Fix log locking problem when using fork() in the library

Ad pointed out by Dan Berrange:
So if some thread in libvirtd is currently executing a logging call,
while another thread calls virExec(), that other thread no longer
exists in the child, but its lock is never released. So when the
child then does virLogReset() it deadlocks.

The only way I see to address this, is for the parent process to call
virLogLock(), immediately before fork(), and then virLogUnlock()
afterwards in both parent & child. This will ensure that no other
thread
can be holding the lock across fork().

* src/util/logging.[ch] src/libvirt_private.syms: export virLogLock() and
  virLogUnlock()
* src/util/util.c: lock just before forking and unlock just after - in
  both parent and child.

src/libvirt_private.syms

@@ -366,6 +366,8 @@ virLogParseOutputs;
 virLogStartup;
 virLogShutdown;
 virLogReset;
+virLogLock;
+virLogUnlock;
 
 
 # memory.h

src/util/logging.c

@@ -133,11 +133,11 @@ static int virLogResetOutputs(void);
  */
 virMutex virLogMutex;
 
-static void virLogLock(void)
+void virLogLock(void)
 {
     virMutexLock(&virLogMutex);
 }
-static void virLogUnlock(void)
+void virLogUnlock(void)
 {
     virMutexUnlock(&virLogMutex);
 }

src/util/logging.h

@@ -128,6 +128,8 @@ extern int virLogDefineOutput(virLogOutputFunc f, virLogCloseFunc c, void *data,
  * Internal logging API
  */
 
+extern void virLogLock(void);
+extern void virLogUnlock(void);
 extern int virLogStartup(void);
 extern int virLogReset(void);
 extern void virLogShutdown(void);

src/util/util.c

@@ -415,7 +415,15 @@ __virExec(virConnectPtr conn,
         childerr = null;
     }
 
-    if ((pid = fork()) < 0) {
+    /* Ensure we hold the logging lock, to protect child processes
+     * from deadlocking on another threads inheirited mutex state */
+    virLogLock();
+    pid = fork();
+
+    /* Unlock for both parent and child process */
+    virLogUnlock();
+
+    if (pid < 0) {
         virReportSystemError(conn, errno,
                              "%s", _("cannot fork child process"));
         goto cleanup;