Make dblink interruptible, via new libpqsrv APIs.

This replaces dblink's blocking libpq calls, allowing cancellation and
allowing DROP DATABASE (of a database not involved in the query).  Apart
from explicit dblink_cancel_query() calls, dblink still doesn't cancel
the remote side.  The replacement for the blocking calls consists of
new, general-purpose query execution wrappers in the libpqsrv facility.
Out-of-tree extensions should adopt these.

The original commit d3c5f37dd5 did not
back-patch.  Back-patch now to v16-v13, bringing coverage to all supported
versions.  This back-patch omits the orignal's refactoring in postgres_fdw.

Discussion: https://postgr.es/m/20231122012945.74@rfd.leadboat.com

contrib/dblink/dblink.c

@@ -49,9 +49,11 @@
 #include "funcapi.h"
 #include "lib/stringinfo.h"
 #include "libpq-fe.h"
+#include "libpq/libpq-be-fe-helpers.h"
 #include "mb/pg_wchar.h"
 #include "miscadmin.h"
 #include "parser/scansup.h"
+#include "pgstat.h"
 #include "utils/acl.h"
 #include "utils/builtins.h"
 #include "utils/fmgroids.h"
@@ -479,7 +481,7 @@ dblink_open(PG_FUNCTION_ARGS)
 	/* If we are not in a transaction, start one */
 	if (PQtransactionStatus(conn) == PQTRANS_IDLE)
 	{
-		res = PQexec(conn, "BEGIN");
+		res = libpqsrv_exec(conn, "BEGIN", PG_WAIT_EXTENSION);
 		if (PQresultStatus(res) != PGRES_COMMAND_OK)
 			dblink_res_internalerror(conn, res, "begin error");
 		PQclear(res);
@@ -498,7 +500,7 @@ dblink_open(PG_FUNCTION_ARGS)
 		(rconn->openCursorCount)++;
 
 	appendStringInfo(&buf, "DECLARE %s CURSOR FOR %s", curname, sql);
-	res = PQexec(conn, buf.data);
+	res = libpqsrv_exec(conn, buf.data, PG_WAIT_EXTENSION);
 	if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
 	{
 		dblink_res_error(conn, conname, res, fail,
@@ -567,7 +569,7 @@ dblink_close(PG_FUNCTION_ARGS)
 	appendStringInfo(&buf, "CLOSE %s", curname);
 
 	/* close the cursor */
-	res = PQexec(conn, buf.data);
+	res = libpqsrv_exec(conn, buf.data, PG_WAIT_EXTENSION);
 	if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
 	{
 		dblink_res_error(conn, conname, res, fail,
@@ -587,7 +589,7 @@ dblink_close(PG_FUNCTION_ARGS)
 		{
 			rconn->newXactForCursor = false;
 
-			res = PQexec(conn, "COMMIT");
+			res = libpqsrv_exec(conn, "COMMIT", PG_WAIT_EXTENSION);
 			if (PQresultStatus(res) != PGRES_COMMAND_OK)
 				dblink_res_internalerror(conn, res, "commit error");
 			PQclear(res);
@@ -669,7 +671,7 @@ dblink_fetch(PG_FUNCTION_ARGS)
 	 * PGresult will be long-lived even though we are still in a short-lived
 	 * memory context.
 	 */
-	res = PQexec(conn, buf.data);
+	res = libpqsrv_exec(conn, buf.data, PG_WAIT_EXTENSION);
 	if (!res ||
 		(PQresultStatus(res) != PGRES_COMMAND_OK &&
 		 PQresultStatus(res) != PGRES_TUPLES_OK))
@@ -817,7 +819,7 @@ dblink_record_internal(FunctionCallInfo fcinfo, bool is_async)
 		else
 		{
 			/* async result retrieval, do it the old way */
-			PGresult   *res = PQgetResult(conn);
+			PGresult   *res = libpqsrv_get_result(conn, PG_WAIT_EXTENSION);
 
 			/* NULL means we're all done with the async results */
 			if (res)
@@ -1131,7 +1133,8 @@ materializeQueryResult(FunctionCallInfo fcinfo,
 		PQclear(sinfo.last_res);
 		PQclear(sinfo.cur_res);
 		/* and clear out any pending data in libpq */
-		while ((res = PQgetResult(conn)) != NULL)
+		while ((res = libpqsrv_get_result(conn, PG_WAIT_EXTENSION)) !=
+			   NULL)
 			PQclear(res);
 		PG_RE_THROW();
 	}
@@ -1158,7 +1161,7 @@ storeQueryResult(volatile storeInfo *sinfo, PGconn *conn, const char *sql)
 	{
 		CHECK_FOR_INTERRUPTS();
 
-		sinfo->cur_res = PQgetResult(conn);
+		sinfo->cur_res = libpqsrv_get_result(conn, PG_WAIT_EXTENSION);
 		if (!sinfo->cur_res)
 			break;
 
@@ -1486,7 +1489,7 @@ dblink_exec(PG_FUNCTION_ARGS)
 		if (!conn)
 			dblink_conn_not_avail(conname);
 
-		res = PQexec(conn, sql);
+		res = libpqsrv_exec(conn, sql, PG_WAIT_EXTENSION);
 		if (!res ||
 			(PQresultStatus(res) != PGRES_COMMAND_OK &&
 			 PQresultStatus(res) != PGRES_TUPLES_OK))
@@ -2771,8 +2774,8 @@ dblink_res_error(PGconn *conn, const char *conname, PGresult *res,
 
 	/*
 	 * If we don't get a message from the PGresult, try the PGconn.  This is
-	 * needed because for connection-level failures, PQexec may just return
-	 * NULL, not a PGresult at all.
+	 * needed because for connection-level failures, PQgetResult may just
+	 * return NULL, not a PGresult at all.
 	 */
 	if (message_primary == NULL)
 		message_primary = pchomp(PQerrorMessage(conn));

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -603,12 +603,9 @@ libpqrcv_readtimelinehistoryfile(WalReceiverConn *conn,
  * Send a query and wait for the results by using the asynchronous libpq
  * functions and socket readiness events.
  *
- * We must not use the regular blocking libpq functions like PQexec()
- * since they are uninterruptible by signals on some platforms, such as
- * Windows.
- *
- * The function is modeled on PQexec() in libpq, but only implements
- * those parts that are in use in the walreceiver api.
+ * The function is modeled on libpqsrv_exec(), with the behavior difference
+ * being that it calls ProcessWalRcvInterrupts().  As an optimization, it
+ * skips try/catch, since all errors terminate the process.
  *
  * May return NULL, rather than an error result, on failure.
  */

src/include/libpq/libpq-be-fe-helpers.h

@@ -49,6 +49,8 @@
 
 static inline void libpqsrv_connect_prepare(void);
 static inline void libpqsrv_connect_internal(PGconn *conn, uint32 wait_event_info);
+static inline PGresult *libpqsrv_get_result_last(PGconn *conn, uint32 wait_event_info);
+static inline PGresult *libpqsrv_get_result(PGconn *conn, uint32 wait_event_info);
 
 
 /*
@@ -239,4 +241,129 @@ libpqsrv_connect_internal(PGconn *conn, uint32 wait_event_info)
 	PG_END_TRY();
 }
 
+/*
+ * PQexec() wrapper that processes interrupts.
+ *
+ * Unless PQsetnonblocking(conn, 1) is in effect, this can't process
+ * interrupts while pushing the query text to the server.  Consider that
+ * setting if query strings can be long relative to TCP buffer size.
+ *
+ * This has the preconditions of PQsendQuery(), not those of PQexec().  Most
+ * notably, PQexec() would silently discard any prior query results.
+ */
+static inline PGresult *
+libpqsrv_exec(PGconn *conn, const char *query, uint32 wait_event_info)
+{
+	if (!PQsendQuery(conn, query))
+		return NULL;
+	return libpqsrv_get_result_last(conn, wait_event_info);
+}
+
+/*
+ * PQexecParams() wrapper that processes interrupts.
+ *
+ * See notes at libpqsrv_exec().
+ */
+static inline PGresult *
+libpqsrv_exec_params(PGconn *conn,
+					 const char *command,
+					 int nParams,
+					 const Oid *paramTypes,
+					 const char *const *paramValues,
+					 const int *paramLengths,
+					 const int *paramFormats,
+					 int resultFormat,
+					 uint32 wait_event_info)
+{
+	if (!PQsendQueryParams(conn, command, nParams, paramTypes, paramValues,
+						   paramLengths, paramFormats, resultFormat))
+		return NULL;
+	return libpqsrv_get_result_last(conn, wait_event_info);
+}
+
+/*
+ * Like PQexec(), loop over PQgetResult() until it returns NULL or another
+ * terminal state.  Return the last non-NULL result or the terminal state.
+ */
+static inline PGresult *
+libpqsrv_get_result_last(PGconn *conn, uint32 wait_event_info)
+{
+	PGresult   *volatile lastResult = NULL;
+
+	/* In what follows, do not leak any PGresults on an error. */
+	PG_TRY();
+	{
+		for (;;)
+		{
+			/* Wait for, and collect, the next PGresult. */
+			PGresult   *result;
+
+			result = libpqsrv_get_result(conn, wait_event_info);
+			if (result == NULL)
+				break;			/* query is complete, or failure */
+
+			/*
+			 * Emulate PQexec()'s behavior of returning the last result when
+			 * there are many.
+			 */
+			PQclear(lastResult);
+			lastResult = result;
+
+			if (PQresultStatus(lastResult) == PGRES_COPY_IN ||
+				PQresultStatus(lastResult) == PGRES_COPY_OUT ||
+				PQresultStatus(lastResult) == PGRES_COPY_BOTH ||
+				PQstatus(conn) == CONNECTION_BAD)
+				break;
+		}
+	}
+	PG_CATCH();
+	{
+		PQclear(lastResult);
+		PG_RE_THROW();
+	}
+	PG_END_TRY();
+
+	return lastResult;
+}
+
+/*
+ * Perform the equivalent of PQgetResult(), but watch for interrupts.
+ */
+static inline PGresult *
+libpqsrv_get_result(PGconn *conn, uint32 wait_event_info)
+{
+	/*
+	 * Collect data until PQgetResult is ready to get the result without
+	 * blocking.
+	 */
+	while (PQisBusy(conn))
+	{
+		int			rc;
+
+		rc = WaitLatchOrSocket(MyLatch,
+							   WL_EXIT_ON_PM_DEATH | WL_LATCH_SET |
+							   WL_SOCKET_READABLE,
+							   PQsocket(conn),
+							   0,
+							   wait_event_info);
+
+		/* Interrupted? */
+		if (rc & WL_LATCH_SET)
+		{
+			ResetLatch(MyLatch);
+			CHECK_FOR_INTERRUPTS();
+		}
+
+		/* Consume whatever data is available from the socket */
+		if (PQconsumeInput(conn) == 0)
+		{
+			/* trouble; expect PQgetResult() to return NULL */
+			break;
+		}
+	}
+
+	/* Now we can collect and return the next PGresult */
+	return PQgetResult(conn);
+}
+
 #endif							/* LIBPQ_BE_FE_HELPERS_H */