Two bugs caught in supervisor review:
1. Stderr was silently dropped when the Details panel was closed during a
crash. _on_spawn_done appended to diag_text only when the panel was
already open, and refresh_dependency() clobbered that text on the next
field change anyway.
Fix: stash the result in self._last_spawn; _full_diag_text() appends
the stderr section whenever diag text is generated; _on_spawn_done
auto-opens the panel on non-ok outcomes (same pattern as the existing
auto_open for missing commands).
2. _drain stopped reading once _STDERR_CAP (4 KB) was reached. A process
that writes more than 4 KB then blocked on a full pipe buffer, never
exited, and was misclassified as "ok" instead of "crashed".
Fix: drain to EOF unconditionally; keep only the first _STDERR_CAP bytes.
Regression test: 64 KB stderr + exit(3) → outcome "crashed".
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Add spawn_test() to bcc_core — spawns a stdio server for up to 3 s,
captures stderr, and reports ok/exited/crashed/not_found. Key design
decisions driven by real MCP server behaviour:
- stdin=PIPE (never written): servers block on JSON-RPC input and stay
alive, so "still running after timeout" reliably signals a healthy
start. stdin=DEVNULL would send EOF, causing well-behaved servers to
exit 0 and be misclassified as "exited".
- Command resolved via shutil.which(augmented_path()) before Popen so
subprocess PATH resolution is unambiguous across platforms.
- start_new_session=True on POSIX + os.killpg on timeout: kills the
whole process group, not just the launcher (npx, uvx), which would
otherwise orphan the actual node/python grandchild process.
- stdout=DEVNULL: draining a PIPE we don't read would deadlock at ~64 KB.
- stderr drained in a daemon thread, capped at 4 KB.
GUI: SpawnTester(QThread) wraps spawn_test; "Test launch" button in
ServerEditor dep row (stdio only, visible when command resolves ok/warn).
Result colours match the existing dep-status palette (green/amber/red).
Stderr appended to the diagnostics panel if it is open.
7 new unit tests cover all outcomes and the stdin-open regression guard.
Ran python bcc.py locally: button appears for stdio servers whose command
resolves, is hidden for remote servers and missing-command servers.
Closes#2
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
AJ