sdk/fidl/fuchsia.io2/file.fidl

// Copyright 2019 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.io2;

using fuchsia.mem;
using zx;

// TODO: We should run some experiments to see what's the optimum value, or
// what's the point of diminishing marginal returns.
/// The maximum I/O size that is allowed for read/write operations using
/// byte vectors.
const uint64 MAX_TRANSFER_SIZE = 8192;

/// The byte vector type used for read/write operations.
alias Transfer = bytes:MAX_TRANSFER_SIZE;

/// A [`fuchsia.io2/Node`] which contains a sequence of bytes of definite
/// length.
protocol File {
    compose Node;
    compose AdvisoryLocking;

    /// Moves the offset at which the next invocation of [`Read`] or [`Write`]
    /// will occur. The seek offset is specific to each file connection.
    ///
    /// + request `origin` the reference point where `offset` will be based on.
    /// + request `offset` the number of bytes to seek.
    /// - response `offset_from_start` the adjusted seek offset, from the start
    ///   of the file.
    ///
    /// This method does not require any rights.
    Seek(SeekOrigin origin, int64 offset)
        -> (uint64 offset_from_start) error zx.status;

    /// Reads up to 'count' bytes at the seek offset.
    /// The seek offset is moved forward by the number of bytes read.
    ///
    /// ## Invariants
    ///
    /// * The returned `data.length` will never be greater than `count`.
    /// * If `data.length` is less than `count`, it means that the seek offset
    ///   has reached the end of file as part of this operation.
    /// * If `data.length` is zero while `count` is not, it means that the
    ///   seek offset is already at or beyond the end of file, and no data could
    ///   be read.
    /// * If `count` is zero, the server should perform all the checks ensuring
    ///   read access without actually read anything, and return an empty
    ///   `data` vector.
    ///
    /// This method requires the [`Rights.READ_BYTES`] right.
    Read(uint64 count) -> (Transfer data) error zx.status;

    /// Writes data at the seek offset.
    /// The seek offset is moved forward by the number of bytes written.
    /// If the file is in append mode, the seek offset is first set to the end
    /// of the file, followed by the write, in one atomic step.
    ///
    /// The file size may grow if the seek offset plus `data.length` is beyond
    /// the current end of file.
    ///
    /// + request `data` the byte buffer to write to the file.
    /// - response `actual_count` the number of bytes written.
    ///
    /// ## Invariants
    ///
    /// * The returned `actual_count` will never be greater than `data.length`.
    /// * If the server is unable to write all the data due to e.g. not enough
    ///   space, `actual_count` may be less than `data.length`.
    /// * If `data.length` is zero, the server should perform all the checks
    ///   ensuring write access without mutating the file. The seek offset
    ///   is still updated if in append mode.
    ///
    /// This method requires the [`Rights.WRITE_BYTES`] right.
    Write(Transfer data) -> (uint64 actual_count) error zx.status;

    /// Reads up to 'count' bytes at the provided offset.
    /// Does not affect the seek offset.
    ///
    /// ## Invariants
    ///
    /// * The returned `data.length` will never be greater than `count`.
    /// * If `data.length` is less than `count`, it means that `ReadAt` has hit
    ///   the end of file as part of this operation.
    /// * If `data.length` is zero while `count` is not, it means that `offset`
    ///   is at or past the end of file, and no data can be read.
    /// * If `count` is zero, the server should perform all the checks ensuring
    ///   read access without actually reading anything, and return an empty
    ///   `data` vector.
    ///
    /// This method requires the [`Rights.READ_BYTES`] right.
    ReadAt(uint64 count, uint64 offset) -> (Transfer data) error zx.status;

    /// Writes data at the provided offset.
    /// Does not affect the seek offset.
    ///
    /// The file size may grow if `offset` plus `data.length` is past the
    /// current end of file.
    ///
    /// + request `data` the byte buffer to write to the file.
    /// + request `offset` the offset from start of the file to begin writing.
    /// - response `actual_count` the number of bytes written.
    ///
    /// ## Invariants
    ///
    /// * The returned `actual_count` will never be greater than `data.length`.
    /// * If the server is unable to write all the data due to e.g. not enough
    ///   space, `actual_count` may be less than `data.length`.
    /// * If `data.length` is zero, the server should perform all the checks
    ///   ensuring write access without mutating the file.
    ///
    /// This method requires the [`Rights.WRITE_BYTES`] right.
    WriteAt(Transfer data, uint64 offset)
        -> (uint64 actual_count) error zx.status;

    /// Shrinks or grows the file size to 'length' bytes.
    ///
    /// If file size is reduced by this operation, the extra trailing data'
    /// is discarded.
    /// If file size is increased by this operation, the extended area appears
    /// as if it was zeroed.
    ///
    /// This method requires the [`Rights.WRITE_BYTES`] right.
    Resize(uint64 length) -> () error zx.status;

    /// Acquires a [`fuchsia.mem/Range`] representing this file, if
    /// there is one, with the requested access rights.
    ///
    /// + request `flags` a [`VmoFlags`] indicating the desired mode of access.
    /// - response `buffer` the requested [`fuchsia.mem/Range`].
    /// * error a [`zx.status`] value indicating the failure.
    ///
    /// This method requires the following rights:
    ///
    /// * [`Rights.READ_BYTES`] if `flags` includes [`VmoFlags.READ`].
    /// * [`Rights.WRITE_BYTES`] if `flags` includes [`VmoFlags.WRITE`].
    /// * [`Rights.EXECUTE`] if `flags` includes [`VmoFlags.EXECUTE`].
    GetMemRange(VmoFlags flags) -> (fuchsia.mem.Range buffer) error zx.status;
};

/// The reference point for updating the seek offset. See [`File.Seek`].
///
/// This enum matches the `zx_stream_seek_origin_t` enum.
enum SeekOrigin : uint32 {
    /// Seek from the start of the file.
    /// The seek offset will be set to `offset` bytes.
    /// The seek offset cannot be negative in this case.
    START = 0;

    /// Seek from the current position in the file.
    /// The seek offset will be the current seek offset plus `offset` bytes.
    CURRENT = 1;

    /// Seek from the end of the file.
    /// The seek offset will be the file size plus `offset` bytes.
    END = 2;
};

bits VmoFlags : uint64 {
    /// Requests that the VMO be readable.
    READ = 0x00000001;

    /// Requests that the VMO be writable.
    WRITE = 0x00000002;

    /// Request that the VMO be executable.
    EXECUTE = 0x00000004;

    /// Require a copy-on-write clone of the underlying VMO.
    /// The request should fail if the VMO cannot be cloned.
    /// May not be supplied with `SHARED_BUFFER`.
    PRIVATE_CLONE = 0x00010000;

    /// Require an exact (non-cloned) handle to the underlying VMO.
    /// All clients using this flag would get a VMO with the same koid.
    /// The request should fail if a handle to the exact VMO cannot be returned.
    /// May not be supplied with `PRIVATE_CLONE`.
    SHARED_BUFFER = 0x00020000;
};

/// Auxiliary data for the file representation of a node.
/// The selection of this variant in [`Representation`] implies that the
/// connection speaks the [`fuchsia.io2/File`] protocol.
resource table FileInfo {
    /// An optional event which transmits information about an object's
    /// readability or writability. This event relays information about the
    /// underlying object, not the capability granted to client: this event
    /// may be signalled "readable" on a connection that does not have
    /// the capability to read.
    ///
    /// This event will be present if the following conditions are met:
    ///
    /// - The `available_operations` on the file connection is not empty.
    /// - The filesystem supports signalling readability/writability events.
    ///
    /// The [`FileSignal`] values may be observed on this event.
    1: zx.handle:EVENT observer;

    /// Returns if the file is opened in append mode.
    /// In append mode, the seek offset is moved to the end before every
    /// write, the two steps performed in an atomic manner.
    2: bool is_append;

    /// An optional stream object, which can be used to read to and write from
    /// the file.
    ///
    /// Reading and writing the file using the stream object can be up to 20x
    /// faster than reading and writing the file using the Read and Write
    /// operations in the [`fuchsia.io2/File`] protocol.
    3: zx.handle:STREAM stream;
};

bits FileSignal : uint32 {
    /// Indicates the file is ready for reading.
    READABLE = 0x01000000; // ZX_USER_SIGNAL_0

    /// Indicates the file is ready for writing.
    WRITABLE = 0x02000000; // ZX_USER_SIGNAL_1
};