Fix some oversights in commit 2455ab48.

The idea was to generate all the junk in a destroyable subcontext rather
than leaking it in the caller's context, but partition_bounds_create was
still being called in the caller's context, allowing plenty of scope for
leakage.  Also, get_rel_relkind() was still being called in the rel's
rd_pdcxt, creating a risk of session-lifespan memory wastage.

Simplify the logic a bit while at it.  Also, reduce rd_pdcxt to
ALLOCSET_SMALL_SIZES, since it seems likely to not usually be big.

Probably something like this needs to be back-patched into v11,
but for now let's get some buildfarm testing on this.

Discussion: https://postgr.es/m/15943.1552601288@sss.pgh.pa.us

src/backend/partitioning/partdesc.c

@@ -67,8 +67,8 @@ RelationBuildPartitionDesc(Relation rel)
 				nparts;
 	PartitionKey key = RelationGetPartitionKey(rel);
 	MemoryContext oldcxt;
+	MemoryContext rbcontext;
 	int		   *mapping;
-	MemoryContext rbcontext = NULL;
 
 	/*
 	 * While building the partition descriptor, we create various temporary
@@ -187,56 +187,50 @@ RelationBuildPartitionDesc(Relation rel)
 	/* Now build the actual relcache partition descriptor */
 	rel->rd_pdcxt = AllocSetContextCreate(CacheMemoryContext,
 										  "partition descriptor",
-										  ALLOCSET_DEFAULT_SIZES);
+										  ALLOCSET_SMALL_SIZES);
 	MemoryContextCopyAndSetIdentifier(rel->rd_pdcxt,
 									  RelationGetRelationName(rel));
 
-	MemoryContextSwitchTo(rel->rd_pdcxt);
-	partdesc = (PartitionDescData *) palloc0(sizeof(PartitionDescData));
+	partdesc = (PartitionDescData *)
+		MemoryContextAllocZero(rel->rd_pdcxt, sizeof(PartitionDescData));
 	partdesc->nparts = nparts;
-	/* oids and boundinfo are allocated below. */
-
-	MemoryContextSwitchTo(oldcxt);
-
-	if (nparts == 0)
+	/* If there are no partitions, the rest of the partdesc can stay zero */
+	if (nparts > 0)
 	{
-		/* We can exit early in this case. */
-		rel->rd_partdesc = partdesc;
-
-		/* Blow away the temporary context. */
-		MemoryContextDelete(rbcontext);
-		return;
-	}
-
-	/* First create PartitionBoundInfo */
+		/* Create PartitionBoundInfo, using the temporary context. */
 		boundinfo = partition_bounds_create(boundspecs, nparts, key, &mapping);
 
-	/* Now copy boundinfo and oids into partdesc. */
-	oldcxt = MemoryContextSwitchTo(rel->rd_pdcxt);
+		/* Now copy all info into relcache's partdesc. */
+		MemoryContextSwitchTo(rel->rd_pdcxt);
 		partdesc->boundinfo = partition_bounds_copy(boundinfo, key);
-	partdesc->oids = (Oid *) palloc(partdesc->nparts * sizeof(Oid));
-	partdesc->is_leaf = (bool *) palloc(partdesc->nparts * sizeof(bool));
+		partdesc->oids = (Oid *) palloc(nparts * sizeof(Oid));
+		partdesc->is_leaf = (bool *) palloc(nparts * sizeof(bool));
 
 		/*
-	 * Now assign OIDs from the original array into mapped indexes of the
+		 * Assign OIDs from the original array into mapped indexes of the
 		 * result array.  The order of OIDs in the former is defined by the
-	 * catalog scan that retrieved them, whereas that in the latter is defined
-	 * by canonicalized representation of the partition bounds.
+		 * catalog scan that retrieved them, whereas that in the latter is
+		 * defined by canonicalized representation of the partition bounds.
+		 *
+		 * Also record leaf-ness of each partition.  For this we use
+		 * get_rel_relkind() which may leak memory, so be sure to run it in
+		 * the temporary context.
 		 */
-	for (i = 0; i < partdesc->nparts; i++)
+		MemoryContextSwitchTo(rbcontext);
+		for (i = 0; i < nparts; i++)
 		{
 			int			index = mapping[i];
 
 			partdesc->oids[index] = oids[i];
-		/* Record if the partition is a leaf partition */
 			partdesc->is_leaf[index] =
 				(get_rel_relkind(oids[i]) != RELKIND_PARTITIONED_TABLE);
 		}
-	MemoryContextSwitchTo(oldcxt);
+	}
 
 	rel->rd_partdesc = partdesc;
 
-	/* Blow away the temporary context. */
+	/* Return to caller's context, and blow away the temporary context. */
+	MemoryContextSwitchTo(oldcxt);
 	MemoryContextDelete(rbcontext);
 }