Request

The payload is the tag byte 6 followed by the path of the directory as JSON: an array of components from the root of the container, empty for the root.

[6][request JSON …]

• path. An array of strings: the components of the directory to serve, from the root of the container. An empty array names the root of the container. No component is empty, . or .., or contains / or NUL; a path with such a component is answered with an error, not as malformed.
• No mode, and nothing left out. The request carries nothing else: a container has no mode, and the server leaves out of a serve what it leaves out of every tree.
• Malformed. A payload that does not decode as stated is answered by a response finish that no response precedes.

The request is defined by diverge-sdk-rs/src/container_proxy/outside/endpoints/filesystem/fuse_serve/client/request/frame.rs:

//! What a client's request frame carries for a FUSE serve.

use serde::{Deserialize, Serialize};

use crate::wire::decode::Decode;
use crate::wire::encode::{Encode, Writer};

/// One directory of the container, served live.
///
/// Nothing but the path: there is no mode, because a container has
/// none, and nothing to leave out, because the proxy leaves out of a
/// serve what it leaves out of every tree.
#[derive(Debug, Clone, PartialEq, Eq, Hash, Serialize, Deserialize, Default)]
pub struct Frame {
    /// The subtree, as path components from the container's root —
    /// the same meaning of "path" as everywhere else in this crate,
    /// and the same frame of reference a
    /// [`filetree`](crate::shared::filetree) stream uses. Empty is the
    /// root itself. Components rather than a joined string: a path is
    /// a sequence, and joining it would invent a separator that then
    /// has to be escaped out of names containing it.
    pub path: Vec<String>,
}

/// This frame's tag among the scope-opening requests.
///
/// One byte at the front of the payload, which is what tells a reader
/// which request it holds. The frame layer does not discriminate them
/// — [`ClientFrame::Request`](crate::wire::frame::client::ClientFrame::Request)
/// is one type carrying bytes — so the distinction has to be in the
/// bytes, and each request owns the value that names it.
///
/// See the table in
/// [`container_proxy_endpoints`](crate::container_proxy::outside) for
/// the whole allocation. The values are chosen across modules that do
/// not know about each other, so the table is the only place they can
/// be seen at once.
const TAG: u8 = 6;

impl Encode for Frame {
    /// The ordinary JSON failure. The tag cannot fail.
    type Error = serde_json::Error;

    fn encode(&self, out: &mut Writer<'_>) -> Result<(), Self::Error> {
        out.extend_from_slice(&[TAG]);
        serde_json::to_writer(out, self)
    }
}

impl Decode<'_> for Frame {
    /// Three ways to fail, and only one of them is JSON.
    type Error = FrameError;

    fn decode(bytes: &[u8]) -> Result<Self, Self::Error> {
        let (tag, rest) = bytes.split_first().ok_or(FrameError::Empty)?;
        if *tag != TAG {
            return Err(FrameError::UnexpectedTag(*tag));
        }
        serde_json::from_slice(rest).map_err(FrameError::Body)
    }
}

/// A FUSE serve request that could not be read.
#[derive(Debug)]
pub enum FrameError {
    /// No bytes at all, so not even a tag.
    Empty,
    /// A tag naming some other request.
    ///
    /// A reader that dispatched on the tag will not see this. One that
    /// assumed which request it held, and was wrong, will — which is
    /// the point of checking a tag rather than skipping it.
    UnexpectedTag(u8),
    /// The request did not parse.
    Body(serde_json::Error),
}

impl std::fmt::Display for FrameError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            FrameError::Empty => {
                f.write_str("FUSE serve request frame is empty")
            }
            FrameError::UnexpectedTag(tag) => {
                write!(f, "expected FUSE serve request tag {TAG}, found {tag}")
            }
            FrameError::Body(error) => {
                write!(f, "FUSE serve request did not parse: {error}")
            }
        }
    }
}

impl std::error::Error for FrameError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match self {
            FrameError::Body(error) => Some(error),
            FrameError::Empty | FrameError::UnexpectedTag(_) => None,
        }
    }
}