOutputEndOfStream() message will be generated only if
    // the server finishes processing the stream before the server sees the
    // client close the stream (such as by starting a new stream).  A way to
    // force the server to finish the stream before closing is to use
    // FlushEndOfStreamAndCloseStream() after QueueInputEndOfStream() before any
    // new stream.  Another way to force the server to finish the stream before
    // closing is to wait for the OnOutputEndOfStream() before taking any action
    // that closes the stream.
    //
    // In addition to serving as an "EndOfStream" marker to make it obvious
    // client-side when all input data has been processed, if a client never
    // sends QueueInputEndOfStream(), no amount of waiting will necessarily
    // result in all input data getting processed through to the output.  Some
    // codecs have some internally-delayed data which only gets pushed through by
    // additional input data _or_ by this EndOfStream marker.  In that sense,
    // this message can be viewed as a flush-through at InputData domain level,
    // but the flush-through only takes effect if the codec even gets that far
    // before the stream is just closed at StreamControl domain level.  This
    // message is not alone sufficient to act as an overall flush-through at
    // StreamControl level. For that, send this message first and then send
    // FlushEndOfStreamAndCloseStream() (at which point it becomes possible to
    // queue input data for a new stream without causing discard of this older
    // stream's data), or wait for the OnOutputEndOfStream() before closing the
    // current stream.
    //
    // If a client sends QueueInputPacket(), QueueInputFormatDetails(),
    // QueueInputEndOfStream() for this stream after the first
    // QueueInputEndOfStream() for this stream, a server should close the Codec
    // channel.
    QueueInputEndOfStream(uint64 stream_lifetime_ordinal);
};