Additional validation for buffer in the ring

If the buffer pool has been shrunk, the buffers in the buffer list may
not be valid anymore. Modify GetBufferFromRing to check if the buffer is
still valid before using it. This makes GetBufferFromRing() a bit more
expensive because of additional boolean condition. That may not be
expensive enough to affect query performance. The alternative to that is
more complex as explained below.

The strategy object is created in CurrentMemoryContext and is not
available in any global structure thus accessible when processing buffer
resizing barriers. We may modify GetAccessStrategy() to register
strategy in a global linked list and then arrange to deregister it once
it's no more in use. Looking at the places which use
GetAccessStrategy(), fixing all those may be some work.

Ashutosh Bapat

Author: ashutosh-bapat

src/backend/storage/buffer/freelist.c

@@ -948,12 +948,13 @@ GetBufferFromRing(BufferAccessStrategy strategy, uint32 *buf_state)
 		strategy->current = 0;
 
 	/*
-	 * If the slot hasn't been filled yet, tell the caller to allocate a new
-	 * buffer with the normal allocation strategy.  He will then fill this
-	 * slot by calling AddBufferToRing with the new buffer.
+	 * If the slot hasn't been filled yet or the buffer in the slot has been
+	 * invalidated when buffer pool was shrunk, tell the caller to allocate a new
+	 * buffer with the normal allocation strategy.  He will then fill this slot
+	 * by calling AddBufferToRing with the new buffer.
 	 */
 	bufnum = strategy->buffers[strategy->current];
-	if (bufnum == InvalidBuffer)
+	if (bufnum == InvalidBuffer || bufnum > NBuffers)
 		return NULL;
 
 	/*