Fix memory leak in pgoutput with publication list cache

michaelpq · michaelpq · commit ba230ce40faa · 2024-12-23T12:48:12.000+09:00
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. Contrary to the HEAD-only commit f0c569d that has changed PGOutputData to track this new child memory context, the context is tracked with a static variable whose state is reset with a MemoryContext reset callback attached to PGOutputData->context, so as ABI compatibility is preserved in stable branches. This approach is based on an suggestion from Amit Kapila. Analyzed-by: Michael Paquier, Jeff Davis Author: Masahiko Sawada Reviewed-by: Amit Kapila, Michael Paquier, Euler Taveira, Hou Zhijie Discussion: https://postgr.es/m/Z0khf9EVMVLOc_YY@paquier.xyz Backpatch-through: 13
diff --git a/src/backend/replication/pgoutput/pgoutput.c b/src/backend/replication/pgoutput/pgoutput.c
@@ -49,6 +49,13 @@ static bool pgoutput_origin_filter(LogicalDecodingContext *ctx,
 
 static bool publications_valid;
 
+/*
+ * Private memory context for publication data, created in
+ * PGOutputData->context when starting pgoutput, and set to NULL when its
+ * parent context is reset via a dedicated MemoryContextCallback.
+ */
+static MemoryContext pubctx = NULL;
+
 static List *LoadPublications(List *pubnames);
 static void publication_invalidation_cb(Datum arg, int cacheid,
 										uint32 hashvalue);
@@ -174,6 +181,15 @@ parse_output_parameters(List *options, uint32 *protocol_version,
 	}
 }
 
+/*
+ * Callback of PGOutputData->context in charge of cleaning pubctx.
+ */
+static void
+pgoutput_pubctx_reset_callback(void *arg)
+{
+	pubctx = NULL;
+}
+
 /*
  * Initialize this plugin
  */
@@ -183,12 +199,22 @@ pgoutput_startup(LogicalDecodingContext *ctx, OutputPluginOptions *opt,
 {
 	PGOutputData *data = palloc0(sizeof(PGOutputData));
 	static bool publication_callback_registered = false;
+	MemoryContextCallback *mcallback;
 
 	/* Create our memory context for private allocations. */
 	data->context = AllocSetContextCreate(ctx->context,
 										  "logical replication output context",
 										  ALLOCSET_DEFAULT_SIZES);
 
+	Assert(pubctx == NULL);
+	pubctx = AllocSetContextCreate(ctx->context,
+								   "logical replication publication list context",
+								   ALLOCSET_SMALL_SIZES);
+
+	mcallback = palloc0(sizeof(MemoryContextCallback));
+	mcallback->func = pgoutput_pubctx_reset_callback;
+	MemoryContextRegisterResetCallback(ctx->context, mcallback);
+
 	ctx->output_plugin_private = data;
 
 	/* This plugin uses binary protocol. */
@@ -587,8 +613,9 @@ pgoutput_origin_filter(LogicalDecodingContext *ctx,
 /*
  * Shutdown the output plugin.
  *
- * Note, we don't need to clean the data->context as it's child context
- * of the ctx->context so it will be cleaned up by logical decoding machinery.
+ * Note, we don't need to clean the data->context and 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)
@@ -598,6 +625,9 @@ pgoutput_shutdown(LogicalDecodingContext *ctx)
 		hash_destroy(RelationSyncCache);
 		RelationSyncCache = NULL;
 	}
+
+	/* Better safe than sorry */
+	pubctx = NULL;
 }
 
 /*
@@ -731,9 +761,10 @@ get_rel_sync_entry(PGOutputData *data, Oid relid)
 		/* Reload publications if needed before use. */
 		if (!publications_valid)
 		{
-			oldctx = MemoryContextSwitchTo(CacheMemoryContext);
-			if (data->publications)
-				list_free_deep(data->publications);
+			Assert(pubctx);
+
+			MemoryContextReset(pubctx);
+			oldctx = MemoryContextSwitchTo(pubctx);
 
 			data->publications = LoadPublications(data->publication_names);
 			MemoryContextSwitchTo(oldctx);
