From: Andreas Karlsson <andreas@proxel.se>
To: Noah Misch <noah@leadboat.com>
To: pgsql-hackers@postgresql.org
Subject: Re: dblink query interruptibility
Date: Thu, 6 Mar 2025 14:57:01 +0100
Message-ID: <bb9f565d-4ca5-4e30-9008-fa538c6895ef@proxel.se> (raw)
In-Reply-To: <20231122012945.74@rfd.leadboat.com>
References: <20231122012945.74@rfd.leadboat.com>
On 11/22/23 2:29 AM, Noah Misch wrote:
> Something as simple as the following doesn't respond to cancellation. In
> v15+, any DROP DATABASE will hang as long as it's running:
Hi,
One of our customers ran into this bug when upgrading from PostgreSQL 14
to PostgreSQL 16. Your commit[1] fixed this issue in PostgreSQL 17 but
the bugfix was not backported with the explanation below.
> Code inspection identified the bug at least thirteen years ago, but
user complaints have not appeared. Hence, no back-patch for now.
But that is as far as I can tell not the case because at least for
CREATE DATABASE the bug was introduced in a commit[2] in PostgeSQL 15.
And now that we actually have a user complaint what do you think about
backporting the fix?
The patch seems small and relatively safe to backport and the functions
have had no bugfixes as far as I could see. And I did a quick git
cherry-pick myself on top of PG 16 (see attached patch) and the only
conflict was related to the introduction of custom wait events which was
easy to fix.
Here is a small snippet which reproduces the bug. It hangs reliably on
PostgreSQL 15 and 16, but not 14, 17 or HEAD.
CREATE EXTENSION dblink;
CREATE FUNCTION f(text, int, text) RETURNS VOID LANGUAGE plpgsql AS $$
BEGIN
PERFORM dblink_connect(format('host=%s port=%s user=%s
dbname=postgres', $1, $2, $3));
RAISE NOTICE 'dblink connected!';
PERFORM dblink_exec('DROP DATABASE test');
RAISE NOTICE 'Database dropped!';
PERFORM dblink_disconnect();
RAISE NOTICE 'dblink disconnected!';
END
$$;
CREATE DATABASE test;
SELECT f(:'HOST', :PORT, :'USER');
Andreas
1.
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=d3c5f37dd543498cc7c678815d3921823...
2.
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=e2f65f42555ff531c6d7c8f151526b4ef...Attachments:0001-Make-dblink-interruptible-via-new-libpqsrv-APIs.patch (text/x-patch, 17.4K) download
From 9fa3af45dcfc6a27d3d2aa5ae29812bbd4740751 Mon Sep 17 00:00:00 2001
From: Noah Misch <noah@leadboat.com>
Date: Mon, 8 Jan 2024 11:39:56 -0800
Subject: [PATCH] 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. Use them in postgres_fdw,
replacing a local implementation from which the libpqsrv implementation
derives. This is a bug fix for dblink. Code inspection identified the
bug at least thirteen years ago, but user complaints have not appeared.
Hence, no back-patch for now.
Discussion: https://postgr.es/m/20231122012945.74@rfd.leadboat.com
---
contrib/dblink/dblink.c | 24 ++--
contrib/postgres_fdw/connection.c | 80 ++---------
contrib/postgres_fdw/deparse.c | 2 +-
contrib/postgres_fdw/postgres_fdw.c | 10 +-
contrib/postgres_fdw/postgres_fdw.h | 2 +-
.../libpqwalreceiver/libpqwalreceiver.c | 9 +-
src/include/libpq/libpq-be-fe-helpers.h | 127 ++++++++++++++++++
7 files changed, 163 insertions(+), 91 deletions(-)
diff --git a/contrib/dblink/dblink.c b/contrib/dblink/dblink.cindex 1ff65d1e521..bd409d7bf1b 100644--- a/contrib/dblink/dblink.c+++ b/contrib/dblink/dblink.c@@ -61,6 +61,7 @@
#include "utils/memutils.h"
#include "utils/rel.h"
#include "utils/varlena.h"
+#include "utils/wait_event.h"
PG_MODULE_MAGIC;
@@ -430,7 +431,7 @@ 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);
@@ -449,7 +450,7 @@ 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,
@@ -518,7 +519,7 @@ 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,
@@ -538,7 +539,7 @@ 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);
@@ -620,7 +621,7 @@ 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))
@@ -768,7 +769,7 @@ 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)
@@ -1076,7 +1077,8 @@ 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();
}
@@ -1103,7 +1105,7 @@ 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;
@@ -1431,7 +1433,7 @@ 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))
@@ -2728,8 +2730,8 @@ 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));
diff --git a/contrib/postgres_fdw/connection.c b/contrib/postgres_fdw/connection.cindex 3246e18ab93..3cf0f4b148e 100644--- a/contrib/postgres_fdw/connection.c+++ b/contrib/postgres_fdw/connection.c@@ -709,7 +709,7 @@ do_sql_command_end(PGconn *conn, const char *sql, bool consume_input)
*/
if (consume_input && !PQconsumeInput(conn))
pgfdw_report_error(ERROR, NULL, conn, false, sql);
- res = pgfdw_get_result(conn, sql);+ res = pgfdw_get_result(conn);
if (PQresultStatus(res) != PGRES_COMMAND_OK)
pgfdw_report_error(ERROR, res, conn, true, sql);
PQclear(res);
@@ -812,7 +812,9 @@ GetPrepStmtNumber(PGconn *conn)
/*
* Submit a query and wait for the result.
*
- * This function is interruptible by signals.+ * Since we don't use non-blocking mode, this can't process interrupts while+ * pushing the query text to the server. That risk is relatively small, so we+ * ignore that for now.
*
* Caller is responsible for the error handling on the result.
*/
@@ -823,77 +825,20 @@ pgfdw_exec_query(PGconn *conn, const char *query, PgFdwConnState *state)
if (state && state->pendingAreq)
process_pending_request(state->pendingAreq);
- /*- * Submit a query. Since we don't use non-blocking mode, this also can- * block. But its risk is relatively small, so we ignore that for now.- */
if (!PQsendQuery(conn, query))
- pgfdw_report_error(ERROR, NULL, conn, false, query);-- /* Wait for the result. */- return pgfdw_get_result(conn, query);+ return NULL;+ return pgfdw_get_result(conn);
}
/*
- * Wait for the result from a prior asynchronous execution function call.- *- * This function offers quick responsiveness by checking for any interruptions.- *- * This function emulates PQexec()'s behavior of returning the last result- * when there are many.+ * Wrap libpqsrv_get_result_last(), adding wait event.
*
* Caller is responsible for the error handling on the result.
*/
PGresult *
-pgfdw_get_result(PGconn *conn, const char *query)+pgfdw_get_result(PGconn *conn)
{
- PGresult *volatile last_res = NULL;-- /* In what follows, do not leak any PGresults on an error. */- PG_TRY();- {- for (;;)- {- PGresult *res;-- while (PQisBusy(conn))- {- int wc;-- /* Sleep until there's something to do */- wc = WaitLatchOrSocket(MyLatch,- WL_LATCH_SET | WL_SOCKET_READABLE |- WL_EXIT_ON_PM_DEATH,- PQsocket(conn),- -1L, PG_WAIT_EXTENSION);- ResetLatch(MyLatch);-- CHECK_FOR_INTERRUPTS();-- /* Data available in socket? */- if (wc & WL_SOCKET_READABLE)- {- if (!PQconsumeInput(conn))- pgfdw_report_error(ERROR, NULL, conn, false, query);- }- }-- res = PQgetResult(conn);- if (res == NULL)- break; /* query is complete */-- PQclear(last_res);- last_res = res;- }- }- PG_CATCH();- {- PQclear(last_res);- PG_RE_THROW();- }- PG_END_TRY();-- return last_res;+ return libpqsrv_get_result_last(conn, PG_WAIT_EXTENSION);
}
/*
@@ -934,8 +879,8 @@ pgfdw_report_error(int elevel, PGresult *res, PGconn *conn,
/*
* 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.+ * is 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));
@@ -1035,7 +980,8 @@ pgfdw_xact_callback(XactEvent event, void *arg)
*/
if (entry->have_prep_stmt && entry->have_error)
{
- res = PQexec(entry->conn, "DEALLOCATE ALL");+ res = pgfdw_exec_query(entry->conn, "DEALLOCATE ALL",+ NULL);
PQclear(res);
}
entry->have_prep_stmt = false;
diff --git a/contrib/postgres_fdw/deparse.c b/contrib/postgres_fdw/deparse.cindex 6cba34350af..dba4dedd932 100644--- a/contrib/postgres_fdw/deparse.c+++ b/contrib/postgres_fdw/deparse.c@@ -3673,7 +3673,7 @@ appendOrderBySuffix(Oid sortop, Oid sortcoltype, bool nulls_first,
* Print the representation of a parameter to be sent to the remote side.
*
* Note: we always label the Param's type explicitly rather than relying on
- * transmitting a numeric type OID in PQexecParams(). This allows us to+ * transmitting a numeric type OID in PQsendQueryParams(). This allows us to
* avoid assuming that types have the same OIDs on the remote side as they
* do locally --- they need only have the same names.
*/
diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.cindex 8657a3b0e23..b18567cf8f4 100644--- a/contrib/postgres_fdw/postgres_fdw.c+++ b/contrib/postgres_fdw/postgres_fdw.c@@ -3767,7 +3767,7 @@ create_cursor(ForeignScanState *node)
* We don't use a PG_TRY block here, so be careful not to throw error
* without releasing the PGresult.
*/
- res = pgfdw_get_result(conn, buf.data);+ res = pgfdw_get_result(conn);
if (PQresultStatus(res) != PGRES_COMMAND_OK)
pgfdw_report_error(ERROR, res, conn, true, fsstate->query);
PQclear(res);
@@ -3817,7 +3817,7 @@ fetch_more_data(ForeignScanState *node)
* The query was already sent by an earlier call to
* fetch_more_data_begin. So now we just fetch the result.
*/
- res = pgfdw_get_result(conn, fsstate->query);+ res = pgfdw_get_result(conn);
/* On error, report the original query, not the FETCH. */
if (PQresultStatus(res) != PGRES_TUPLES_OK)
pgfdw_report_error(ERROR, res, conn, false, fsstate->query);
@@ -4166,7 +4166,7 @@ execute_foreign_modify(EState *estate,
* We don't use a PG_TRY block here, so be careful not to throw error
* without releasing the PGresult.
*/
- res = pgfdw_get_result(fmstate->conn, fmstate->query);+ res = pgfdw_get_result(fmstate->conn);
if (PQresultStatus(res) !=
(fmstate->has_returning ? PGRES_TUPLES_OK : PGRES_COMMAND_OK))
pgfdw_report_error(ERROR, res, fmstate->conn, true, fmstate->query);
@@ -4236,7 +4236,7 @@ prepare_foreign_modify(PgFdwModifyState *fmstate)
* We don't use a PG_TRY block here, so be careful not to throw error
* without releasing the PGresult.
*/
- res = pgfdw_get_result(fmstate->conn, fmstate->query);+ res = pgfdw_get_result(fmstate->conn);
if (PQresultStatus(res) != PGRES_COMMAND_OK)
pgfdw_report_error(ERROR, res, fmstate->conn, true, fmstate->query);
PQclear(res);
@@ -4578,7 +4578,7 @@ execute_dml_stmt(ForeignScanState *node)
* We don't use a PG_TRY block here, so be careful not to throw error
* without releasing the PGresult.
*/
- dmstate->result = pgfdw_get_result(dmstate->conn, dmstate->query);+ dmstate->result = pgfdw_get_result(dmstate->conn);
if (PQresultStatus(dmstate->result) !=
(dmstate->has_returning ? PGRES_TUPLES_OK : PGRES_COMMAND_OK))
pgfdw_report_error(ERROR, dmstate->result, dmstate->conn, true,
diff --git a/contrib/postgres_fdw/postgres_fdw.h b/contrib/postgres_fdw/postgres_fdw.hindex 02c11523199..8e8a36ad14c 100644--- a/contrib/postgres_fdw/postgres_fdw.h+++ b/contrib/postgres_fdw/postgres_fdw.h@@ -158,7 +158,7 @@ extern void ReleaseConnection(PGconn *conn);
extern unsigned int GetCursorNumber(PGconn *conn);
extern unsigned int GetPrepStmtNumber(PGconn *conn);
extern void do_sql_command(PGconn *conn, const char *sql);
-extern PGresult *pgfdw_get_result(PGconn *conn, const char *query);+extern PGresult *pgfdw_get_result(PGconn *conn);
extern PGresult *pgfdw_exec_query(PGconn *conn, const char *query,
PgFdwConnState *state);
extern void pgfdw_report_error(int elevel, PGresult *res, PGconn *conn,
diff --git a/src/backend/replication/libpqwalreceiver/libpqwalreceiver.c b/src/backend/replication/libpqwalreceiver/libpqwalreceiver.cindex b4038e114d8..568024ec974 100644--- a/src/backend/replication/libpqwalreceiver/libpqwalreceiver.c+++ b/src/backend/replication/libpqwalreceiver/libpqwalreceiver.c@@ -705,12 +705,9 @@ 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.
*/
diff --git a/src/include/libpq/libpq-be-fe-helpers.h b/src/include/libpq/libpq-be-fe-helpers.hindex 41e3bb4376a..a4b3e805b9d 100644--- a/src/include/libpq/libpq-be-fe-helpers.h+++ b/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 */
--
2.47.2
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To: pgsql-hackers@postgresql.org
Cc: andreas@proxel.se, noah@leadboat.com
Subject: Re: dblink query interruptibility
In-Reply-To: <bb9f565d-4ca5-4e30-9008-fa538c6895ef@proxel.se>
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