Response

The server sends exactly one response, whose payload is exactly the byte 0 when the volume no longer exists, exactly the byte 1 when the volume is mounted and was not deleted, or the byte 2 followed by an error as one JSON value, and the response finish follows it.

The server sends exactly one response on the scope. The response finish follows the response, and no frame follows the finish.

[0]
[1]
[2][error JSON …]

• The sequence. Exactly one response precedes the response finish, and nothing follows the finish. A response finish that no response precedes states that the request was not served, as Endpoints provides; whether the volume exists is then not stated, and the client learns it from a volumes::list request.
• Deleted. A payload whose first byte is 0 states that the volume no longer exists. The server sends it after the volume is gone. From the moment the server sends it, every volumes::list request by the client omits the volume: a listing the client receives after receiving the byte 0 never contains it, and no interval in which the volume is deleted and still listed exists. The server sends nothing after the byte 0, and a client ignores every byte that follows it.
• Mounted. A payload whose first byte is 1 states that the volume is mounted in a running container at the time of the request and was not deleted. The volume, its content, and the client’s listing are as they were before the request. The server sends nothing after the byte 1, and a client ignores every byte that follows it.
• The error. A payload whose first byte is 2 carries an error after that byte, running to the end of the payload. The error is exactly one JSON value, in the form defined on the volumes::list response page. This revision prescribes nothing about its content. The error states that the volume exists as a result of the request, as it did before it, and that the reason is not a mount.
• Malformed. A payload with no byte, a payload whose first byte is none of 0, 1 and 2, and a payload whose bytes after 2 are not one JSON value are malformed.

The response is defined by diverge-provider-sdk/src/endpoints/volumes/delete/server/response/frame.rs:

//! What a server's response frame carries for a volume deletion.

use std::fmt;

use crate::decode::Decode;
use crate::encode::{Encode, Writer};
use crate::shared::error::Error;

/// The volume is gone, or it is mounted and stays, or it is not gone
/// for some other reason.
///
/// One of these on channel `0`, then the scope finishes. A payload
/// leads with one byte saying which — `0` for
/// [`Deleted`](Self::Deleted), `1` for [`Mounted`](Self::Mounted), `2`
/// for [`Error`](Self::Error) — and for the first two there is nothing
/// after it, because saying so IS the whole message.
///
/// | the scope ends with | means |
/// |----------------------|-------|
/// | [`Deleted`](Self::Deleted), then a finish | the volume is gone and a listing will not show it |
/// | [`Mounted`](Self::Mounted), then a finish | it is mounted in a running container, and nothing was changed |
/// | an [`Error`](Self::Error), then a finish | it is not gone, for some other reason, and a caller should assume it is intact |
///
/// # Mounted is an answer, not an error
///
/// A provider MUST refuse to delete a volume that is mounted in a
/// running container at the time of the request, and it says so with
/// [`Mounted`](Self::Mounted) rather than with an
/// [`Error`](Self::Error) a caller could not tell from any other
/// failure. The distinction is what a caller acts on: a mounted
/// volume is one to stop the container over and ask again, and a
/// failure is not.
///
/// # Gone means gone, not emptied
///
/// The volume itself no longer exists. A caller that wanted the space
/// back with the name kept deletes and creates — which is two asks
/// because they are two things, and a provider that emptied one in
/// place would be doing something this frame has no way to distinguish
/// from the other.
///
/// # There is no partial deletion to report
///
/// A provider either finishes destroying the volume or reports an
/// error. What it leaves behind on failure is its own business and not
/// a state a caller can observe: a
/// [`list`](crate::endpoints::volumes::list) shows the volume or it
/// does not, and that is the only answer this protocol offers about
/// whether something exists.
#[derive(Debug, Clone, PartialEq)]
pub enum Frame {
    /// The volume is gone. Tag `0`.
    Deleted,
    /// The volume is mounted in a running container, and was not
    /// deleted. Tag `1`.
    ///
    /// Nothing was changed: the volume, its content, and the listing
    /// are as they were.
    Mounted,
    /// A failure. Tag `2`.
    ///
    /// See [`shared::error::Error`](crate::shared::error::Error) for
    /// why it says so little.
    Error(Error),
}

/// Tag for [`Frame::Deleted`].
const DELETED: u8 = 0;

/// Tag for [`Frame::Mounted`].
const MOUNTED: u8 = 1;

/// Tag for [`Frame::Error`].
const ERROR: u8 = 2;

/// A tag, and for a failure the JSON after it. Postcard encodes the
/// rest of [`volumes`](crate::endpoints::volumes) and encodes nothing
/// here — an [`Error`](Frame::Error) is a
/// [`serde_json::Value`], which deserializes through
/// `deserialize_any` and so cannot come back out of a format with no
/// self-description. The tag chooses the format, one variant at a
/// time.
impl Encode for Frame {
    /// The ordinary JSON failure, from the one variant that has one.
    /// A lone tag byte cannot fail.
    type Error = serde_json::Error;

    // Spelled out rather than `Self::Error`: this enum has a variant
    // called `Error`, so the associated type is ambiguous by that name.
    fn encode(
        &self,
        out: &mut Writer<'_>,
    ) -> Result<(), serde_json::Error> {
        match self {
            Frame::Deleted => {
                out.extend_from_slice(&[DELETED]);
                Ok(())
            }
            Frame::Mounted => {
                out.extend_from_slice(&[MOUNTED]);
                Ok(())
            }
            Frame::Error(error) => {
                out.extend_from_slice(&[ERROR]);
                error.encode(out)
            }
        }
    }
}

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

    // Spelled out for the same reason as `encode` above.
    fn decode(bytes: &[u8]) -> Result<Self, FrameError> {
        let (tag, rest) = bytes.split_first().ok_or(FrameError::Empty)?;
        match *tag {
            DELETED => Ok(Frame::Deleted),
            MOUNTED => Ok(Frame::Mounted),
            ERROR => {
                Error::decode(rest).map(Frame::Error).map_err(FrameError::Error)
            }
            tag => Err(FrameError::UnknownTag(tag)),
        }
    }
}

/// A volume deletion result that could not be read.
#[derive(Debug)]
pub enum FrameError {
    /// No bytes at all, so not even a tag.
    Empty,
    /// A tag that is none of this frame's three.
    UnknownTag(u8),
    /// The error did not parse.
    Error(serde_json::Error),
}

impl fmt::Display for FrameError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            FrameError::Empty => {
                f.write_str("volume deletion result frame is empty")
            }
            FrameError::UnknownTag(tag) => {
                write!(f, "unknown volume deletion result frame tag {tag}")
            }
            FrameError::Error(error) => {
                write!(f, "volume deletion error did not parse: {error}")
            }
        }
    }
}

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