Fix memory leak in pgoutput with publication list cache

The pgoutput module caches publication names in a list and frees it upon
invalidation.  However, the code forgot to free the actual publication
names within the list elements, as publication names are pstrdup()'d in
GetPublication().  This would cause memory to leak in
CacheMemoryContext, bloating it over time as this context is not
cleaned.

This is a problem for WAL senders running for a long time, as an
accumulation of invalidation requests would bloat its cache memory
usage.  A second case, where this leak is easier to see, involves a
backend calling SQL functions like pg_logical_slot_{get,peek}_changes()
which create a new decoding context with each execution.  More
publications create more bloat.

To address this, this commit adds a new memory context within the
logical decoding context and resets it each time the publication names
cache is invalidated, based on a suggestion from Amit Kapila.  This
ensures that the lifespan of the publication names aligns with that of
the logical decoding context.

This solution changes PGOutputData, which is fine for HEAD but it could
cause an ABI breakage in stable branches as the structure size would
change, so these are left out for now.

Analyzed-by: Michael Paquier, Jeff Davis
Author: Zhijie Hou
Reviewed-by: Michael Paquier, Masahiko Sawada, Euler Taveira
Discussion: https://postgr.es/m/Z0khf9EVMVLOc_YY@paquier.xyz

Author: michaelpq

src/backend/replication/pgoutput/pgoutput.c

@@ -436,6 +436,10 @@ pgoutput_startup(LogicalDecodingContext *ctx, OutputPluginOptions *opt,
 										   "logical replication cache context",
 										   ALLOCSET_DEFAULT_SIZES);
 
+	data->pubctx = AllocSetContextCreate(ctx->context,
+										 "logical replication publication list context",
+										 ALLOCSET_SMALL_SIZES);
+
 	ctx->output_plugin_private = data;
 
 	/* This plugin uses binary protocol. */
@@ -1734,9 +1738,9 @@ pgoutput_origin_filter(LogicalDecodingContext *ctx,
 /*
  * Shutdown the output plugin.
  *
- * Note, we don't need to clean the data->context and data->cachectx as
- * they are child contexts of the ctx->context so they will be cleaned up by
- * logical decoding machinery.
+ * Note, we don't need to clean the data->context, data->cachectx, and
+ * data->pubctx as they are child contexts of the ctx->context so they
+ * will be cleaned up by logical decoding machinery.
  */
 static void
 pgoutput_shutdown(LogicalDecodingContext *ctx)
@@ -2063,12 +2067,9 @@ get_rel_sync_entry(PGOutputData *data, Relation relation)
 		/* Reload publications if needed before use. */
 		if (!publications_valid)
 		{
-			oldctx = MemoryContextSwitchTo(CacheMemoryContext);
-			if (data->publications)
-			{
-				list_free_deep(data->publications);
-				data->publications = NIL;
-			}
+			MemoryContextReset(data->pubctx);
+
+			oldctx = MemoryContextSwitchTo(data->pubctx);
 			data->publications = LoadPublications(data->publication_names);
 			MemoryContextSwitchTo(oldctx);
 			publications_valid = true;

src/include/replication/pgoutput.h

@@ -20,6 +20,7 @@ typedef struct PGOutputData
 	MemoryContext context;		/* private memory context for transient
 								 * allocations */
 	MemoryContext cachectx;		/* private memory context for cache data */
+	MemoryContext pubctx;		/* private memory context for publication data */
 
 	bool		in_streaming;	/* true if we are streaming a chunk of
 								 * transaction */