Remove volatile from latch API

This was no longer useful since the latch functions use memory
barriers already, which are also compiler barriers, and volatile does
not help with cross-process access.

Discussion: https://www.postgresql.org/message-id/flat/20190218202511.qsfpuj5sy4dbezcw%40alap3.anarazel.de#18783c27d73e9e40009c82f6e0df0974

Author: petere

src/backend/storage/ipc/latch.c

@@ -225,7 +225,7 @@ InitializeLatchSupport(void)
  * Initialize a process-local latch.
  */
 void
-InitLatch(volatile Latch *latch)
+InitLatch(Latch *latch)
 {
 	latch->is_set = false;
 	latch->owner_pid = MyProcPid;
@@ -257,7 +257,7 @@ InitLatch(volatile Latch *latch)
  * process references to postmaster-private latches or WaitEventSets.
  */
 void
-InitSharedLatch(volatile Latch *latch)
+InitSharedLatch(Latch *latch)
 {
 #ifdef WIN32
 	SECURITY_ATTRIBUTES sa;
@@ -293,7 +293,7 @@ InitSharedLatch(volatile Latch *latch)
  * as shared latches use SIGUSR1 for inter-process communication.
  */
 void
-OwnLatch(volatile Latch *latch)
+OwnLatch(Latch *latch)
 {
 	/* Sanity checks */
 	Assert(latch->is_shared);
@@ -313,7 +313,7 @@ OwnLatch(volatile Latch *latch)
  * Disown a shared latch currently owned by the current process.
  */
 void
-DisownLatch(volatile Latch *latch)
+DisownLatch(Latch *latch)
 {
 	Assert(latch->is_shared);
 	Assert(latch->owner_pid == MyProcPid);
@@ -341,7 +341,7 @@ DisownLatch(volatile Latch *latch)
  * we return all of them in one call, but we will return at least one.
  */
 int
-WaitLatch(volatile Latch *latch, int wakeEvents, long timeout,
+WaitLatch(Latch *latch, int wakeEvents, long timeout,
 		  uint32 wait_event_info)
 {
 	return WaitLatchOrSocket(latch, wakeEvents, PGINVALID_SOCKET, timeout,
@@ -366,7 +366,7 @@ WaitLatch(volatile Latch *latch, int wakeEvents, long timeout,
  * WaitEventSet instead; that's more efficient.
  */
 int
-WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
+WaitLatchOrSocket(Latch *latch, int wakeEvents, pgsocket sock,
 				  long timeout, uint32 wait_event_info)
 {
 	int			ret = 0;
@@ -381,7 +381,7 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 
 	if (wakeEvents & WL_LATCH_SET)
 		AddWaitEventToSet(set, WL_LATCH_SET, PGINVALID_SOCKET,
-						  (Latch *) latch, NULL);
+						  latch, NULL);
 
 	/* Postmaster-managed callers must handle postmaster death somehow. */
 	Assert(!IsUnderPostmaster ||
@@ -433,7 +433,7 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
  * throwing an error is not a good idea.
  */
 void
-SetLatch(volatile Latch *latch)
+SetLatch(Latch *latch)
 {
 #ifndef WIN32
 	pid_t		owner_pid;
@@ -516,7 +516,7 @@ SetLatch(volatile Latch *latch)
  * the latch is set again before the WaitLatch call.
  */
 void
-ResetLatch(volatile Latch *latch)
+ResetLatch(Latch *latch)
 {
 	/* Only the owner should reset the latch */
 	Assert(latch->owner_pid == MyProcPid);

src/include/storage/latch.h

@@ -156,12 +156,12 @@ typedef struct WaitEventSet WaitEventSet;
  * prototypes for functions in latch.c
  */
 extern void InitializeLatchSupport(void);
-extern void InitLatch(volatile Latch *latch);
-extern void InitSharedLatch(volatile Latch *latch);
-extern void OwnLatch(volatile Latch *latch);
-extern void DisownLatch(volatile Latch *latch);
-extern void SetLatch(volatile Latch *latch);
-extern void ResetLatch(volatile Latch *latch);
+extern void InitLatch(Latch *latch);
+extern void InitSharedLatch(Latch *latch);
+extern void OwnLatch(Latch *latch);
+extern void DisownLatch(Latch *latch);
+extern void SetLatch(Latch *latch);
+extern void ResetLatch(Latch *latch);
 
 extern WaitEventSet *CreateWaitEventSet(MemoryContext context, int nevents);
 extern void FreeWaitEventSet(WaitEventSet *set);
@@ -172,9 +172,9 @@ extern void ModifyWaitEvent(WaitEventSet *set, int pos, uint32 events, Latch *la
 extern int WaitEventSetWait(WaitEventSet *set, long timeout,
 				 WaitEvent *occurred_events, int nevents,
 				 uint32 wait_event_info);
-extern int WaitLatch(volatile Latch *latch, int wakeEvents, long timeout,
+extern int WaitLatch(Latch *latch, int wakeEvents, long timeout,
 		  uint32 wait_event_info);
-extern int WaitLatchOrSocket(volatile Latch *latch, int wakeEvents,
+extern int WaitLatchOrSocket(Latch *latch, int wakeEvents,
 				  pgsocket sock, long timeout, uint32 wait_event_info);
 
 /*