Subject: Re: memory leak when serializing TRUNCATE in reorderbuffer
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>,
 pgsql-hackers@lists.postgresql.org
References: <66175a41-9342-2845-652f-1bd4c3ee50aa@2ndquadrant.com>
 <04cfcf19-1a43-5d85-918d-56a8620377c4@2ndquadrant.com>
 <a6e5cf90-bee9-8b57-5f13-bed59cf6014f@2ndquadrant.com>
Message-ID: <9b17d019-7261-a5b0-1d65-caf30f69a891@2ndquadrant.com>
Date: Thu, 16 Aug 2018 14:24:38 +0200
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On 08/08/2018 09:47 PM, Tomas Vondra wrote:
> 
> IMHO the cleanest way is to add a method like
> ReorderBufferGetChange, which does the allocation internally. That
> way the memory context choice is up to reorderbuffer, not decode.c.
> That's at least consistent with what the rest of decode.c does.
> 

OK, attached is a patch doing it along these lines. It introduces
ReorderBufferGetRelids and ReorderBufferReturnRelids, which make the
decision which context to use (so it's contained in reorderbuffer.c).
Currently the main reorderbuffer context is used - we can't use the SLAB
contexts, and the tup_context is meant for something else.

I've considered adding yet another special-purpose context into
reorderbuffer, but it seems like an overkill considering that TRUNCATE
usually is not particularly common operation. So using the main context
seems OK.

I plan to commit this over the next couple of days, and backpatch it to
pg11 where TRUNCATE decoding was introduced. It's clearly a memory leak,
and there is no behavior change.

Two more things I noticed while working on this:

1) We're using very different data types for nrelids on various places
in the decoding. xl_heap_truncate uses uint32, logicalrep_write_truncate
uses int and ReorderBufferChange uses Size. That seems somewhat strange,
although it's unlikely to overflow (Who would truncate that many rels at
the same time?).

2) The tup_context is allocated like this:

  buffer->tup_context = GenerationContextCreate(new_ctx,
                                                "Tuples",
                                                SLAB_LARGE_BLOCK_SIZE);

Using SLAB_ constant for GenerationContextCreate is not a bug, but it's
a bit confusing.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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diff --git a/src/backend/replication/logical/decode.c b/src/backend/replication/logical/decode.c
index 59c003de9c..6e46884194 100644
--- a/src/backend/replication/logical/decode.c
+++ b/src/backend/replication/logical/decode.c
@@ -859,7 +859,7 @@ DecodeTruncate(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
 	if (xlrec->flags & XLH_TRUNCATE_RESTART_SEQS)
 		change->data.truncate.restart_seqs = true;
 	change->data.truncate.nrelids = xlrec->nrelids;
-	change->data.truncate.relids = palloc(xlrec->nrelids * sizeof(Oid));
+	change->data.truncate.relids = ReorderBufferGetRelids(ctx->reorder, xlrec->nrelids);
 	memcpy(change->data.truncate.relids, xlrec->relids,
 		   xlrec->nrelids * sizeof(Oid));
 	ReorderBufferQueueChange(ctx->reorder, XLogRecGetXid(r),
diff --git a/src/backend/replication/logical/reorderbuffer.c b/src/backend/replication/logical/reorderbuffer.c
index 1d43a165ad..0e41040ac2 100644
--- a/src/backend/replication/logical/reorderbuffer.c
+++ b/src/backend/replication/logical/reorderbuffer.c
@@ -409,10 +409,16 @@ ReorderBufferReturnChange(ReorderBuffer *rb, ReorderBufferChange *change)
 			}
 			break;
 			/* no data in addition to the struct itself */
+		case REORDER_BUFFER_CHANGE_TRUNCATE:
+			if (change->data.truncate.relids != NULL)
+			{
+				ReorderBufferReturnRelids(rb, change->data.truncate.relids);
+				change->data.truncate.relids = NULL;
+			}
+			break;
 		case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_CONFIRM:
 		case REORDER_BUFFER_CHANGE_INTERNAL_COMMAND_ID:
 		case REORDER_BUFFER_CHANGE_INTERNAL_TUPLECID:
-		case REORDER_BUFFER_CHANGE_TRUNCATE:
 			break;
 	}
 
@@ -451,6 +457,35 @@ ReorderBufferReturnTupleBuf(ReorderBuffer *rb, ReorderBufferTupleBuf *tuple)
 }
 
 /*
+ * Get an array for relids of truncated relations. We use the global context
+ * for the whole reorder buffer - none of the existing ones seems like a good
+ * match (change/txn are SLAB, so we can't use them anyway, and tup_context
+ * is meant for tuple data, not for relids). We could add yet another context,
+ * but it seems like an overkill. So just use rb->context directly.
+ */
+Oid *
+ReorderBufferGetRelids(ReorderBuffer *rb, int nrelids)
+{
+	Oid	   *relids;
+	Size	alloc_len;
+
+	alloc_len = sizeof(Oid) * nrelids;
+
+	relids = (Oid *) MemoryContextAlloc(rb->context, alloc_len);
+
+	return relids;
+}
+
+/*
+ * Free an array of relids.
+ */
+void
+ReorderBufferReturnRelids(ReorderBuffer *rb, Oid *relids)
+{
+	pfree(relids);
+}
+
+/*
  * Return the ReorderBufferTXN from the given buffer, specified by Xid.
  * If create is true, and a transaction doesn't already exist, create it
  * (with the given LSN, and as top transaction if that's specified);
@@ -2412,6 +2447,26 @@ ReorderBufferSerializeChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
 				break;
 			}
 		case REORDER_BUFFER_CHANGE_TRUNCATE:
+			{
+				Size	size;
+				char   *data;
+
+				/* account for the OIDs of truncated relations */
+				size = sizeof(Oid) * change->data.truncate.nrelids;
+				sz += size;
+
+				/* make sure we have enough space */
+				ReorderBufferSerializeReserve(rb, sz);
+
+				data = ((char *) rb->outbuf) + sizeof(ReorderBufferDiskChange);
+				/* might have been reallocated above */
+				ondisk = (ReorderBufferDiskChange *) rb->outbuf;
+
+				memcpy(data, change->data.truncate.relids, size);
+				data += size;
+
+				break;
+			}
 		case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_CONFIRM:
 		case REORDER_BUFFER_CHANGE_INTERNAL_COMMAND_ID:
 		case REORDER_BUFFER_CHANGE_INTERNAL_TUPLECID:
@@ -2695,6 +2750,16 @@ ReorderBufferRestoreChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
 			}
 			/* the base struct contains all the data, easy peasy */
 		case REORDER_BUFFER_CHANGE_TRUNCATE:
+			{
+				Oid	   *relids;
+
+				relids = ReorderBufferGetRelids(rb,
+												change->data.truncate.nrelids);
+				memcpy(relids, data, change->data.truncate.nrelids * sizeof(Oid));
+				change->data.truncate.relids = relids;
+
+				break;
+			}
 		case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_CONFIRM:
 		case REORDER_BUFFER_CHANGE_INTERNAL_COMMAND_ID:
 		case REORDER_BUFFER_CHANGE_INTERNAL_TUPLECID:
diff --git a/src/include/replication/reorderbuffer.h b/src/include/replication/reorderbuffer.h
index 1f52f6bde7..7787edf7b6 100644
--- a/src/include/replication/reorderbuffer.h
+++ b/src/include/replication/reorderbuffer.h
@@ -402,6 +402,9 @@ void		ReorderBufferReturnTupleBuf(ReorderBuffer *, ReorderBufferTupleBuf *tuple)
 ReorderBufferChange *ReorderBufferGetChange(ReorderBuffer *);
 void		ReorderBufferReturnChange(ReorderBuffer *, ReorderBufferChange *);
 
+Oid * ReorderBufferGetRelids(ReorderBuffer *, int nrelids);
+void ReorderBufferReturnRelids(ReorderBuffer *, Oid *relids);
+
 void		ReorderBufferQueueChange(ReorderBuffer *, TransactionId, XLogRecPtr lsn, ReorderBufferChange *);
 void ReorderBufferQueueMessage(ReorderBuffer *, TransactionId, Snapshot snapshot, XLogRecPtr lsn,
 						  bool transactional, const char *prefix,

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