Allow parent's WaitEventSets to be freed after fork().

An epoll fd belonging to the parent should be closed in the child.  A
kqueue fd is automatically closed by fork(), but we should still adjust
our counter.  For poll and Windows systems, nothing special is required.
On all systems we free the memory.

No caller yet, but we'll need this if we start using WaitEventSet in the
postmaster as planned.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKG%2BZ-HpOj1JsO9eWUP%2Bar7npSVinsC_npxSy%2BjdOMsx%3DGg%40mail.gmail.com

Author: macdice

src/backend/storage/ipc/latch.c

@@ -869,6 +869,23 @@ FreeWaitEventSet(WaitEventSet *set)
 	pfree(set);
 }
 
+/*
+ * Free a previously created WaitEventSet in a child process after a fork().
+ */
+void
+FreeWaitEventSetAfterFork(WaitEventSet *set)
+{
+#if defined(WAIT_USE_EPOLL)
+	close(set->epoll_fd);
+	ReleaseExternalFD();
+#elif defined(WAIT_USE_KQUEUE)
+	/* kqueues are not normally inherited by child processes */
+	ReleaseExternalFD();
+#endif
+
+	pfree(set);
+}
+
 /* ---
  * Add an event to the set. Possible events are:
  * - WL_LATCH_SET: Wait for the latch to be set

src/include/storage/latch.h

@@ -175,6 +175,7 @@ extern void ShutdownLatchSupport(void);
 
 extern WaitEventSet *CreateWaitEventSet(MemoryContext context, int nevents);
 extern void FreeWaitEventSet(WaitEventSet *set);
+extern void FreeWaitEventSetAfterFork(WaitEventSet *set);
 extern int	AddWaitEventToSet(WaitEventSet *set, uint32 events, pgsocket fd,
 							  Latch *latch, void *user_data);
 extern void ModifyWaitEvent(WaitEventSet *set, int pos, uint32 events, Latch *latch);