=== Background
Something as simple as the following doesn't respond to cancellation. In
v15+, any DROP DATABASE will hang as long as it's running:
SELECT dblink_exec(
$$dbname='$$||current_database()||$$' port=$$||current_setting('port'),
'SELECT pg_sleep(15)');
https://postgr.es/m/4b584c99.8090...@enterprisedb.com proposed a fix back in
2010. Latches and the libpqsrv facility have changed the server programming
environment since that patch. The problem statement also came up here:
On Thu, Dec 08, 2022 at 06:08:15PM -0800, Andres Freund wrote:
> dblink.c uses a lot of other blocking libpq functions, which obviously also
> isn't ok.
=== Key decisions
This patch adds to libpqsrv facility. It dutifully follows the existing
naming scheme. For greppability, I am favoring renaming new and old functions
such that the libpq name is a substring of this facility's name. That is,
rename libpqsrv_disconnect to srvPQfinish or maybe libpqsrv_PQfinish(). Now
is better than later, while pgxn contains no references to libpqsrv. Does
anyone else have a preference between naming schemes? If nobody does, I'll
keep today's libpqsrv_disconnect() style.
I was tempted to add a timeout argument to each libpqsrv function, which would
allow libpqsrv_get_result_last() to replace pgfdw_get_cleanup_result(). We
can always add a timeout-accepting function later and make this thread's
function name a thin wrapper around it. Does anyone feel a mandatory timeout
argument, accepting -1 for no timeout, would be the right thing?
=== Minor topics
It would be nice to replace libpqrcv_PQgetResult() and friends with the new
functions. I refrained since they use ProcessWalRcvInterrupts(), not
CHECK_FOR_INTERRUPTS(). Since walreceiver already reaches
CHECK_FOR_INTERRUPTS() via libpqsrv_connect_params(), things might just work.
This patch contains a PQexecParams() wrapper, called nowhere in
postgresql.git. It's inessential, but twelve pgxn modules do mention
PQexecParams. Just one mentions PQexecPrepared, and none mention PQdescribe*.
The patch makes postgres_fdw adopt its functions, as part of confirming the
functions are general enough. postgres_fdw create_cursor() has been passing
the "DECLARE CURSOR FOR inner_query" string for some error messages and just
inner_query for others. I almost standardized on the longer one, but the test
suite checks that. Hence, I standardized on just inner_query.
I wrote this because pglogical needs these functions to cooperate with v15+
DROP DATABASE (https://github.com/2ndQuadrant/pglogical/issues/418).
Thanks,
nm
Author: Noah Misch <n...@leadboat.com>
Commit: Noah Misch <n...@leadboat.com>
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.
Reviewed by FIXME.
Discussion: https://postgr.es/m/FIXME
diff --git a/contrib/dblink/dblink.c b/contrib/dblink/dblink.c
index 195b278..4624e53 100644
--- a/contrib/dblink/dblink.c
+++ b/contrib/dblink/dblink.c
@@ -133,6 +133,7 @@ static HTAB *remoteConnHash = NULL;
/* custom wait event values, retrieved from shared memory */
static uint32 dblink_we_connect = 0;
static uint32 dblink_we_get_conn = 0;
+static uint32 dblink_we_get_result = 0;
/*
* Following is list that holds multiple remote connections.
@@ -252,6 +253,9 @@ dblink_init(void)
{
if (!pconn)
{
+ if (dblink_we_get_result == 0)
+ dblink_we_get_result =
WaitEventExtensionNew("DblinkGetResult");
+
pconn = (remoteConn *) MemoryContextAlloc(TopMemoryContext,
sizeof(remoteConn));
pconn->conn = NULL;
pconn->openCursorCount = 0;
@@ -442,7 +446,7 @@ 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", dblink_we_get_result);
if (PQresultStatus(res) != PGRES_COMMAND_OK)
dblink_res_internalerror(conn, res, "begin error");
PQclear(res);
@@ -461,7 +465,7 @@ 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, dblink_we_get_result);
if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
{
dblink_res_error(conn, conname, res, fail,
@@ -530,7 +534,7 @@ dblink_close(PG_FUNCTION_ARGS)
appendStringInfo(&buf, "CLOSE %s", curname);
/* close the cursor */
- res = PQexec(conn, buf.data);
+ res = libpqsrv_exec(conn, buf.data, dblink_we_get_result);
if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
{
dblink_res_error(conn, conname, res, fail,
@@ -550,7 +554,7 @@ dblink_close(PG_FUNCTION_ARGS)
{
rconn->newXactForCursor = false;
- res = PQexec(conn, "COMMIT");
+ res = libpqsrv_exec(conn, "COMMIT",
dblink_we_get_result);
if (PQresultStatus(res) != PGRES_COMMAND_OK)
dblink_res_internalerror(conn, res, "commit
error");
PQclear(res);
@@ -632,7 +636,7 @@ 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, dblink_we_get_result);
if (!res ||
(PQresultStatus(res) != PGRES_COMMAND_OK &&
PQresultStatus(res) != PGRES_TUPLES_OK))
@@ -780,7 +784,7 @@ 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,
dblink_we_get_result);
/* NULL means we're all done with the async results */
if (res)
@@ -1088,7 +1092,8 @@ 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, dblink_we_get_result))
!=
+ NULL)
PQclear(res);
PG_RE_THROW();
}
@@ -1115,7 +1120,7 @@ storeQueryResult(volatile storeInfo *sinfo, PGconn *conn,
const char *sql)
{
CHECK_FOR_INTERRUPTS();
- sinfo->cur_res = PQgetResult(conn);
+ sinfo->cur_res = libpqsrv_get_result(conn,
dblink_we_get_result);
if (!sinfo->cur_res)
break;
@@ -1443,7 +1448,7 @@ dblink_exec(PG_FUNCTION_ARGS)
if (!conn)
dblink_conn_not_avail(conname);
- res = PQexec(conn, sql);
+ res = libpqsrv_exec(conn, sql, dblink_we_get_result);
if (!res ||
(PQresultStatus(res) != PGRES_COMMAND_OK &&
PQresultStatus(res) != PGRES_TUPLES_OK))
@@ -2740,8 +2745,8 @@ 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.c
index 5800c6a..8755244 100644
--- a/contrib/postgres_fdw/connection.c
+++ b/contrib/postgres_fdw/connection.c
@@ -187,6 +187,10 @@ GetConnection(UserMapping *user, bool will_prep_stmt,
PgFdwConnState **state)
{
HASHCTL ctl;
+ if (pgfdw_we_get_result == 0)
+ pgfdw_we_get_result =
+ WaitEventExtensionNew("PostgresFdwGetResult");
+
ctl.keysize = sizeof(ConnCacheKey);
ctl.entrysize = sizeof(ConnCacheEntry);
ConnectionHash = hash_create("postgres_fdw connections", 8,
@@ -716,7 +720,7 @@ 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);
@@ -819,7 +823,9 @@ 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.
*/
@@ -830,81 +836,20 @@ 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;
-
- /* first time, allocate or get the custom wait
event */
- if (pgfdw_we_get_result == 0)
- pgfdw_we_get_result =
WaitEventExtensionNew("PostgresFdwGetResult");
-
- /* Sleep until there's something to do */
- wc = WaitLatchOrSocket(MyLatch,
-
WL_LATCH_SET | WL_SOCKET_READABLE |
-
WL_EXIT_ON_PM_DEATH,
-
PQsocket(conn),
- -1L,
pgfdw_we_get_result);
- 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, pgfdw_we_get_result);
}
/*
@@ -945,8 +890,8 @@ 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));
@@ -1046,7 +991,8 @@ 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.c
index 09fd489..5084aa6 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.c
index 6de2bec..7d509c6 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -3759,7 +3759,7 @@ 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);
@@ -3809,7 +3809,7 @@ 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);
@@ -4158,7 +4158,7 @@ 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);
@@ -4228,7 +4228,7 @@ 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);
@@ -4570,7 +4570,7 @@ 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.h
index 47157ac..3e94d51 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/doc/src/sgml/dblink.sgml b/doc/src/sgml/dblink.sgml
index e8de5a6..81f3598 100644
--- a/doc/src/sgml/dblink.sgml
+++ b/doc/src/sgml/dblink.sgml
@@ -37,6 +37,15 @@
</para>
</listitem>
</varlistentry>
+
+ <varlistentry>
+ <term><literal>DblinkGetResult</literal></term>
+ <listitem>
+ <para>
+ Waiting to receive the results of a query from a remote server.
+ </para>
+ </listitem>
+ </varlistentry>
</variablelist>
<para>
diff --git a/src/backend/replication/libpqwalreceiver/libpqwalreceiver.c
b/src/backend/replication/libpqwalreceiver/libpqwalreceiver.c
index 60d5c1f..2825001 100644
--- a/src/backend/replication/libpqwalreceiver/libpqwalreceiver.c
+++ b/src/backend/replication/libpqwalreceiver/libpqwalreceiver.c
@@ -648,12 +648,9 @@ 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.h
index 41e3bb4..a4b3e80 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 */
