Closes#1100.
## What
`handleAsync` in `internal/jsonrpc2/conn.go` reported *why* an in-flight
request had been canceled by inspecting the connection-level
`s.writeErr`, guarded by a TODO waiting on golang/go#51365:
```go
// Only deliver to the Handler if not already canceled.
if err := req.ctx.Err(); err != nil {
c.updateInFlight(func(s *inFlightState) {
if s.writeErr != nil {
// Assume that req.ctx was canceled due to s.writeErr.
// TODO(#51365): use a Context API to plumb this through req.ctx.
err = fmt.Errorf("%w: %v", ErrServerClosing, s.writeErr)
}
})
...
}
```
That API shipped as `context.WithCancelCause` / `context.Cause` in Go
1.21, and this module already targets Go 1.25, so the TODO is now
actionable.
This PR carries the cancellation cause on each request's context instead
of inferring it from global state:
- `incomingRequest.cancel` becomes a `context.CancelCauseFunc` (built
via `context.WithCancelCause`).
- The write-failure and read-side teardowns cancel each in-flight
request with a cause wrapping `ErrServerClosing`.
- The remaining cancel sites (peer cancel via `Cancel`, normal
completion) pass `nil`.
- `handleAsync` reads `context.Cause(req.ctx)` instead of looking up
`s.writeErr`.
## Why
Besides resolving the TODO, this fixes a latent mis-attribution.
`s.writeErr` is connection-level state, not per-request: a request
canceled for an unrelated reason (e.g. a peer cancel notification via
`Cancel`) was reported as `ErrServerClosing` whenever `s.writeErr`
happened to be non-nil at that moment. Tying the cause to the specific
request's context removes that coupling.
`ctx.Err()` still returns `context.Canceled` in all cases, so external
behavior is unchanged; only the reported cause becomes accurate and
per-request.
## Test plan
- `go build ./...`
- `go test ./internal/... ./mcp/` (including the longer `mcp`
integration tests) — all pass.
- Verified `context.WithCancelCause` semantics: with a cause, `Err()`
stays `context.Canceled` while `Cause()` returns the wrapped
`ErrServerClosing`; with `nil`, both resolve to `context.Canceled` —
matching prior behavior.
## Note
Per the discussion in #1100, the read-side teardown also carries a
descriptive cause (wrapping `ErrServerClosing`) rather than a bare
`context.Canceled`.
---------
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
Co-authored-by: Yaroslav <yarolegovich@gmail.com>
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