logical decoding: Fix handling of large old tuples with replica identity full.

When decoding the old version of an UPDATE or DELETE change, and if that
tuple was bigger than MaxHeapTupleSize, we either Assert'ed out, or
failed in more subtle ways in non-assert builds.  Normally individual
tuples aren't bigger than MaxHeapTupleSize, with big datums toasted.
But that's not the case for the old version of a tuple for logical
decoding; the replica identity is logged as one piece. With the default
replica identity btree limits that to small tuples, but that's not the
case for FULL.

Change the tuple buffer infrastructure to separate allocate over-large
tuples, instead of always going through the slab cache.

This unfortunately requires changing the ReorderBufferTupleBuf
definition, we need to store the allocated size someplace. To avoid
requiring output plugins to recompile, don't store HeapTupleHeaderData
directly after HeapTupleData, but point to it via t_data; that leaves
rooms for the allocated size.  As there's no reason for an output plugin
to look at ReorderBufferTupleBuf->t_data.header, remove the field. It
was just a minor convenience having it directly accessible.

Reported-By: Adam Dratwiński
Discussion: CAKg6ypLd7773AOX4DiOGRwQk1TVOQKhNwjYiVjJnpq8Wo+i62Q@mail.gmail.com

1 files changed, 11 insertions, 4 deletions

diff --git a/src/include/replication/reorderbuffer.h b/src/include/replication/reorderbuffer.h
index 00183fa16d6..9092c9b4ff6 100644
--- a/src/include/replication/reorderbuffer.h
+++ b/src/include/replication/reorderbuffer.h
@@ -26,12 +26,19 @@ typedef struct ReorderBufferTupleBuf
 	/* position in preallocated list */
 	slist_node	node;
 
-	/* tuple, stored sequentially */
+	/* tuple header, the interesting bit for users of logical decoding */
 	HeapTupleData tuple;
-	HeapTupleHeaderData header;
-	char		data[MaxHeapTupleSize];
+
+	/* pre-allocated size of tuple buffer, different from tuple size */
+	Size	alloc_tuple_size;
+
+	/* actual tuple data follows */
 } ReorderBufferTupleBuf;
 
+/* pointer to the data stored in a TupleBuf */
+#define ReorderBufferTupleBufData(p) \
+	((HeapTupleHeader) MAXALIGN(((char *) p) + sizeof(ReorderBufferTupleBuf)))
+
 /*
  * Types of the change passed to a 'change' callback.
  *
@@ -327,7 +334,7 @@ struct ReorderBuffer
 ReorderBuffer *ReorderBufferAllocate(void);
 void		ReorderBufferFree(ReorderBuffer *);
 
-ReorderBufferTupleBuf *ReorderBufferGetTupleBuf(ReorderBuffer *);
+ReorderBufferTupleBuf *ReorderBufferGetTupleBuf(ReorderBuffer *, Size tuple_len);
 void		ReorderBufferReturnTupleBuf(ReorderBuffer *, ReorderBufferTupleBuf *tuple);
 ReorderBufferChange *ReorderBufferGetChange(ReorderBuffer *);
 void		ReorderBufferReturnChange(ReorderBuffer *, ReorderBufferChange *);