Subject: [PATCH v2 1/3] tell client when prep stmts are deallocated

From: Nathan Bossart <[email-masked]>
Date: Thu, 28 May 2026 15:42:20 -0500

---
 src/backend/commands/prepare.c                | 30 +++++++++++++
 src/include/libpq/pqcomm.h                    |  2 +-
 src/include/libpq/protocol.h                  |  1 +
 src/interfaces/libpq/exports.txt              |  1 +
 src/interfaces/libpq/fe-connect.c             | 30 +++++++++++++
 src/interfaces/libpq/fe-protocol3.c           | 43 +++++++++++++++++++
 src/interfaces/libpq/fe-trace.c               | 10 +++++
 src/interfaces/libpq/libpq-fe.h               |  6 +++
 src/interfaces/libpq/libpq-int.h              |  2 +
 .../modules/libpq_pipeline/libpq_pipeline.c   | 12 +++---
 .../libpq_pipeline/traces/prepared.trace      |  1 +
 11 files changed, 131 insertions(+), 7 deletions(-)

diff --git a/src/backend/commands/prepare.c b/src/backend/commands/prepare.c
index 876aad2100a..4ca85b10f9e 100644
--- a/src/backend/commands/prepare.c
+++ b/src/backend/commands/prepare.c
@@ -26,6 +26,9 @@
 #include "commands/explain_state.h"
 #include "commands/prepare.h"
 #include "funcapi.h"
+#include "libpq/libpq-be.h"
+#include "libpq/pqformat.h"
+#include "miscadmin.h"
 #include "nodes/nodeFuncs.h"
 #include "parser/parse_coerce.h"
 #include "parser/parse_collate.h"
@@ -512,6 +515,27 @@ DeallocateQuery(DeallocateStmt *stmt)
 		DropAllPreparedStatements();
 }
 
+/*
+ * Tell the client that a prepared statement has been deallocated.  Pass an
+ * empty string to indicate that all statements were deallocated.
+ *
+ * This is only sent to clients that are using protocol version 3.3 or later.
+ */
+static void
+SendStmtDeallocMsg(const char *name)
+{
+	StringInfoData buf;
+
+	if (whereToSendOutput != DestRemote)
+		return;
+	if (!MyProcPort || MyProcPort->proto < PG_PROTOCOL(3, 3))
+		return;
+
+	pq_beginmessage(&buf, PqMsg_PrepStmtDeallocated);
+	pq_sendstring(&buf, name);
+	pq_endmessage(&buf);
+}
+
 /*
  * Internal version of DEALLOCATE
  *
@@ -530,6 +554,9 @@ DropPreparedStatement(const char *stmt_name, bool showError)
 		/* Release the plancache entry */
 		DropCachedPlan(entry->plansource);
 
+		/* Alert the client */
+		SendStmtDeallocMsg(entry->stmt_name);
+
 		/* Now we can remove the hash table entry */
 		hash_search(prepared_queries, entry->stmt_name, HASH_REMOVE, NULL);
 	}
@@ -548,6 +575,9 @@ DropAllPreparedStatements(void)
 	if (!prepared_queries)
 		return;
 
+	/* Alert the client */
+	SendStmtDeallocMsg("");
+
 	/* walk over cache */
 	hash_seq_init(&seq, prepared_queries);
 	while ((entry = hash_seq_search(&seq)) != NULL)
diff --git a/src/include/libpq/pqcomm.h b/src/include/libpq/pqcomm.h
index a29c9c94d79..28e7944cdf4 100644
--- a/src/include/libpq/pqcomm.h
+++ b/src/include/libpq/pqcomm.h
@@ -92,7 +92,7 @@ is_unixsock_path(const char *path)
  * The earliest and latest frontend/backend protocol version supported.
  */
 #define PG_PROTOCOL_EARLIEST	PG_PROTOCOL(3,0)
-#define PG_PROTOCOL_LATEST		PG_PROTOCOL(3,2)
+#define PG_PROTOCOL_LATEST		PG_PROTOCOL(3,3)
 
 /*
  * Reserved protocol numbers, which have special semantics:
diff --git a/src/include/libpq/protocol.h b/src/include/libpq/protocol.h
index eae8f0e7238..7ea331f7210 100644
--- a/src/include/libpq/protocol.h
+++ b/src/include/libpq/protocol.h
@@ -53,6 +53,7 @@
 #define PqMsg_FunctionCallResponse	'V'
 #define PqMsg_CopyBothResponse		'W'
 #define PqMsg_ReadyForQuery			'Z'
+#define PqMsg_PrepStmtDeallocated	'i'
 #define PqMsg_NoData				'n'
 #define PqMsg_PortalSuspended		's'
 #define PqMsg_ParameterDescription	't'
diff --git a/src/interfaces/libpq/exports.txt b/src/interfaces/libpq/exports.txt
index 1e3d5bd5867..effd73ca3e6 100644
--- a/src/interfaces/libpq/exports.txt
+++ b/src/interfaces/libpq/exports.txt
@@ -211,3 +211,4 @@ PQdefaultAuthDataHook     208
 PQfullProtocolVersion     209
 appendPQExpBufferVA       210
 PQgetThreadLock           211
+PQaddPrepStmtDeallocCallback 212
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 4272d386e64..5e41c21c6f6 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -8374,6 +8374,11 @@ pqParseProtocolVersion(const char *value, ProtocolVersion *result, PGconn *conn,
 		*result = PG_PROTOCOL(3, 2);
 		return true;
 	}
+	if (strcmp(value, "3.3") == 0)
+	{
+		*result = PG_PROTOCOL(3, 3);
+		return true;
+	}
 
 	libpq_append_conn_error(conn, "invalid %s value: \"%s\"",
 							context, value);
@@ -8426,3 +8431,28 @@ PQgetThreadLock(void)
 	Assert(pg_g_threadlock);
 	return pg_g_threadlock;
 }
+
+/*
+ * Adds a prepared statement deallocation callback to the connection's list of
+ * callbacks.  These are invoked when the server sends us
+ * PqMsg_PrepStmtDeallocated messages.
+ */
+bool
+PQaddPrepStmtDeallocCallback(PGconn *conn, PQprepStmtDeallocCallback cb)
+{
+	if (!conn)
+		return false;
+
+	/* Add to end to preserve registration order */
+	for (int i = 0; i < lengthof(conn->prepStmtDeallocCallbacks); i++)
+	{
+		if (conn->prepStmtDeallocCallbacks[i])
+			continue;
+
+		conn->prepStmtDeallocCallbacks[i] = cb;
+		return true;
+	}
+
+	libpq_append_conn_error(conn, "maximum number of prepared statement deallocation callbacks already registered");
+	return false;
+}
diff --git a/src/interfaces/libpq/fe-protocol3.c b/src/interfaces/libpq/fe-protocol3.c
index 840e018cd18..0407d10362d 100644
--- a/src/interfaces/libpq/fe-protocol3.c
+++ b/src/interfaces/libpq/fe-protocol3.c
@@ -61,6 +61,32 @@ static size_t build_startup_packet(const PGconn *conn, char *packet,
 								   const PQEnvironmentOption *options);

+/*
+ * Attempt to read a PrepStmtDeallocated message and invoke the connection's
+ * registered callbacks.  This is possible in several places, so we break it
+ * out as a subroutine.
+ *
+ * Entry: 'i' message type and length have already been consumed.
+ * Exit: returns 0 if successfully consumed message and invoked callbacks, or
+ *       EOF if not enough data.
+ */
+static int
+getPrepStmtDeallocated(PGconn *conn)
+{
+	if (pqGets(&conn->workBuffer, conn))
+		return EOF;
+
+	for (int i = 0; i < lengthof(conn->prepStmtDeallocCallbacks); i++)
+	{
+		if (!conn->prepStmtDeallocCallbacks[i])
+			break;
+
+		(conn->prepStmtDeallocCallbacks[i]) (conn, conn->workBuffer.data);
+	}
+
+	return 0;
+}
+
 /*
  * parseInput: if appropriate, parse input data from backend
  * until input is exhausted or a stopping state is reached.
@@ -184,6 +210,11 @@ pqParseInput3(PGconn *conn)
 				if (getParameterStatus(conn))
 					return;
 			}
+			else if (id == PqMsg_PrepStmtDeallocated)
+			{
+				if (getPrepStmtDeallocated(conn))
+					return;
+			}
 			else
 			{
 				/* Any other case is unexpected and we summarily skip it */
@@ -305,6 +336,10 @@ pqParseInput3(PGconn *conn)
 					if (getParameterStatus(conn))
 						return;
 					break;
+				case PqMsg_PrepStmtDeallocated:
+					if (getPrepStmtDeallocated(conn))
+						return;
+					break;
 				case PqMsg_BackendKeyData:
 
 					/*
@@ -1905,6 +1940,10 @@ getCopyDataMessage(PGconn *conn)
 				if (getParameterStatus(conn))
 					return 0;
 				break;
+			case PqMsg_PrepStmtDeallocated:
+				if (getPrepStmtDeallocated(conn))
+					return 0;
+				break;
 			case PqMsg_CopyData:
 				return msgLength;
 			case PqMsg_CopyDone:
@@ -2409,6 +2448,10 @@ pqFunctionCall3(PGconn *conn, Oid fnid,
 				if (getParameterStatus(conn))
 					continue;
 				break;
+			case PqMsg_PrepStmtDeallocated:
+				if (getPrepStmtDeallocated(conn))
+					continue;
+				break;
 			default:
 				/* The backend violates the protocol. */
 				libpq_append_conn_error(conn, "protocol error: id=0x%x", id);
diff --git a/src/interfaces/libpq/fe-trace.c b/src/interfaces/libpq/fe-trace.c
index c348b08c39b..e9f734187a2 100644
--- a/src/interfaces/libpq/fe-trace.c
+++ b/src/interfaces/libpq/fe-trace.c
@@ -543,6 +543,13 @@ pqTraceOutput_ParameterStatus(FILE *f, const char *message, int *cursor)
 	pqTraceOutputString(f, message, cursor, false);
 }
 
+static void
+pqTraceOutput_PrepStmtDeallocated(FILE *f, const char *message, int *cursor)
+{
+	fprintf(f, "PrepStmtDeallocated\t");
+	pqTraceOutputString(f, message, cursor, false);
+}
+
 static void
 pqTraceOutput_ParameterDescription(FILE *f, const char *message, int *cursor, bool regress)
 {
@@ -793,6 +800,9 @@ pqTraceOutputMessage(PGconn *conn, const char *message, bool toServer)
 			else
 				pqTraceOutput_ParameterStatus(conn->Pfdebug, message, &logCursor);
 			break;
+		case PqMsg_PrepStmtDeallocated:
+			pqTraceOutput_PrepStmtDeallocated(conn->Pfdebug, message, &logCursor);
+			break;
 		case PqMsg_ParameterDescription:
 			pqTraceOutput_ParameterDescription(conn->Pfdebug, message, &logCursor, regress);
 			break;
diff --git a/src/interfaces/libpq/libpq-fe.h b/src/interfaces/libpq/libpq-fe.h
index 8ecb9b4a4c7..c57bb8806cf 100644
--- a/src/interfaces/libpq/libpq-fe.h
+++ b/src/interfaces/libpq/libpq-fe.h
@@ -486,6 +486,12 @@ typedef void (*pgthreadlock_t) (int acquire);
 extern pgthreadlock_t PQregisterThreadLock(pgthreadlock_t newhandler);
 extern pgthreadlock_t PQgetThreadLock(void);
 
+/* callbacks for prepared statement deallocation notifications */
+typedef void (*PQprepStmtDeallocCallback) (PGconn *conn, const char *name);
+
+extern bool PQaddPrepStmtDeallocCallback(PGconn *conn,
+										 PQprepStmtDeallocCallback cb);
+
 /* === in fe-trace.c === */
 extern void PQtrace(PGconn *conn, FILE *debug_port);
 extern void PQuntrace(PGconn *conn);
diff --git a/src/interfaces/libpq/libpq-int.h b/src/interfaces/libpq/libpq-int.h
index 461b39620c3..7eca941ddcc 100644
--- a/src/interfaces/libpq/libpq-int.h
+++ b/src/interfaces/libpq/libpq-int.h
@@ -532,6 +532,8 @@ struct pg_conn
 	void		(*cleanup_async_auth) (PGconn *conn);
 	pgsocket	altsock;		/* alternative socket for client to poll */
 
+	/* Callbacks for prep stmt deallocs (16 ought to be enough for anybody) */
+	PQprepStmtDeallocCallback prepStmtDeallocCallbacks[16];
 
 	/* Transient state needed while establishing connection */
 	PGTargetServerType target_server_type;	/* desired session properties */
diff --git a/src/test/modules/libpq_pipeline/libpq_pipeline.c b/src/test/modules/libpq_pipeline/libpq_pipeline.c
index ee3e2ec7570..b61f33e7cd9 100644
--- a/src/test/modules/libpq_pipeline/libpq_pipeline.c
+++ b/src/test/modules/libpq_pipeline/libpq_pipeline.c
@@ -1363,7 +1363,7 @@ test_protocol_version(PGconn *conn)
 	Assert(max_protocol_version_index >= 0);
 
 	/*
-	 * Test default protocol_version (GREASE - should negotiate down to 3.2)
+	 * Test default protocol_version (GREASE - should negotiate down to 3.3)
 	 */
 	vals[max_protocol_version_index] = "";
 	conn = PQconnectdbParams(keywords, vals, false);
@@ -1373,8 +1373,8 @@ test_protocol_version(PGconn *conn)
 				 PQerrorMessage(conn));
 
 	protocol_version = PQfullProtocolVersion(conn);
-	if (protocol_version != 30002)
-		pg_fatal("expected 30002, got %d", protocol_version);
+	if (protocol_version != 30003)
+		pg_fatal("expected 30003, got %d", protocol_version);
 
 	PQfinish(conn);
 
@@ -1423,7 +1423,7 @@ test_protocol_version(PGconn *conn)
 	PQfinish(conn);
 
 	/*
-	 * Test max_protocol_version=latest. 'latest' currently means '3.2'.
+	 * Test max_protocol_version=latest. 'latest' currently means '3.3'.
 	 */
 	vals[max_protocol_version_index] = "latest";
 	conn = PQconnectdbParams(keywords, vals, false);
@@ -1433,8 +1433,8 @@ test_protocol_version(PGconn *conn)
 				 PQerrorMessage(conn));
 
 	protocol_version = PQfullProtocolVersion(conn);
-	if (protocol_version != 30002)
-		pg_fatal("expected 30002, got %d", protocol_version);
+	if (protocol_version != 30003)
+		pg_fatal("expected 30003, got %d", protocol_version);
 
 	PQfinish(conn);
 
diff --git a/src/test/modules/libpq_pipeline/traces/prepared.trace b/src/test/modules/libpq_pipeline/traces/prepared.trace
index aeb5de109e0..5d36fb0056d 100644
--- a/src/test/modules/libpq_pipeline/traces/prepared.trace
+++ b/src/test/modules/libpq_pipeline/traces/prepared.trace
@@ -7,6 +7,7 @@ B	113	RowDescription	 4 "?column?" NNNN 0 NNNN 4 -1 0 "?column?" NNNN 0 NNNN 655
 B	5	ReadyForQuery	 I
 F	16	Close	 S "select_one"
 F	4	Sync
+B	15	PrepStmtDeallocated	 "select_one"
 B	4	CloseComplete
 B	5	ReadyForQuery	 I
 F	16	Describe	 S "select_one"
-- 
2.50.1 (Apple Git-155)

--tbByXeXYMjI7URMk--