Test honestly for <sys/signalfd.h>.

Commit 6a2a70a02 supposed that any platform having <sys/epoll.h>
would also have <sys/signalfd.h>.  It turns out there are still a
few people using platforms where that's not so, so we'd better make
a separate configure probe for it.  But since it took this long to
notice, I'm content with the decision to not have a separate code
path for epoll-only machines; we'll just fall back to using poll()
for these stragglers.

Per gripe from Gabriela Serventi.  Back-patch to v14 where this
code came in.

Discussion: https://postgr.es/m/CAHOHWE-JjJDfcYuLAAEO7Jk07atFAU47z8TzHzg71gbC0aMy=g@mail.gmail.com

src/backend/storage/ipc/latch.c

@@ -43,6 +43,9 @@
 #ifdef HAVE_SYS_EVENT_H
 #include <sys/event.h>
 #endif
+#ifdef HAVE_SYS_SIGNALFD_H
+#include <sys/signalfd.h>
+#endif
 #ifdef HAVE_POLL_H
 #include <poll.h>
 #endif
@@ -69,7 +72,7 @@
 #if defined(WAIT_USE_EPOLL) || defined(WAIT_USE_POLL) || \
 	defined(WAIT_USE_KQUEUE) || defined(WAIT_USE_WIN32)
 /* don't overwrite manual choice */
-#elif defined(HAVE_SYS_EPOLL_H)
+#elif defined(HAVE_SYS_EPOLL_H) && defined(HAVE_SYS_SIGNALFD_H)
 #define WAIT_USE_EPOLL
 #elif defined(HAVE_KQUEUE)
 #define WAIT_USE_KQUEUE
@@ -81,10 +84,6 @@
 #error "no wait set implementation available"
 #endif
 
-#ifdef WAIT_USE_EPOLL
-#include <sys/signalfd.h>
-#endif
-
 /* typedef in latch.h */
 struct WaitEventSet
 {

src/include/pg_config.h.in

@@ -640,6 +640,9 @@
 /* Define to 1 if you have the <sys/shm.h> header file. */
 #undef HAVE_SYS_SHM_H
 
+/* Define to 1 if you have the <sys/signalfd.h> header file. */
+#undef HAVE_SYS_SIGNALFD_H
+
 /* Define to 1 if you have the <sys/sockio.h> header file. */
 #undef HAVE_SYS_SOCKIO_H
 

src/tools/msvc/Solution.pm

@@ -406,6 +406,7 @@ sub GenerateFiles
 		HAVE_SYS_SELECT_H                        => undef,
 		HAVE_SYS_SEM_H                           => undef,
 		HAVE_SYS_SHM_H                           => undef,
+		HAVE_SYS_SIGNALFD_H                      => undef,
 		HAVE_SYS_SOCKIO_H                        => undef,
 		HAVE_SYS_STAT_H                          => 1,
 		HAVE_SYS_TAS_H                           => undef,