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Make WaitLatch's WL_POSTMASTER_DEATH result trustworthy; simplify callers.

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Per a suggestion from Peter Geoghegan, make WaitLatch responsible for
verifying that the WL_POSTMASTER_DEATH bit it returns is truthful (by
testing PostmasterIsAlive).  Then simplify its callers, who no longer
need to do that for themselves.  Remove weasel wording about falsely-set
result bits from WaitLatch's API contract.
This commit is contained in:
Tom Lane
2012-05-10 14:34:22 -04:00
parent 586d356bc5
commit f40022f1ad
7 changed files with 47 additions and 37 deletions
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35
src/backend/port/unix_latch.c
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@ -50,6 +50,7 @@
#include "miscadmin.h"
#include "postmaster/postmaster.h"
#include "storage/latch.h"
#include "storage/pmsignal.h"
#include "storage/shmem.h"
/* Are we currently in WaitLatch? The signal handler would like to know. */
@ -160,15 +161,7 @@ DisownLatch(volatile Latch *latch)
*
* Returns bit mask indicating which condition(s) caused the wake-up. Note
* that if multiple wake-up conditions are true, there is no guarantee that
* we return all of them in one call, but we will return at least one. Also,
* according to the select(2) man page on Linux, select(2) may spuriously
* return and report a file descriptor as readable, when it's not. We use
* select(2), so WaitLatch can also spuriously claim that a socket is
* readable, or postmaster has died, even when none of the wake conditions
* have been satisfied. That should be rare in practice, but the caller
* should not use the return value for anything critical, re-checking the
* situation with PostmasterIsAlive() or read() on a socket as necessary.
* The latch and timeout flag bits can be trusted, however.
* we return all of them in one call, but we will return at least one.
*/
int
WaitLatch(volatile Latch *latch, int wakeEvents, long timeout)
@ -318,7 +311,17 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
if ((wakeEvents & WL_POSTMASTER_DEATH) &&
(pfds[nfds - 1].revents & (POLLHUP | POLLIN | POLLERR | POLLNVAL)))
{
result |= WL_POSTMASTER_DEATH;
/*
* According to the select(2) man page on Linux, select(2) may
* spuriously return and report a file descriptor as readable,
* when it's not; and presumably so can poll(2). It's not clear
* that the relevant cases would ever apply to the postmaster
* pipe, but since the consequences of falsely returning
* WL_POSTMASTER_DEATH could be pretty unpleasant, we take the
* trouble to positively verify EOF with PostmasterIsAlive().
*/
if (!PostmasterIsAlive())
result |= WL_POSTMASTER_DEATH;
}
#else /* !HAVE_POLL */
@ -380,7 +383,17 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
if ((wakeEvents & WL_POSTMASTER_DEATH) &&
FD_ISSET(postmaster_alive_fds[POSTMASTER_FD_WATCH], &input_mask))
{
result |= WL_POSTMASTER_DEATH;
/*
* According to the select(2) man page on Linux, select(2) may
* spuriously return and report a file descriptor as readable,
* when it's not; and presumably so can poll(2). It's not clear
* that the relevant cases would ever apply to the postmaster
* pipe, but since the consequences of falsely returning
* WL_POSTMASTER_DEATH could be pretty unpleasant, we take the
* trouble to positively verify EOF with PostmasterIsAlive().
*/
if (!PostmasterIsAlive())
result |= WL_POSTMASTER_DEATH;
}
#endif /* HAVE_POLL */
} while (result == 0);