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Assert that WaitLatchOrSocket callers cannot wait only for writability.

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Since we have chosen to report socket EOF and error conditions via the
WL_SOCKET_READABLE flag bit, it's unsafe to wait only for
WL_SOCKET_WRITEABLE; the caller would never be notified of the socket
condition, and in some of these implementations WaitLatchOrSocket would
busy-wait until something else happens.  Add this restriction to the API
specification, and add Asserts to check that callers don't try to do that.

At some point we might want to consider adjusting the API to relax this
restriction, but until we have an actual use case for waiting on a
write-only socket, it seems premature to design a solution.
This commit is contained in:
Tom Lane
2012-05-14 16:11:59 -04:00
parent ff4628f37a
commit e42a21b9e6
2 changed files with 10 additions and 2 deletions
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2
src/backend/port/win32_latch.c
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@@ -106,6 +106,8 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
wakeEvents &= ~(WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE);
Assert(wakeEvents != 0); /* must have at least one wake event */
/* Cannot specify WL_SOCKET_WRITEABLE without WL_SOCKET_READABLE */
Assert((wakeEvents & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE)) != WL_SOCKET_WRITEABLE);
if ((wakeEvents & WL_LATCH_SET) && latch->owner_pid != MyProcPid)
elog(ERROR, "cannot wait on a latch owned by another process");