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Commitff181d1

Browse files
jchampiomacdice
andcommitted
oauth: Remove stale events from the kqueue multiplexer
If a socket is added to the kqueue, becomes readable/writable, andsubsequently becomes non-readable/writable again, the kqueue itself willremain readable until either the socket registration is removed, or thestale event is cleared via a call to kevent().In many simple cases, Curl itself will remove the socket registrationquickly, but in real-world usage, this is not guaranteed to happen. Thekqueue can then remain stuck in a permanently readable state until therequest ends, which results in pointless wakeups for the client andwasted CPU time.Implement comb_multiplexer() to call kevent() and unstick any staleevents that would cause unnecessary callbacks. This is called rightafter drive_request(), before we return control to the client to wait.Suggested-by: Thomas Munro <thomas.munro@gmail.com>Co-authored-by: Thomas Munro <thomas.munro@gmail.com>Reviewed-by: Thomas Munro <thomas.munro@gmail.com>Backpatch-through: 18Discussion:https://postgr.es/m/CAOYmi+nDZxJHaWj9_jRSyf8uMToCADAmOfJEggsKW-kY7aUwHA@mail.gmail.com
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‎src/interfaces/libpq-oauth/oauth-curl.c‎

Lines changed: 61 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -1376,6 +1376,53 @@ register_socket(CURL *curl, curl_socket_t socket, int what, void *ctx,
13761376
#endif
13771377
}
13781378

1379+
/*
1380+
* If there is no work to do on any of the descriptors in the multiplexer, then
1381+
* this function must ensure that the multiplexer is not readable.
1382+
*
1383+
* Unlike epoll descriptors, kqueue descriptors only transition from readable to
1384+
* unreadable when kevent() is called and finds nothing, after removing
1385+
* level-triggered conditions that have gone away. We therefore need a dummy
1386+
* kevent() call after operations might have been performed on the monitored
1387+
* sockets or timer_fd. Any event returned is ignored here, but it also remains
1388+
* queued (being level-triggered) and leaves the descriptor readable. This is a
1389+
* no-op for epoll descriptors.
1390+
*/
1391+
staticbool
1392+
comb_multiplexer(structasync_ctx*actx)
1393+
{
1394+
#if defined(HAVE_SYS_EPOLL_H)
1395+
/* The epoll implementation doesn't hold onto stale events. */
1396+
return true;
1397+
#elif defined(HAVE_SYS_EVENT_H)
1398+
structtimespectimeout= {0};
1399+
structkeventev;
1400+
1401+
/*
1402+
* Try to read a single pending event. We can actually ignore the result:
1403+
* either we found an event to process, in which case the multiplexer is
1404+
* correctly readable for that event at minimum, and it doesn't matter if
1405+
* there are any stale events; or we didn't find any, in which case the
1406+
* kernel will have discarded any stale events as it traveled to the end
1407+
* of the queue.
1408+
*
1409+
* Note that this depends on our registrations being level-triggered --
1410+
* even the timer, so we use a chained kqueue for that instead of an
1411+
* EVFILT_TIMER on the top-level mux. If we used edge-triggered events,
1412+
* this call would improperly discard them.
1413+
*/
1414+
if (kevent(actx->mux,NULL,0,&ev,1,&timeout)<0)
1415+
{
1416+
actx_error(actx,"could not comb kqueue: %m");
1417+
return false;
1418+
}
1419+
1420+
return true;
1421+
#else
1422+
#error comb_multiplexer is not implemented on this platform
1423+
#endif
1424+
}
1425+
13791426
/*
13801427
* Enables or disables the timer in the multiplexer set. The timeout value is
13811428
* in milliseconds (negative values disable the timer).
@@ -2755,13 +2802,21 @@ pg_fe_run_oauth_flow_impl(PGconn *conn)
27552802

27562803
if (status==PGRES_POLLING_FAILED)
27572804
gotoerror_return;
2758-
elseif (status!=PGRES_POLLING_OK)
2759-
{
2760-
/* not done yet */
2761-
returnstatus;
2762-
}
2805+
elseif (status==PGRES_POLLING_OK)
2806+
break;/* done! */
27632807

2764-
break;
2808+
/*
2809+
* This request is still running.
2810+
*
2811+
* Make sure that stale events don't cause us to come back
2812+
* early. (Currently, this can occur only with kqueue.) If
2813+
* this is forgotten, the multiplexer can get stuck in a
2814+
* signaled state and we'll burn CPU cycles pointlessly.
2815+
*/
2816+
if (!comb_multiplexer(actx))
2817+
gotoerror_return;
2818+
2819+
returnstatus;
27652820
}
27662821

27672822
caseOAUTH_STEP_WAIT_INTERVAL:

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