Introduce symbolic names for FeBeWaitSet positions.

Previously we used 0 and 1 to refer to the socket and latch in far flung
parts of the tree, without any explanation.  Also use PGINVALID_SOCKET
rather than -1 in a couple of places that didn't already do that.
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGJAC4Oqao%3DqforhNey20J8CiG2R%3DoBPqvfR0vOJrFysGw%40mail.gmail.com

src/backend/libpq/be-secure.c

@@ -180,7 +180,7 @@ retry:
 
 		Assert(waitfor);
 
-		ModifyWaitEvent(FeBeWaitSet, 0, waitfor, NULL);
+		ModifyWaitEvent(FeBeWaitSet, FeBeWaitSetSocketPos, waitfor, NULL);
 
 		WaitEventSetWait(FeBeWaitSet, -1 /* no timeout */ , &event, 1,
 						 WAIT_EVENT_CLIENT_READ);
@@ -292,7 +292,7 @@ retry:
 
 		Assert(waitfor);
 
-		ModifyWaitEvent(FeBeWaitSet, 0, waitfor, NULL);
+		ModifyWaitEvent(FeBeWaitSet, FeBeWaitSetSocketPos, waitfor, NULL);
 
 		WaitEventSetWait(FeBeWaitSet, -1 /* no timeout */ , &event, 1,
 						 WAIT_EVENT_CLIENT_WRITE);

src/backend/libpq/pqcomm.c

@@ -191,6 +191,9 @@ WaitEventSet *FeBeWaitSet;
 void
 pq_init(void)
 {
+	int		socket_pos PG_USED_FOR_ASSERTS_ONLY;
+	int		latch_pos PG_USED_FOR_ASSERTS_ONLY;
+
 	/* initialize state variables */
 	PqSendBufferSize = PQ_SEND_BUFFER_SIZE;
 	PqSendBuffer = MemoryContextAlloc(TopMemoryContext, PqSendBufferSize);
@@ -219,10 +222,19 @@ pq_init(void)
 #endif
 
 	FeBeWaitSet = CreateWaitEventSet(TopMemoryContext, 3);
-	AddWaitEventToSet(FeBeWaitSet, WL_SOCKET_WRITEABLE, MyProcPort->sock,
+	socket_pos = AddWaitEventToSet(FeBeWaitSet, WL_SOCKET_WRITEABLE,
+								   MyProcPort->sock, NULL, NULL);
+	latch_pos = AddWaitEventToSet(FeBeWaitSet, WL_LATCH_SET, PGINVALID_SOCKET,
+								  MyLatch, NULL);
+	AddWaitEventToSet(FeBeWaitSet, WL_POSTMASTER_DEATH, PGINVALID_SOCKET,
 					  NULL, NULL);
-	AddWaitEventToSet(FeBeWaitSet, WL_LATCH_SET, -1, MyLatch, NULL);
-	AddWaitEventToSet(FeBeWaitSet, WL_POSTMASTER_DEATH, -1, NULL, NULL);
+
+	/*
+	 * The event positions match the order we added them, but let's sanity
+	 * check them to be sure.
+	 */
+	Assert(socket_pos == FeBeWaitSetSocketPos);
+	Assert(latch_pos == FeBeWaitSetLatchPos);
 }
 
 /* --------------------------------

src/backend/utils/init/miscinit.c

@@ -202,7 +202,8 @@ SwitchToSharedLatch(void)
 	MyLatch = &MyProc->procLatch;
 
 	if (FeBeWaitSet)
-		ModifyWaitEvent(FeBeWaitSet, 1, WL_LATCH_SET, MyLatch);
+		ModifyWaitEvent(FeBeWaitSet, FeBeWaitSetLatchPos, WL_LATCH_SET,
+						MyLatch);
 
 	/*
 	 * Set the shared latch as the local one might have been set. This
@@ -221,7 +222,8 @@ SwitchBackToLocalLatch(void)
 	MyLatch = &LocalLatchData;
 
 	if (FeBeWaitSet)
-		ModifyWaitEvent(FeBeWaitSet, 1, WL_LATCH_SET, MyLatch);
+		ModifyWaitEvent(FeBeWaitSet, FeBeWaitSetLatchPos, WL_LATCH_SET,
+						MyLatch);
 
 	SetLatch(MyLatch);
 }

src/include/libpq/libpq.h

@@ -55,6 +55,9 @@ extern const PGDLLIMPORT PQcommMethods *PqCommMethods;
  */
 extern WaitEventSet *FeBeWaitSet;
 
+#define FeBeWaitSetSocketPos 0
+#define FeBeWaitSetLatchPos 1
+
 extern int	StreamServerPort(int family, const char *hostName,
 							 unsigned short portNumber, const char *unixSocketDir,
 							 pgsocket ListenSocket[], int MaxListen);