Fix double KeyboardInterrupt on PL/Python cancel

src/pl/plpython/plpy_main.c

@@ -60,6 +60,8 @@ void PLy_handle_cancel_interrupt(void);
 
 bool PLy_enter_python_intepreter = false;
 
+static volatile sig_atomic_t plpy_cancel_callback_queued = false;
+
 void
 _PG_init(void)
 {
@@ -147,6 +149,12 @@ PLy_initialize(void)
 int
 PLy_python_cancel_handler(void *arg)
 {
+	plpy_cancel_callback_queued = false;
+
+	/* A queued callback can outlive the cancellation that scheduled it. */
+	if (!QueryCancelPending)
+		return 0;
+
 	PyErr_SetNone(PyExc_KeyboardInterrupt);
 
 	/* return -1 to indicate that we set an exception. */
@@ -166,11 +174,17 @@ PLy_handle_cancel_interrupt(void)
 	 * We can't do much in a signal handler, so just tell the Python
 	 * interpreter to call us back when possible.
 	 *
-	 * We don't bother to check the return value, as there's nothing we could
-	 * do if it fails for some reason.
+	 * A single timeout or cancel request can deliver SIGINT twice, to the
+	 * process group and to the backend itself.  Keep at most one callback
+	 * queued, so that a duplicate KeyboardInterrupt cannot abort the Python
+	 * code handling the first one.
 	 */
-	if (PLy_enter_python_intepreter)
-		(void) Py_AddPendingCall(PLy_python_cancel_handler, NULL);
+	if (PLy_enter_python_intepreter && !plpy_cancel_callback_queued)
+	{
+		plpy_cancel_callback_queued = true;
+		if (Py_AddPendingCall(PLy_python_cancel_handler, NULL) != 0)
+			plpy_cancel_callback_queued = false;
+	}
 
 	if (prev_cancel_pending_hook)
 		prev_cancel_pending_hook();

src/test/isolation2/expected/cancel_plpython.out

@@ -100,3 +100,38 @@ PL/Python function "pynestsleep2"
 7q: ... <quitting>
 -- end_ignore
 
+-- A timeout must deliver only one KeyboardInterrupt and let its handler
+-- finish.  The handler makes a call so that, on any Python version, a
+-- duplicate callback would run inside it.
+CREATE OR REPLACE FUNCTION pysleephandler() RETURNS text AS $$ import time try: while True: time.sleep(0.1) except KeyboardInterrupt: time.sleep(0.1) GD['stage'] = 'handled' raise $$ LANGUAGE plpython3u;
+CREATE
+
+CREATE OR REPLACE FUNCTION pystage() RETURNS text AS $$ return GD.get('stage') $$ LANGUAGE plpython3u;
+CREATE
+
+SET statement_timeout = '1s';
+SET
+SELECT pysleephandler();
+ERROR:  canceling statement due to statement timeout
+CONTEXT:  PL/Python function "pysleephandler"
+RESET statement_timeout;
+RESET
+SELECT pystage();
+ pystage 
+---------
+ handled 
+(1 row)
+
+-- Once a KeyboardInterrupt has been raised, a later SIGINT must raise
+-- another one.
+CREATE OR REPLACE FUNCTION pycancelhandler() RETURNS text AS $$ import os import signal try: os.kill(os.getpid(), signal.SIGINT) while True: pass except KeyboardInterrupt: pass try: os.kill(os.getpid(), signal.SIGINT) except KeyboardInterrupt: GD['stage'] = 'cancelled twice' raise GD['stage'] = 'second cancel ignored' $$ LANGUAGE plpython3u;
+CREATE
+
+SELECT pycancelhandler();
+ERROR:  canceling statement due to user request
+CONTEXT:  PL/Python function "pycancelhandler"
+SELECT pystage();
+ pystage         
+-----------------
+ cancelled twice 
+(1 row)

src/test/isolation2/sql/cancel_plpython.sql

@@ -75,3 +75,47 @@ FROM pg_stat_activity WHERE query LIKE 'select pynestsleep2()%' ORDER BY pid LIM
 7q:
 -- end_ignore
 
+-- A timeout must deliver only one KeyboardInterrupt and let its handler
+-- finish.  The handler makes a call so that, on any Python version, a
+-- duplicate callback would run inside it.
+CREATE OR REPLACE FUNCTION pysleephandler() RETURNS text AS $$
+import time
+try:
+    while True:
+        time.sleep(0.1)
+except KeyboardInterrupt:
+    time.sleep(0.1)
+    GD['stage'] = 'handled'
+    raise
+$$ LANGUAGE plpython3u;
+
+CREATE OR REPLACE FUNCTION pystage() RETURNS text AS $$
+return GD.get('stage')
+$$ LANGUAGE plpython3u;
+
+SET statement_timeout = '1s';
+SELECT pysleephandler();
+RESET statement_timeout;
+SELECT pystage();
+
+-- Once a KeyboardInterrupt has been raised, a later SIGINT must raise
+-- another one.
+CREATE OR REPLACE FUNCTION pycancelhandler() RETURNS text AS $$
+import os
+import signal
+try:
+    os.kill(os.getpid(), signal.SIGINT)
+    while True:
+        pass
+except KeyboardInterrupt:
+    pass
+try:
+    os.kill(os.getpid(), signal.SIGINT)
+except KeyboardInterrupt:
+    GD['stage'] = 'cancelled twice'
+    raise
+GD['stage'] = 'second cancel ignored'
+$$ LANGUAGE plpython3u;
+
+SELECT pycancelhandler();
+SELECT pystage();