Redesign the API for list sorting (list_qsort becomes list_sort).

In the wake of commit 1cff1b95a, the obvious way to sort a List
is to apply qsort() directly to the array of ListCells.  list_qsort
was building an intermediate array of pointers-to-ListCells, which
we no longer need, but getting rid of it forces an API change:
the comparator functions need to do one less level of indirection.

Since we're having to touch the callers anyway, let's do two additional
changes: sort the given list in-place rather than making a copy (as
none of the existing callers have any use for the copying behavior),
and rename list_qsort to list_sort.  It was argued that the old name
exposes more about the implementation than it should, which I find
pretty questionable, but a better reason to rename it is to be sure
we get the attention of any external callers about the need to fix
their comparator functions.

While we're at it, change four existing callers of qsort() to use
list_sort instead; previously, they all had local reinventions
of list_qsort, ie build-an-array-from-a-List-and-qsort-it.
(There are some other places where changing to list_sort perhaps
would be worthwhile, but they're less obviously wins.)

Discussion: https://postgr.es/m/29361.1563220190@sss.pgh.pa.us

1 files changed, 8 insertions, 19 deletions

diff --git a/src/backend/replication/logical/reorderbuffer.c b/src/backend/replication/logical/reorderbuffer.c
index 591377d2cd7..e8ffa0492f1 100644
--- a/src/backend/replication/logical/reorderbuffer.c
+++ b/src/backend/replication/logical/reorderbuffer.c
@@ -3236,7 +3236,7 @@ ReorderBufferToastReset(ReorderBuffer *rb, ReorderBufferTXN *txn)
  * -------------------------------------------------------------------------
  */
 
-/* struct for qsort()ing mapping files by lsn somewhat efficiently */
+/* struct for sorting mapping files by LSN efficiently */
 typedef struct RewriteMappingFile
 {
 	XLogRecPtr	lsn;
@@ -3378,13 +3378,13 @@ TransactionIdInArray(TransactionId xid, TransactionId *xip, Size num)
 }
 
 /*
- * qsort() comparator for sorting RewriteMappingFiles in LSN order.
+ * list_sort() comparator for sorting RewriteMappingFiles in LSN order.
  */
 static int
-file_sort_by_lsn(const void *a_p, const void *b_p)
+file_sort_by_lsn(const ListCell *a_p, const ListCell *b_p)
 {
-	RewriteMappingFile *a = *(RewriteMappingFile **) a_p;
-	RewriteMappingFile *b = *(RewriteMappingFile **) b_p;
+	RewriteMappingFile *a = (RewriteMappingFile *) lfirst(a_p);
+	RewriteMappingFile *b = (RewriteMappingFile *) lfirst(b_p);
 
 	if (a->lsn < b->lsn)
 		return -1;
@@ -3404,8 +3404,6 @@ UpdateLogicalMappings(HTAB *tuplecid_data, Oid relid, Snapshot snapshot)
 	struct dirent *mapping_de;
 	List	   *files = NIL;
 	ListCell   *file;
-	RewriteMappingFile **files_a;
-	size_t		off;
 	Oid			dboid = IsSharedRelation(relid) ? InvalidOid : MyDatabaseId;
 
 	mapping_dir = AllocateDir("pg_logical/mappings");
@@ -3459,21 +3457,12 @@ UpdateLogicalMappings(HTAB *tuplecid_data, Oid relid, Snapshot snapshot)
 	}
 	FreeDir(mapping_dir);
 
-	/* build array we can easily sort */
-	files_a = palloc(list_length(files) * sizeof(RewriteMappingFile *));
-	off = 0;
-	foreach(file, files)
-	{
-		files_a[off++] = lfirst(file);
-	}
-
 	/* sort files so we apply them in LSN order */
-	qsort(files_a, list_length(files), sizeof(RewriteMappingFile *),
-		  file_sort_by_lsn);
+	list_sort(files, file_sort_by_lsn);
 
-	for (off = 0; off < list_length(files); off++)
+	foreach(file, files)
 	{
-		RewriteMappingFile *f = files_a[off];
+		RewriteMappingFile *f = (RewriteMappingFile *) lfirst(file);
 
 		elog(DEBUG1, "applying mapping: \"%s\" in %u", f->fname,
 			 snapshot->subxip[0]);