* fix(mcp): give stdio subprocess a real stderr fd under redirected streams (#418)
On Windows the Textual TUI redirects sys.stderr to an in-memory capture
(textual.app._PrintCapture) whose fileno() returns -1. The MCP SDK forwards
that stderr to stdio server subprocesses via subprocess.Popen(stderr=...),
and Popen rejects the invalid handle with OSError: [Errno 9] Bad file
descriptor — so only stdio servers fail to load (HTTP/SSE are unaffected).
Wrap mcp.client.stdio.stdio_client so that, whenever the configured errlog
has no usable fileno, it falls back to sys.__stderr__ (or os.devnull in GUI
hosts). Idempotent, no-op when the SDK is absent, warns if the SDK renames
stdio_client. Adds 9 regression tests and a troubleshooting note.
* fix(mcp): validate live fd and close fallback errlog after stdio session
Address CodeRabbit review on #423:
- _stdio_errlog_is_usable now os.fstat()s the fd to reject closed streams
that still report their former positive fileno (prevents a deferred
[Errno 9] from subprocess.Popen).
- The stdio_client wrapper owns the devnull fallback it allocates and
closes it once the session exits, so repeated MCP reloads no longer leak
file descriptors. Caller-provided usable errlogs pass through untouched.
- Tests cover the closed-fd case, the fd-leak/closure invariant, and
confirm langchain-mcp-adapters binds the patched stdio_client.
* fix(mcp): rebind adapter stdio_client, forward errlog by kw, harden tests
Address CodeRabbit round-2 review on #423:
- The patch now also rebinds langchain_mcp_adapters.sessions.stdio_client,
which the adapter captures via a 'from' import at module load — so the
wrapped function reaches the adapter regardless of import order.
- errlog is forwarded to the SDK by keyword (original(server, *args,
errlog=errlog, **kwargs)) so a future SDK inserting a positional
parameter before errlog can't mis-bind the fallback.
- The fallback stream is now allocated inside the async context manager,
so it is closed on session exit even if the CM is constructed but never
entered (narrower fd-leak path).
- test_closed_fd_rejected now reaches the os.fstat branch (stale positive
fd stub) instead of the ValueError path; test_adapter_binds_patched_stdio_client
documents and asserts the import-order-independent rebind.
* fix(mcp): close fallback errlog when stdio_client construction fails
Address CodeRabbit round-3 review on #423: move the original(server, *args,
errlog=errlog, **kwargs) construction inside the try block so a failure
during subprocess/client setup still reaches the finally and closes the
wrapper-owned os.devnull stream. Added test_fallback_closed_when_construction_fails
covering the path.
* refactor(mcp): track fallback ownership via (stream, opened_by_us)
Address din0s review on #423:
- _safe_stdio_errlog() now returns (stream, opened_by_us); the wrapper closes
the fallback only when opened_by_us is True, instead of inferring ownership
from needs_fallback + an identity check against sys.__stderr__. Simpler and
less likely to regress.
- Removed dead try/finally in test_closed_fd_rejected.
- Added test_wrapped_stdio_client_swaps_explicit_bad_errlog covering the
'not _stdio_errlog_is_usable(errlog)' branch (explicit bad errlog, not the
default sentinel).
- Updated test_safe_errlog_returns_usable_stream for the tuple return.
---------
Co-authored-by: Xi Zhang <106144707+X-iZhang@users.noreply.github.com>
houren Antony
932c934485