Fix calculation of maximum statistics-message size.

The PGSTAT_NUM_TABENTRIES macro should have been updated when new fields
were added to struct PgStat_MsgTabstat in commit 64482890, but it wasn't.
Fix that.

Also, add a static assertion that we didn't overrun the intended size limit
on stats messages.  This will not necessarily catch every mistake in
computing the maximum array size for stats messages, but it will catch ones
that have practical consequences.  (The assertion in fact doesn't complain
about the aforementioned error in PGSTAT_NUM_TABENTRIES, because that was
not big enough to cause the array length to increase.)

No back-patch, as there's no actual bug in existing releases; this is just
in the nature of future-proofing.

Mark Dilger and Tom Lane

src/backend/postmaster/pgstat.c

@@ -329,6 +329,16 @@ pgstat_init(void)
 
 #define TESTBYTEVAL ((char) 199)
 
+	/*
+	 * This static assertion verifies that we didn't mess up the calculations
+	 * involved in selecting maximum payload sizes for our UDP messages.
+	 * Because the only consequence of overrunning PGSTAT_MAX_MSG_SIZE would
+	 * be silent performance loss from fragmentation, it seems worth having a
+	 * compile-time cross-check that we didn't.
+	 */
+	StaticAssertStmt(sizeof(PgStat_Msg) <= PGSTAT_MAX_MSG_SIZE,
+				   'maximum stats message size exceeds PGSTAT_MAX_MSG_SIZE');
+
 	/*
 	 * Create the UDP socket for sending and receiving statistic messages
 	 */

src/include/pgstat.h

@@ -177,11 +177,13 @@ typedef struct PgStat_MsgHdr
 
 /* ----------
  * Space available in a message.  This will keep the UDP packets below 1K,
- * which should fit unfragmented into the MTU of the lo interface on most
- * platforms. Does anybody care for platforms where it doesn't?
+ * which should fit unfragmented into the MTU of the loopback interface.
+ * (Larger values of PGSTAT_MAX_MSG_SIZE would work for that on most
+ * platforms, but we're being conservative here.)
  * ----------
  */
-#define PGSTAT_MSG_PAYLOAD	(1000 - sizeof(PgStat_MsgHdr))
+#define PGSTAT_MAX_MSG_SIZE 1000
+#define PGSTAT_MSG_PAYLOAD	(PGSTAT_MAX_MSG_SIZE - sizeof(PgStat_MsgHdr))
 
 
 /* ----------
@@ -225,7 +227,7 @@ typedef struct PgStat_TableEntry
  * ----------
  */
 #define PGSTAT_NUM_TABENTRIES  \
-	((PGSTAT_MSG_PAYLOAD - sizeof(Oid) - 3 * sizeof(int))  \
+	((PGSTAT_MSG_PAYLOAD - sizeof(Oid) - 3 * sizeof(int) - 2 * sizeof(PgStat_Counter))	\
 	 / sizeof(PgStat_TableEntry))
 
 typedef struct PgStat_MsgTabstat