Flush the stdio subprocess's coverage data before the clean-exit line (#2840)

Author: maxisbey

tests/interaction/transports/_stdio_server.py

@@ -9,6 +9,7 @@
 import sys
 
 import anyio
+import coverage
 
 from mcp.server import Server, ServerRequestContext
 from mcp.server.stdio import stdio_server
@@ -54,9 +55,27 @@ async def set_logging_level(ctx: ServerRequestContext, params: SetLevelRequestPa
 async def main() -> None:
     async with stdio_server() as (read_stream, write_stream):
         await server.run(read_stream, write_stream, server.create_initialization_options())
+    # Flush this process's coverage data before the clean-exit line below. Without this, the
+    # data is only written by coverage's atexit hook during interpreter teardown -- and on a
+    # slow Windows runner that can overrun the transport's termination grace, so the kill
+    # silently destroys the data file and the 100% gate trips on this module's subprocess-only
+    # lines. Saving here puts the write before the line the test synchronizes on: once the
+    # parent has seen "clean exit", the data is durably on disk and the escalation is harmless.
+    # Nothing measured may execute after the save (it would be unrecordable by construction),
+    # hence the excluded lines below. The branch is pragma'd because under coverage the
+    # instance always exists, and without coverage nothing is measured anyway.
+    cov = getattr(coverage.process_startup, "coverage", None)
+    if cov is not None:  # pragma: no branch
+        # stop() is load-bearing twice over: it ends tracing, making itself the last
+        # recordable line, and it leaves nothing new for coverage's atexit re-save to flush --
+        # so a kill landing during interpreter teardown cannot corrupt the file save() wrote
+        # (coverage opens it with sqlite journaling off; a torn rewrite would not roll back).
+        cov.stop()
+        cov.save()  # pragma: lax no cover - untraced: stop() above already ended measurement
     # Reached only when the run loop exits because stdin closed; if the process were terminated
-    # the test's stderr capture would not see this line.
-    print("stdio-echo: clean exit", file=sys.stderr, flush=True)
+    # the test's stderr capture would not see this line. lax no cover: runs after the coverage
+    # save by design, so it can never appear covered.
+    print("stdio-echo: clean exit", file=sys.stderr, flush=True)  # pragma: lax no cover
 
 
 if __name__ == "__main__":

tests/interaction/transports/test_stdio.py

@@ -56,10 +56,12 @@ async def test_tool_call_and_notification_round_trip_over_a_stdio_subprocess(
     notification before the call returns; the server exits when the transport closes its
     stdin.
     """
-    # After stdin closes, the child must unwind, write the clean-exit line, and let coverage's
-    # atexit hook persist its subprocess data file before escalation. The production 2s default
-    # was too tight on slow Windows runners: the child was killed mid-atexit (test stayed green)
-    # and the silently missing data file tripped the 100% coverage gate. Not under test.
+    # After stdin closes, the child must unwind, flush its subprocess coverage data, and write
+    # the clean-exit line before escalation (the server saves coverage *before* printing, so a
+    # post-print kill can no longer silently lose the data file -- see _stdio_server.main). The
+    # production 2s default is too tight for the unwind+save tail on loaded Windows runners
+    # (measured in-situ p99 of the whole test is ~7s); a kill before the print fails the stderr
+    # assertion below loudly rather than tripping the coverage gate. Not under test.
     monkeypatch.setattr(stdio, "PROCESS_TERMINATION_TIMEOUT", 10.0)
 
     received: list[LoggingMessageNotificationParams] = []