Response

The server sends Ok once the mount is serving and nothing more on the scope for its own life, or an error, UTF-8, and the finish.

[0]             ok: the mount is serving
[1][message …]  not made: why not, UTF-8

• The sequence. Exactly one response whose first byte is 0, and no response finish for the life of the server; or exactly one response whose first byte is 1, followed by the response finish.
• The message. The bytes after the byte 1 are UTF-8, not JSON: the reason the mount was not made.
• A finish with nothing. A response finish that no response precedes states that the server could not read the request.

The response is defined by diverge-sdk-rs/src/container_proxy/outside/endpoints/fuse/mount/server/response/frame.rs:

//! What a server's response frame carries for a FUSE mount.

/// `Ok` once the mount is complete — the mount point made, the
/// session started, the path serving — and then the scope stays open
/// for the proxy's life; or `Error` with why not, then the finish:
/// the path empty or the root, a path the proxy already mounted, a
/// mount point it could not make, no FUSE on the host, a session that
/// would not start. See [`ack::Frame`](crate::shared::containers::fuse::ack::Frame).
///
/// An alias rather than a re-export because this module is real: the
/// path says this scope's answer lives here, and it does, rather than
/// naming somewhere else and hoping a reader follows. What the alias
/// points at is right there in the signature.
pub type Frame<'a> = crate::shared::containers::fuse::ack::Frame<'a>;

and by diverge-sdk-rs/src/shared/containers/fuse/ack/frame.rs:

//! Ok, or why not.

use std::convert::Infallible;

use super::super::ResponseError;
use crate::wire::encode::{Encode, Writer};

/// The one message that answers a mutation.
///
/// ```text
/// [kind: u8][message…]
/// ```
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Frame<'a> {
    /// Kind `0`. It happened: the caller holds the result.
    Ok,
    /// Kind `1`. It did not, and this says why, for a reader rather
    /// than a program: what a caller can refuse — a mount it keeps
    /// unchangeable, a directory it will not empty, a path it will not
    /// serve — is its policy and not this specification's.
    Error(&'a str),
    /// Kind `2`. It did not, because the volume behind the mount is
    /// read only: its mode is
    /// [`ReadOnly`](crate::provider::endpoints::volumes::Mode::ReadOnly), and
    /// no change is made to it. Nothing after the kind. The program
    /// sees a read-only filesystem, `EROFS`, and every immutable ask
    /// on the same mount goes through.
    ReadOnly,
}

impl Encode for Frame<'_> {
    /// [`Infallible`]: a kind byte and bytes copied.
    type Error = Infallible;

    fn encode(&self, out: &mut Writer<'_>) -> Result<(), Infallible> {
        match self {
            Frame::Ok => out.extend_from_slice(&[0]),
            Frame::Error(message) => {
                out.extend_from_slice(&[1]);
                out.extend_from_slice(message.as_bytes());
            }
            Frame::ReadOnly => out.extend_from_slice(&[2]),
        }
        Ok(())
    }
}

impl<'a> Frame<'a> {
    /// Decode the one message. The message borrows from `bytes`.
    pub fn decode(bytes: &'a [u8]) -> Result<Self, ResponseError> {
        let (kind, rest) = bytes.split_first().ok_or(ResponseError::Empty)?;
        match *kind {
            0 => Ok(Frame::Ok),
            1 => std::str::from_utf8(rest)
                .map(Frame::Error)
                .map_err(|_| ResponseError::MessageUtf8),
            2 => Ok(Frame::ReadOnly),
            other => Err(ResponseError::UnknownKind(other)),
        }
    }
}