libcamera: base: semaphore: Do not unlock prematurely

In `Semaphore::release()`, unlocking the mutex before signalling the condition
variable can be problematic, especially with "temporary" objects such as the
ones `BoundMethodBase::activatePack()` uses to handle `ConnectionTypeBlocking`.

Specifically, `Semaphore::acquire()` might lock the mutex after `Semaphore::release()`
has unlocked it, but before it had the chance to notify the condition variable.
In that case `Semaphore::acquire()` can succeed, and execution may proceed to
destroy the `Semaphore` object while the other thread is in the process of
running `std::condition_variable::notify_all()`.

Bug: https://bugs.libcamera.org/show_bug.cgi?id=225
Fixes: 66e7c5b774 ("libcamera: Add Semaphore class")
Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/base/semaphore.cpp

@@ -93,11 +93,9 @@ bool Semaphore::tryAcquire(unsigned int n)
  */
 void Semaphore::release(unsigned int n)
 {
-	{
-		MutexLocker locker(mutex_);
-		available_ += n;
-	}
+	MutexLocker locker(mutex_);
 
+	available_ += n;
 	cv_.notify_all();
 }