virthread: Introduce virLockGuard

Locks a virMutex on creation and unlocks it in its destructor.

The VIR_LOCK_GUARD macro is used instead of "g_auto(virLockGuard)" to
work around a clang issue (see https://bugs.llvm.org/show_bug.cgi?id=3888
and https://bugs.llvm.org/show_bug.cgi?id=43482).

Typical usage:

    void function(virMutex *m)
    {
        VIR_LOCK_GUARD lock = virLockGuardLock(m);
        /* `m` is locked, and released automatically on scope exit */

        ...
        while (expression) {
            VIR_LOCK_GUARD lock2 = virLockGuardLock(...);
            /* similar */
        }
    }

Signed-off-by: Tim Wiederhake <twiederh@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

src/libvirt_private.syms

@@ -3388,6 +3388,8 @@ virCondInit;
 virCondSignal;
 virCondWait;
 virCondWaitUntil;
+virLockGuardLock;
+virLockGuardUnlock;
 virMutexDestroy;
 virMutexInit;
 virMutexInitRecursive;

src/util/virthread.c

@@ -96,6 +96,21 @@ void virMutexUnlock(virMutex *m)
     pthread_mutex_unlock(&m->lock);
 }
 
+virLockGuard virLockGuardLock(virMutex *m)
+{
+    virLockGuard l = { m };
+    virMutexLock(m);
+    return l;
+}
+
+void virLockGuardUnlock(virLockGuard *l)
+{
+    if (!l || !l->mutex)
+        return;
+
+    virMutexUnlock(g_steal_pointer(&l->mutex));
+}
+
 
 int virRWLockInit(virRWLock *m)
 {

src/util/virthread.h

@@ -31,6 +31,11 @@ struct virMutex {
     pthread_mutex_t lock;
 };
 
+typedef struct virLockGuard virLockGuard;
+struct virLockGuard {
+    virMutex *mutex;
+};
+
 typedef struct virRWLock virRWLock;
 struct virRWLock {
     pthread_rwlock_t lock;
@@ -121,6 +126,11 @@ void virMutexLock(virMutex *m);
 void virMutexUnlock(virMutex *m);
 
 
+virLockGuard virLockGuardLock(virMutex *m);
+void virLockGuardUnlock(virLockGuard *l);
+G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(virLockGuard, virLockGuardUnlock);
+#define VIR_LOCK_GUARD g_auto(virLockGuard) G_GNUC_UNUSED
+
 int virRWLockInit(virRWLock *m) G_GNUC_WARN_UNUSED_RESULT;
 void virRWLockDestroy(virRWLock *m);