avutil/buffer: free all pooled buffers immediately after uninitializing the pool

No buffer will be fetched from the pool after it's uninitialized, so there's
no benefit from waiting until every single buffer has been returned to it
before freeing them all.
This should free some memory in certain scenarios, which can be beneficial in
low memory systems.

Based on a patch by Jonas Karlman.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/buffer.c

@@ -279,11 +279,7 @@ AVBufferPool *av_buffer_pool_init(int size, AVBufferRef* (*alloc)(int size))
     return pool;
 }
 
-/*
- * This function gets called when the pool has been uninited and
- * all the buffers returned to it.
- */
-static void buffer_pool_free(AVBufferPool *pool)
+static void buffer_pool_flush(AVBufferPool *pool)
 {
     while (pool->pool) {
         BufferPoolEntry *buf = pool->pool;
@@ -292,6 +288,15 @@ static void buffer_pool_free(AVBufferPool *pool)
         buf->free(buf->opaque, buf->data);
         av_freep(&buf);
     }
+}
+
+/*
+ * This function gets called when the pool has been uninited and
+ * all the buffers returned to it.
+ */
+static void buffer_pool_free(AVBufferPool *pool)
+{
+    buffer_pool_flush(pool);
     ff_mutex_destroy(&pool->mutex);
 
     if (pool->pool_free)
@@ -309,6 +314,10 @@ void av_buffer_pool_uninit(AVBufferPool **ppool)
     pool   = *ppool;
     *ppool = NULL;
 
+    ff_mutex_lock(&pool->mutex);
+    buffer_pool_flush(pool);
+    ff_mutex_unlock(&pool->mutex);
+
     if (atomic_fetch_sub_explicit(&pool->refcount, 1, memory_order_acq_rel) == 1)
         buffer_pool_free(pool);
 }