Fix pipe read race in exec_service server

SetReadDeadline(time.Now()) can fire before the read loop consumes
buffered pipe data. Go's poller checks the deadline before attempting
syscall.Read, so ErrDeadlineExceeded is returned and buffered output
is silently dropped.

Fix: kill the process group after cmd.Wait() so background children
close their inherited pipe fds, letting the read loop reach EOF
without any deadline. A 100ms deadline fallback handles the rare case
of children that escaped the process group (e.g. via setsid).

Reproducer (fails 10/10 without the fix):

    bazel test //exec_service/cmd/exec_client:exec_client_test \
        --test_arg=-test.run=TestStderrOutput \
        --test_arg=-test.count=500 \
        --test_arg=-test.cpu=1 \
        --runs_per_test=10 --local_test_jobs=10

Author: afq984

exec_service/server/server.go

@@ -61,13 +61,17 @@ func (s *ExecServer) RunCommand(req *pb.StartCommandRequest, stream pb.ExecServi
 		syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL)
 	}
 
-	// Wait for the command concurrently. When it exits, expire the pipe
-	// read deadline so the read loop drains buffered output without
-	// blocking on background children that inherited the pipe fds.
+	// Wait for the command concurrently. When it exits, kill the
+	// process group so that background children close their inherited
+	// pipe fds, allowing the read loop to reach EOF cleanly. The
+	// deadline is a fallback for children that escaped the group
+	// (e.g. via setsid); it is not reached in the normal case
+	// because SIGKILL closes the pipe before the deadline fires.
 	waitCh := make(chan error, 1)
 	go func() {
 		err := cmd.Wait()
-		pr.SetReadDeadline(time.Now())
+		killGroup()
+		pr.SetReadDeadline(time.Now().Add(100 * time.Millisecond))
 		waitCh <- err
 	}()