Fix thread-safety of qt_ignore_sigpipe

The testAndSet operation would mean another thread could see the value
of 1 and proceed to write(2)/sendto(2) before SIGPIPE had been ignored.
If the pipe or socket were already closed by then, a SIGPIPE would be
delivered to the application with its default action: terminate.

(cherry-picked from qtbase commit 3f970c20f9afd5c9a1cc14d7f69882e13f6aaf1b)
Change-Id: I62dc8f5fa14c1dd453d13e4053c642bd78fbc468
Reviewed-by: Qt Doc Bot <qt_docbot@qt-project.org>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>
Reviewed-by: Peter Hartmann <phartmann@rim.com>

2 files changed, 10 insertions, 2 deletions

diff --git a/src/corelib/io/qprocess_unix.cpp b/src/corelib/io/qprocess_unix.cpp
index 82d80dc..2feb6a5 100644
--- a/src/corelib/io/qprocess_unix.cpp
+++ b/src/corelib/io/qprocess_unix.cpp
@@ -962,11 +962,15 @@ static void qt_ignore_sigpipe()
 {
     // Set to ignore SIGPIPE once only.
     static QBasicAtomicInt atom = Q_BASIC_ATOMIC_INITIALIZER(0);
-    if (atom.testAndSetRelaxed(0, 1)) {
+    if (!atom) {
+        // More than one thread could turn off SIGPIPE at the same time
+        // But that's acceptable because they all would be doing the same
+        // action
         struct sigaction noaction;
         memset(&noaction, 0, sizeof(noaction));
         noaction.sa_handler = SIG_IGN;
         ::sigaction(SIGPIPE, &noaction, 0);
+        atom = 1;
     }
 }
 
diff --git a/src/network/socket/qnativesocketengine_unix.cpp b/src/network/socket/qnativesocketengine_unix.cpp
index b4e5126..640a627 100644
--- a/src/network/socket/qnativesocketengine_unix.cpp
+++ b/src/network/socket/qnativesocketengine_unix.cpp
@@ -106,11 +106,15 @@ static void qt_ignore_sigpipe()
 #ifndef Q_NO_POSIX_SIGNALS
     // Set to ignore SIGPIPE once only.
     static QBasicAtomicInt atom = Q_BASIC_ATOMIC_INITIALIZER(0);
-    if (atom.testAndSetRelaxed(0, 1)) {
+    if (!atom) {
+        // More than one thread could turn off SIGPIPE at the same time
+        // But that's acceptable because they all would be doing the same
+        // action
         struct sigaction noaction;
         memset(&noaction, 0, sizeof(noaction));
         noaction.sa_handler = SIG_IGN;
         ::sigaction(SIGPIPE, &noaction, 0);
+        atom = 1;
     }
 #else
     // Posix signals are not supported by the underlying platform