[tests] Fix flaky pty log probe test on macOS (#18681)

tests/unit_tests/test_log.py

@@ -1,5 +1,4 @@
 from collections.abc import Generator
-import errno
 import io
 import logging
 import os
@@ -178,37 +177,34 @@ def _run_probe_on_pty(
     output = b""
     deadline = time.monotonic() + 60
     try:
-        try:
-            proc = subprocess.Popen(
-                _probe_command(fixture_path),
-                stdout=follower,
-                stderr=follower if stderr_to_pty else subprocess.PIPE,
-                stdin=follower,
-                env=probe_env,
-            )
-        finally:
-            os.close(follower)
-        while True:
-            timeout = deadline - time.monotonic()
-            if timeout <= 0 or not select.select([controller], [], [], timeout)[0]:
-                pytest.fail(f"pty probe produced no EOF in time; got {output!r}")
-            try:
-                chunk = os.read(controller, 1024)
-            except OSError as err:
-                # macOS raises EIO once the child closes its end of the pty;
-                # anything else is a real failure, not end-of-stream.
-                if err.errno != errno.EIO:
-                    raise
-                break
-            if not chunk:
-                break
+        proc = subprocess.Popen(
+            _probe_command(fixture_path),
+            stdout=follower,
+            stderr=follower if stderr_to_pty else subprocess.PIPE,
+            stdin=follower,
+            env=probe_env,
+        )
+        # The parent keeps the follower open until the child has exited and
+        # the controller is drained: macOS discards buffered pty output once
+        # the last follower closes, so closing it early loses the probe's
+        # output whenever the child finishes before the first read.
+        while proc.poll() is None:
+            if time.monotonic() > deadline:
+                pytest.fail(f"pty probe did not exit in time; got {output!r}")
+            if select.select([controller], [], [], 0.01)[0]:
+                output += os.read(controller, 4096)
+        # Everything the child wrote is already buffered, so drain without waiting.
+        while select.select([controller], [], [], 0)[0] and (
+            chunk := os.read(controller, 4096)
+        ):
             output += chunk
         stderr_text = ""
         if proc.stderr is not None:
             stderr_text = proc.stderr.read().decode(errors="replace")
             proc.stderr.close()
-        assert proc.wait(60) == 0, stderr_text
+        assert proc.returncode == 0, stderr_text
     finally:
+        os.close(follower)
         os.close(controller)
         if proc is not None and proc.poll() is None:
             proc.kill()