From: Christian Babeux <christian.babeux@efficios.com>
Date: Thu, 6 Dec 2012 04:33:17 +0000 (-0500)
Subject: Cygwin: Fix racy read to detect applications sockets close
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=bcccc1bfd7e1586259ff1c516a458e04321d0e78;p=lttng-tools.git

Cygwin: Fix racy read to detect applications sockets close

The mechanism to detect the applications sockets close is racy and
could potentially discard data. A read(3) is performed on the application
socket and the returned len is compared to 0 to detect the case where an
application closed the socket. If a read(3) occur in the thread_manage_apps
*before* the consumption of the data in the consumer, one byte of data would
be discarded.

To fix this race, perform a recv(3) on the socket with the MSG_PEEK flag.
This has no impact on the data delivery to the consumer and can successfully
detect the applications sockets close.
---

diff --git a/src/bin/lttng-sessiond/main.c b/src/bin/lttng-sessiond/main.c
index 722549554..236341d27 100644
--- a/src/bin/lttng-sessiond/main.c
+++ b/src/bin/lttng-sessiond/main.c
@@ -1293,9 +1293,10 @@ static void *thread_manage_apps(void *data)
 					char dummy;
 
 					do {
-						readlen = read(pollfd, &dummy, 1);
+						readlen = recv(pollfd, &dummy, 1, MSG_PEEK);
 					} while (readlen == -1 && errno == EINTR);
 
+					/* Peer has performed an orderly shutdown */
 					if (readlen == 0) {
 						/* Removing from the poll set */
 						ret = lttng_poll_del(&events, pollfd);