// 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;

/// Options for [`Directory.Open`] and [`Node.Reopen`].
table ConnectionOptions {
    /// Flags which can affect the behavior when opening and reopening.
    /// If absent, assumes a default of zero.
    1: ConnectionFlags flags;

    /// Specifies the set of representations accepted by the client, to support
    /// a form of protocol negotiation on the node being opened.
    /// Refer to the definition of [`NodeProtocols`] for more details.
    /// It cannot be zero.
    ///
    /// In addition, clients may assert the type of the object by setting
    /// the protocol corresponding to the expected type:
    ///
    /// * If the caller expected a directory but the node cannot be accessed
    ///   as a directory, the error is `ZX_ERR_NOT_DIR`.
    /// * If the caller expected a file but the node cannot be accessed as a
    ///   file, the error is `ZX_ERR_NOT_FILE`.
    /// * In other mismatched cases, the error is `ZX_ERR_WRONG_TYPE`.
    ///
    /// During [`Directory.Open`], if a new object is to be created, `protocols`
    /// determines the type of object to create; it must be present.
    /// If a valid object type cannot be unambiguously inferred e.g.
    /// both `DIRECTORY` and `FILE` were set, the request must fail.
    ///
    /// During [`Node.Reopen`], clients may specify a different but compatible
    /// `protocols` to do a "protocol upgrade".
    ///
    /// If more than one protocol is present in `protocols`, the resultant
    /// protocol may become any one of them. Clients should specify
    /// [`ConnectionFlags.GET_CONNECTION_INFO`] to receive a
    /// [`Node.OnConnectionInfo`] event, in order to ascertain the protocol.
    ///
    /// If absent, indicates that the caller accepts any type of node, and
    /// the resulting protocol is unspecified.
    2: NodeProtocols protocols;

    /// Requests rights on the new connection according to the specified rules.
    /// See [`fuchsia.io2/RightsRequest`].
    ///
    /// ## Rights Hierarchy
    ///
    /// Respecting principles of least privileges, rights in general must meet
    /// the following restrictions:
    ///
    /// * A connection must have nonzero rights.
    /// * From the perspective of a client, rights must never increase in a
    ///   derived connection.
    /// * From the perspective of a directory proxy, it must ensure that
    ///   new connections opened through it cannot have more rights than
    ///   the connection where the proxy received the `Open`/`Reopen` call.
    ///
    /// The proper enforcement of the rights hierarchy is a powerful refinement
    /// over the existing access control facilities offered by directory
    /// sandboxing.
    ///
    /// ## Rights Inheritance
    ///
    /// If `rights_request` is absent, inherits at most the rights on the source
    /// connection:
    ///
    /// * During [`Node.Reopen`], the new connection would have the same rights
    ///   as the connection where the `Reopen` call is made.
    /// * During [`Directory.Open`], the rights on the connection would inherit
    ///   from the connection where the `Open` call is made. If the path crosses
    ///   intermediate proxies, a proxy may strip elements from the resulting
    ///   rights if the intermediate connection does not have the corresponding
    ///   rights.
    ///
    /// ## Rights vs Abilities
    ///
    /// The rights on a connection limits the set of operations allowed on that
    /// connection, but does not guarantee their availability, because the
    /// object may not support it. For convenience, clients may query the
    /// [`ConnectionInfo.available_operations`] field on a new connection,
    /// which is the intersection of the rights and abilities and indicates the
    /// guaranteed set of available operations.
    ///
    /// See [`fuchsia.io2/Rights`] and [`fuchsia.io2/Abilities`].
    ///
    /// ## Implementation Notes
    ///
    /// When a directory proxy encounters an absent `rights` field, let `r` be
    /// the rights on the connection where it received this request, the proxy
    /// should fill in this field with the following:
    ///
    /// ```
    /// RightsRequest {
    ///     at_most: r,
    ///     at_least: 0,
    ///     resolution: RightsResolution.MAXIMIZE,
    /// }
    /// ```
    ///
    /// before forwarding the request to the remote party.
    3: RightsRequest rights_request;
};

/// Options for requesting rights on the new connection. Because opening a
/// new connection may involve multiple hops through directory proxies,
/// we require the client to set an upper bound and lower bound on the
/// rights request, and intermediate proxies to refine these bounds.
///
/// The rights manipulation should be implemented mechanically
/// without knowledge of any specific rights, and servers should propagate
/// unknown bits members, to gracefully handle future rights extensions.
///
/// ## Implementation Notes
///
/// It could be common for a client to request an exact set of rights.
/// We recommend client libraries to define a helper function like follows:
///
/// ```
/// fn Exact(exact_rights: Rights) -> RightsRequest {
///     RightsRequest {
///         at_most: exact_rights,
///         at_least: exact_rights,
///         resolution: RightsResolution.MAXIMIZE,
///     }
/// }
/// ```
struct RightsRequest {
    /// Sets an upper bound on the resulting rights. The exact rights will
    /// depend on `resolution`.
    ///
    /// ## Implementation Notes
    ///
    /// When a directory proxy encounters this variant, it should compute the
    /// intersection between this and the rights on the connection where it
    /// received the request, to shrink the rights.
    ///
    /// * If the intersection is empty, or not a superset of `at_least`,
    ///   the proxy should close `object_request` with the
    ///   `ZX_ERR_ACCESS_DENIED` epitaph.
    /// * Otherwise, the proxy should forward the `Open` call as usual,
    ///   but update `at_most` with the shrunk rights.
    Rights at_most;

    /// Sets a lower bound on the resulting rights. The exact rights will
    /// depend on `resolution`.
    ///
    /// + During [`Directory.Open`], you may only specify the same rights as
    ///   what the directory connection already has, or a subset of those.
    /// + During [`Node.Reopen`], similarly, you may only specify the same or
    ///   a subset of rights possessed by the original connection.
    /// + Exceeding those rights causes `object_request` to be closed with a
    ///   `ZX_ERR_ACCESS_DENIED` epitaph.
    ///
    /// Therefore there are these invariants which should be maintained:
    ///
    /// ```
    /// at_most ⊋ {}
    /// at_most ⊃ at_least
    /// rights_on_connection_where_open_is_received ⊋ {}
    /// rights_on_connection_where_open_is_received ⊃ at_least
    /// ```
    ///
    /// using the superset (`⊃`), proper superset (`⊋`),
    /// and empty set (`{}`) notations.
    Rights at_least;

    /// Controls how the rights on the new connection are computed from
    /// `at_most` and `at_least`. See [`fuchsia.io2/RightsResolution`].
    RightsResolution resolution;
};

/// When an `Open`/`Reopen` request reaches its final remote server, it should
/// assign rights on the new connection based on one of these modes.
enum RightsResolution : uint32 {
    /// The rights will be the intersection between [`RightsRequest.at_most`]
    /// and the connection where the `Open`/`Reopen` request was received,
    /// closing `object_request` with `ZX_ERR_ACCESS_DENIED` if it is empty.
    MAXIMIZE = 1;

    /// The rights will be determined by the following rules:
    ///
    /// * If the negotiated protocol on the new connection is
    ///   [`fuchsia.io2/Directory`], the rules from the `MAXIMIZE` case applies.
    /// * Otherwise, the rights will be [`RightsRequest.at_least`] if it
    ///   does not exceed rights on the current connection.
    /// * Otherwise, `object_request` should be closed with
    ///   `ZX_ERR_ACCESS_DENIED`.
    ///
    /// The motivation for this enum is to facilitate implementing POSIX
    /// compatibility layers. The POSIX file permission model relies on ambient
    /// authority: access control on files are resolved based on the `mode` of
    /// the file, and the current user. There is no concept of hierarchical
    /// permissions. Fuchsia, on the other hand, restricts rights on file
    /// connections to never exceed that of its containing directory connection.
    POSIX = 2;
};

/// Flags applicable to both [`Directory.Open`] and [`Node.Reopen`].
bits ConnectionFlags : uint64 {
    /// Requests that an [`Node.OnConnectionInfo`] event be sent as the first
    /// message on the protocol request. Requests all fields in the
    /// [`ConnectionInfo`] table. Doing so is more efficient than calling
    /// [`Node.Describe`] later on the connection.
    GET_CONNECTION_INFO = 0x01;

    /// Connects to the exposed protocol if the node is a Connector.
    /// It is an error to use this flag with other types of nodes.
    ///
    /// If both `GET_CONNECTION_INFO` and `CONNECT` are specified, the channel
    /// will receive exactly one [`Node.OnConnectionInfo`] event, after which
    /// the protocol switches from [`fuchsia.io2/Node`] to the intended protocol.
    /// Message sent by the client prior to receiving [`Node.OnConnectionInfo`]
    /// are queued and processed after the protocol switch.
    ///
    /// `CONNECT` cannot be supplied together with `APPEND`.
    /// `CONNECT` cannot be supplied together with `TRUNCATE`.
    ///
    /// Requires the [`Rights.CONNECT`] right on the connection.
    CONNECT = 0x02;

    /// Opens the node in append mode, i.e. the connection should seek to the
    /// end of the node before every write.
    ///
    /// If the node does not support appending, it should result in a
    /// `ZX_ERR_NOT_SUPPORTED` epitaph.
    /// Currently, only [`fuchsia.io2/NodeProtocols.FILE`] connections
    /// may be configured for appending.
    APPEND = 0x04;

    /// Truncates the object before usage, by setting its length to 0.
    /// Requires the [`Rights.WRITE_BYTES`] right on the connection.
    ///
    /// If the node does not support truncating, it should result in a
    /// `ZX_ERR_NOT_SUPPORTED` epitaph.
    TRUNCATE = 0x08;
};