[lttng-dev] [PATCH lttng-tools v3 6/8] Change lttng_poll_wait behaviour of compat-poll to match compat-epoll

[Yannick Lamarre]

This removes the need to verify for idle file descriptors and mitigates
risks of bug due to behaviour mismatch.

Signed-off-by: Yannick Lamarre <ylamarre at efficios.com>
---
 src/common/compat/compat-poll.c | 39 +++++++++++++++++++++++++++++++++++----
 1 file changed, 35 insertions(+), 4 deletions(-)

diff --git a/src/common/compat/compat-poll.c b/src/common/compat/compat-poll.c
index be655728..8263b64b 100644
--- a/src/common/compat/compat-poll.c
+++ b/src/common/compat/compat-poll.c
@@ -293,7 +293,8 @@ error:
  */
 int compat_poll_wait(struct lttng_poll_event *events, int timeout)
 {
-	int ret;
+	int ret, active_fd_count;
+	int idle_pfd_index = 0;
 
 	if (events == NULL || events->current.events == NULL) {
 		ERR("poll wait arguments error");
@@ -325,11 +326,41 @@ int compat_poll_wait(struct lttng_poll_event *events, int timeout)
 		goto error;
 	}
 
+	active_fd_count = ret;
+
 	/*
-	 * poll() should always iterate on all FDs since we handle the pollset in
-	 * user space and after poll returns, we have to try every fd for a match.
+	 * Swap all active pollfd structs to the beginning of the
+	 * array to emulate compat-epoll behaviour. This algorithm takes
+	 * advantage of poll's returned value and the burst nature of active
+	 * events on the file descriptors. The while loop guarantees that
+	 * idle_pfd will always point to an idle fd.
 	 */
-	return events->wait.nb_fd;
+	if (active_fd_count == events->wait.nb_fd) {
+		goto end;
+	}
+	while (idle_pfd_index < active_fd_count &&
+			events->wait.events[idle_pfd_index].revents != 0) {
+		idle_pfd_index++;
+	}
+
+	for (int i = idle_pfd_index + 1; idle_pfd_index < active_fd_count;
+			i++) {
+		struct pollfd swap_pfd;
+		struct pollfd *idle_pfd = &events->wait.events[idle_pfd_index];
+		struct pollfd *current_pfd = &events->wait.events[i];
+
+		assert(i < events->wait.nb_fd);
+
+		if (ipfd->revents != 0) {
+			swap_pfd = *current_pfd;
+			*current_pfd = *idle_pfd;
+			*idle_pfd = swap_pfd;
+			idle_pfd_index++;
+		}
+	}
+
+end:
+	return ret;
 
 error:
 	return -1;
-- 
2.11.0