Fix bug in bulk extending temp relation after failure

A ResourceOwnerEnlarge() call was missing. That led to an error:

ERROR:  ResourceOwnerRemember called but array was full

and an assertion failure, if you tried to extend a temp relation again
after a failure. Alexander's test case used running out of disk space
to trigger the original failure.

This bug was introduced in the large ResourceOwner rewrite commit
b8bff07daa. Before that, the UnpinLocalBuffer() call guaranteed that
the subsequent PinLocalBuffer() will succeed, but after the rewrite,
releasing an old resource doesn't guarantee that there is space for a
new one.

Add a comment explaining why the UnpinBuffer + PinBuffer calls in
BufferAlloc(), with no ResourceOwnerEnlarge() in between, are safe.

Reported-by: Alexander Lakhin
Discussion: https://www.postgresql.org/message-id/dc574fea-c83e-a600-08cd-10881762e4fa@gmail.com

src/backend/storage/buffer/localbuf.c

@@ -374,6 +374,9 @@ ExtendBufferedRelLocal(BufferManagerRelation bmr,
 		victim_buf_id = -buffers[i] - 1;
 		victim_buf_hdr = GetLocalBufferDescriptor(victim_buf_id);
 
+		/* in case we need to pin an existing buffer below */
+		ResourceOwnerEnlarge(CurrentResourceOwner);
+
 		InitBufferTag(&tag, &bmr.smgr->smgr_rlocator.locator, fork, first_block + i);
 
 		hresult = (LocalBufferLookupEnt *)
@@ -646,6 +649,8 @@ InitLocalBuffers(void)
  * XXX: We could have a slightly more efficient version of PinLocalBuffer()
  * that does not support adjusting the usagecount - but so far it does not
  * seem worth the trouble.
+ *
+ * Note that ResourceOwnerEnlarge() must have been done already.
  */
 bool
 PinLocalBuffer(BufferDesc *buf_hdr, bool adjust_usagecount)