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libpq-be-fe-helpers.h: wrap new cancel APIs

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Commit 61461a300c introduced new functions to libpq for cancelling
queries.  This commit introduces a helper function that backend-side
libraries and extensions can use to invoke those.  This function takes a
timeout and can itself be interrupted while it is waiting for a cancel
request to be sent and processed, instead of being blocked.

This replaces the usage of the old functions in postgres_fdw and dblink.

Finally, it also adds some test coverage for the cancel support in
postgres_fdw.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://postgr.es/m/CAGECzQT_VgOWWENUqvUV9xQmbaCyXjtRRAYO8W07oqashk_N+g@mail.gmail.com
This commit is contained in:
Alvaro Herrera
2024-03-28 11:31:03 +01:00
parent 427005742b
commit 2466d6654f
5 changed files with 140 additions and 39 deletions
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89
src/include/libpq/libpq-be-fe-helpers.h
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@@ -44,6 +44,8 @@
#include "miscadmin.h"
#include "storage/fd.h"
#include "storage/latch.h"
#include "utils/timestamp.h"
#include "utils/wait_event.h"
static inline void libpqsrv_connect_prepare(void);
@@ -365,4 +367,91 @@ libpqsrv_get_result(PGconn *conn, uint32 wait_event_info)
return PQgetResult(conn);
}
/*
* Submit a cancel request to the given connection, waiting only until
* the given time.
*
* We sleep interruptibly until we receive confirmation that the cancel
* request has been accepted, and if it is, return NULL; if the cancel
* request fails, return an error message string (which is not to be
* freed).
*
* For other problems (to wit: OOM when strdup'ing an error message from
* libpq), this function can ereport(ERROR).
*
* Note: this function leaks a string's worth of memory when reporting
* libpq errors. Make sure to call it in a transient memory context.
*/
static inline char *
libpqsrv_cancel(PGconn *conn, TimestampTz endtime)
{
PGcancelConn *cancel_conn;
char *error = NULL;
cancel_conn = PQcancelCreate(conn);
if (cancel_conn == NULL)
return _("out of memory");
/* In what follows, do not leak any PGcancelConn on any errors. */
PG_TRY();
{
if (!PQcancelStart(cancel_conn))
{
error = pchomp(PQcancelErrorMessage(cancel_conn));
goto exit;
}
for (;;)
{
PostgresPollingStatusType pollres;
TimestampTz now;
long cur_timeout;
int waitEvents = WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH;
pollres = PQcancelPoll(cancel_conn);
if (pollres == PGRES_POLLING_OK)
break; /* success! */
/* If timeout has expired, give up, else get sleep time. */
now = GetCurrentTimestamp();
cur_timeout = TimestampDifferenceMilliseconds(now, endtime);
if (cur_timeout <= 0)
{
error = _("cancel request timed out");
break;
}
switch (pollres)
{
case PGRES_POLLING_READING:
waitEvents |= WL_SOCKET_READABLE;
break;
case PGRES_POLLING_WRITING:
waitEvents |= WL_SOCKET_WRITEABLE;
break;
default:
error = pchomp(PQcancelErrorMessage(cancel_conn));
goto exit;
}
/* Sleep until there's something to do */
WaitLatchOrSocket(MyLatch, waitEvents, PQcancelSocket(cancel_conn),
cur_timeout, PG_WAIT_CLIENT);
ResetLatch(MyLatch);
CHECK_FOR_INTERRUPTS();
}
exit: ;
}
PG_FINALLY();
{
PQcancelFinish(cancel_conn);
}
PG_END_TRY();
return error;
}
#endif /* LIBPQ_BE_FE_HELPERS_H */