Remove a race condition in SIGCHLD handler.

If the SIGCHLD is delivered *just* as we're installing our handler,
there's a race condition in the setting of the old handler. So instead
rely on the OS to set it properly before it calls our handler.

Reviewed-by: Olivier Goffart

1 files changed, 10 insertions, 15 deletions

diff --git a/src/corelib/io/qprocess_unix.cpp b/src/corelib/io/qprocess_unix.cpp
index ba61bda..b93fce9 100644
--- a/src/corelib/io/qprocess_unix.cpp
+++ b/src/corelib/io/qprocess_unix.cpp
@@ -120,7 +120,7 @@ static inline char *strdup(const char *data)
 #endif
 
 static int qt_qprocess_deadChild_pipe[2];
-static void (*qt_sa_old_sigchld_handler)(int) = 0;
+static struct sigaction qt_sa_old_sigchld_handler;
 static void qt_sa_sigchld_handler(int signum)
 {
     qt_safe_write(qt_qprocess_deadChild_pipe[1], "", 1);
@@ -128,8 +128,10 @@ static void qt_sa_sigchld_handler(int signum)
     fprintf(stderr, "*** SIGCHLD\n");
 #endif
 
-    if (qt_sa_old_sigchld_handler && qt_sa_old_sigchld_handler != SIG_IGN)
-        qt_sa_old_sigchld_handler(signum);
+    // load it as volatile
+    void (*oldAction)(int) = ((volatile struct sigaction *)&qt_sa_old_sigchld_handler)->sa_handler;
+    if (oldAction && oldAction != SIG_IGN)
+        oldAction(signum);
 }
 
 static inline void add_fd(int &nfds, int fd, fd_set *fdset)
@@ -190,14 +192,11 @@ QProcessManager::QProcessManager()
 
     // set up the SIGCHLD handler, which writes a single byte to the dead
     // child pipe every time a child dies.
-    struct sigaction oldAction;
     struct sigaction action;
     memset(&action, 0, sizeof(action));
     action.sa_handler = qt_sa_sigchld_handler;
     action.sa_flags = SA_NOCLDSTOP;
-    ::sigaction(SIGCHLD, &action, &oldAction);
-    if (oldAction.sa_handler != qt_sa_sigchld_handler)
-	qt_sa_old_sigchld_handler = oldAction.sa_handler;
+    ::sigaction(SIGCHLD, &action, &qt_sa_old_sigchld_handler);
 }
 
 QProcessManager::~QProcessManager()
@@ -217,14 +216,10 @@ QProcessManager::~QProcessManager()
     qDeleteAll(children.values());
     children.clear();
 
-    struct sigaction oldAction;
-    struct sigaction action;
-    memset(&action, 0, sizeof(action));
-    action.sa_handler = qt_sa_old_sigchld_handler;
-    action.sa_flags = SA_NOCLDSTOP;
-    ::sigaction(SIGCHLD, &action, &oldAction);
-    if (oldAction.sa_handler != qt_sa_sigchld_handler) {
-        ::sigaction(SIGCHLD, &oldAction, 0);
+    struct sigaction currentAction;
+    ::sigaction(SIGCHLD, 0, &currentAction);
+    if (currentAction.sa_handler == qt_sa_sigchld_handler) {
+        ::sigaction(SIGCHLD, &qt_sa_old_sigchld_handler, 0);
     }
 }