Fix unsafe references to errno within error messaging logic.

Various places were supposing that errno could be expected to hold still
within an ereport() nest or similar contexts.  This isn't true necessarily,
though in some cases it accidentally failed to fail depending on how the
compiler chanced to order the subexpressions.  This class of thinko
explains recent reports of odd failures on clang-built versions, typically
missing or inappropriate HINT fields in messages.

Problem identified by Christian Kruse, who also submitted the patch this
commit is based on.  (I fixed a few issues in his patch and found a couple
of additional places with the same disease.)

Back-patch as appropriate to all supported branches.

src/backend/port/sysv_sema.c

@@ -91,15 +91,17 @@ InternalIpcSemaphoreCreate(IpcSemaphoreKey semKey, int numSems)
 
 	if (semId < 0)
 	{
+		int			saved_errno = errno;
+
 		/*
 		 * Fail quietly if error indicates a collision with existing set. One
 		 * would expect EEXIST, given that we said IPC_EXCL, but perhaps we
 		 * could get a permission violation instead?  Also, EIDRM might occur
 		 * if an old set is slated for destruction but not gone yet.
 		 */
-		if (errno == EEXIST || errno == EACCES
+		if (saved_errno == EEXIST || saved_errno == EACCES
 #ifdef EIDRM
-			|| errno == EIDRM
+			|| saved_errno == EIDRM
 #endif
 			)
 			return -1;
@@ -112,7 +114,7 @@ InternalIpcSemaphoreCreate(IpcSemaphoreKey semKey, int numSems)
 				 errdetail("Failed system call was semget(%lu, %d, 0%o).",
 						   (unsigned long) semKey, numSems,
 						   IPC_CREAT | IPC_EXCL | IPCProtection),
-				 (errno == ENOSPC) ?
+				 (saved_errno == ENOSPC) ?
 				 errhint("This error does *not* mean that you have run out of disk space.  "
 		  "It occurs when either the system limit for the maximum number of "
 			 "semaphore sets (SEMMNI), or the system wide maximum number of "
@@ -136,13 +138,17 @@ IpcSemaphoreInitialize(IpcSemaphoreId semId, int semNum, int value)
 
 	semun.val = value;
 	if (semctl(semId, semNum, SETVAL, semun) < 0)
+	{
+		int			saved_errno = errno;
+
 		ereport(FATAL,
 				(errmsg_internal("semctl(%d, %d, SETVAL, %d) failed: %m",
 								 semId, semNum, value),
-				 (errno == ERANGE) ?
+				 (saved_errno == ERANGE) ?
 				 errhint("You possibly need to raise your kernel's SEMVMX value to be at least "
 				  "%d.  Look into the PostgreSQL documentation for details.",
 						 value) : 0));
+	}
 }
 
 /*