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Commit3b790d2

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Fix corner-case bug in WaitEventSetWaitBlock on Windows.
If we do not reset the FD_READ event, WaitForMultipleObjects won'treturn it again again unless we've meanwhile read from the socket,which is generally true but not guaranteed. WaitEventSetWaitBlockitself may fail to return the event to the caller if the latch isalso set, and even if we changed that, the caller isn't obliged tohandle all returned events at once. On non-Windows systems, thesocket-read event is purely level-triggered, so this issue doesnot exist. To fix, make Windows reset the event when needed.This bug was introduced by98a64d0,and causes hangs when trying to use the pldebugger extension.Patch by Amit Kapial. Reported and tested by Ashutosh Sharma, whoalso provided some analysis. Further analysis by Michael Paquier.
1 parent59649c3 commit3b790d2

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‎src/backend/storage/ipc/latch.c

Lines changed: 27 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -643,6 +643,9 @@ AddWaitEventToSet(WaitEventSet *set, uint32 events, pgsocket fd, Latch *latch,
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event->fd=fd;
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event->events=events;
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event->user_data=user_data;
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#ifdefWIN32
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event->reset= false;
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#endif
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if (events==WL_LATCH_SET)
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{
@@ -1389,6 +1392,18 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
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DWORDrc;
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WaitEvent*cur_event;
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/* Reset any wait events that need it */
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for (cur_event=set->events;
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cur_event< (set->events+set->nevents);
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cur_event++)
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{
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if (cur_event->reset)
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{
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WaitEventAdjustWin32(set,cur_event);
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cur_event->reset= false;
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}
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}
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/*
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* Sleep.
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*
@@ -1472,6 +1487,18 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
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{
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/* data available in socket */
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occurred_events->events |=WL_SOCKET_READABLE;
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/*------
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* WaitForMultipleObjects doesn't guarantee that a read event will
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* be returned if the latch is set at the same time. Even if it
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* did, the caller might drop that event expecting it to reoccur
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* on next call. So, we must force the event to be reset if this
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* WaitEventSet is used again in order to avoid an indefinite
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* hang. Refer https://msdn.microsoft.com/en-us/library/windows/desktop/ms741576(v=vs.85).aspx
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* for the behavior of socket events.
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*------
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*/
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cur_event->reset= true;
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}
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if ((cur_event->events&WL_SOCKET_WRITEABLE)&&
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(resEvents.lNetworkEvents&FD_WRITE))

‎src/include/storage/latch.h

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -133,6 +133,9 @@ typedef struct WaitEvent
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uint32events;/* triggered events */
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pgsocketfd;/* socket fd associated with event */
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void*user_data;/* pointer provided in AddWaitEventToSet */
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#ifdefWIN32
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boolreset;/* Is reset of the event required? */
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#endif
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}WaitEvent;
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/* forward declaration to avoid exposing latch.c implementation details */

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