Fix memory leak when de-toasting compressed values in VACUUM FULL/CLUSTER

VACUUM FULL and CLUSTER can be used to enforce the use of the existing
compression method of a toastable column if a value currently stored is
compressed with a method that does not match the column's defined
method.  The code in charge of decompressing and recompressing toast
values at rewrite left around the detoasted values, causing an
accumulation of memory allocated in TopTransactionContext.

When processing large relations, this could cause the system to run out
of memory.  The detoasted values are not needed once their tuple is
rewritten, and this commit ensures that the necessary cleanup happens.

Issue introduced by bbe0a81.  The comments of the area are reordered a
bit while on it.

Reported-by: Andres Freund
Analyzed-by: Andres Freund
Author: Michael Paquier
Reviewed-by: Dilip Kumar
Discussion: https://postgr.es/m/20210521211929.pcehg6f23icwstdb@alap3.anarazel.de

Author: michaelpq

src/backend/access/heap/heapam_handler.c

@@ -2463,24 +2463,27 @@ reform_and_rewrite_tuple(HeapTuple tuple,
 	TupleDesc	newTupDesc = RelationGetDescr(NewHeap);
 	HeapTuple	copiedTuple;
 	int			i;
+	bool		values_free[MaxTupleAttributeNumber];
+
+	memset(values_free, 0, newTupDesc->natts * sizeof(bool));
 
 	heap_deform_tuple(tuple, oldTupDesc, values, isnull);
 
-	/* Be sure to null out any dropped columns */
 	for (i = 0; i < newTupDesc->natts; i++)
 	{
+		/* Be sure to null out any dropped columns */
 		if (TupleDescAttr(newTupDesc, i)->attisdropped)
 			isnull[i] = true;
-
-		/*
-		 * Use this opportunity to force recompression of any data that's
-		 * compressed with some TOAST compression method other than the one
-		 * configured for the column.  We don't actually need to perform the
-		 * compression here; we just need to decompress. That will trigger
-		 * recompression later on.
-		 */
 		else if (!isnull[i] && TupleDescAttr(newTupDesc, i)->attlen == -1)
 		{
+			/*
+			 * Use this opportunity to force recompression of any data that's
+			 * compressed with some TOAST compression method other than the
+			 * one configured for the column.  We don't actually need to
+			 * perform the compression here; we just need to decompress.  That
+			 * will trigger recompression later on.
+			 */
+
 			struct varlena *new_value;
 			ToastCompressionId cmid;
 			char		cmethod;
@@ -2507,7 +2510,10 @@ reform_and_rewrite_tuple(HeapTuple tuple,
 
 			/* if compression method doesn't match then detoast the value */
 			if (TupleDescAttr(newTupDesc, i)->attcompression != cmethod)
+			{
 				values[i] = PointerGetDatum(detoast_attr(new_value));
+				values_free[i] = true;
+			}
 		}
 	}
 
@@ -2516,6 +2522,13 @@ reform_and_rewrite_tuple(HeapTuple tuple,
 	/* The heap rewrite module does the rest */
 	rewrite_heap_tuple(rwstate, tuple, copiedTuple);
 
+	/* Free any value detoasted previously */
+	for (i = 0; i < newTupDesc->natts; i++)
+	{
+		if (values_free[i])
+			pfree(DatumGetPointer(values[i]));
+	}
+
 	heap_freetuple(copiedTuple);
 }