Add assert to ensure that page locks don't participate in deadlock cycle.

Assert that we don't acquire any other heavyweight lock while holding the
page lock except for relation extension.  However, these locks are never
taken in reverse order which implies that page locks will never
participate in the deadlock cycle.

Similar to relation extension, page locks are also held for a short
duration, so imposing such a restriction won't hurt.

Author: Dilip Kumar, with few changes by Amit Kapila
Reviewed-by: Amit Kapila, Kuntal Ghosh and Sawada Masahiko
Discussion: https://postgr.es/m/CAD21AoCmT3cFQUN4aVvzy5chw7DuzXrJCbrjTU05B+Ss=Gn1LA@mail.gmail.com

src/backend/storage/lmgr/lock.c

@@ -185,6 +185,18 @@ static int	FastPathLocalUseCount = 0;
  */
 static bool IsRelationExtensionLockHeld PG_USED_FOR_ASSERTS_ONLY = false;
 
+/*
+ * Flag to indicate if the page lock is held by this backend.  We don't
+ * acquire any other heavyweight lock while holding the page lock except for
+ * relation extension.  However, these locks are never taken in reverse order
+ * which implies that page locks will also never participate in the deadlock
+ * cycle.
+ *
+ * Similar to relation extension, page locks are also held for a short
+ * duration, so imposing such a restriction won't hurt.
+ */
+static bool IsPageLockHeld PG_USED_FOR_ASSERTS_ONLY = false;
+
 /* Macros for manipulating proc->fpLockBits */
 #define FAST_PATH_BITS_PER_SLOT			3
 #define FAST_PATH_LOCKNUMBER_OFFSET		1
@@ -862,6 +874,13 @@ LockAcquireExtended(const LOCKTAG *locktag,
 	 */
 	Assert(!IsRelationExtensionLockHeld);
 
+	/*
+	 * We don't acquire any other heavyweight lock while holding the page lock
+	 * except for relation extension.
+	 */
+	Assert(!IsPageLockHeld ||
+		   (locktag->locktag_type == LOCKTAG_RELATION_EXTEND));
+
 	/*
 	 * Prepare to emit a WAL record if acquisition of this lock needs to be
 	 * replayed in a standby server.
@@ -1310,10 +1329,10 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 }
 
 /*
- * Check and set/reset the flag that we hold the relation extension lock.
+ * Check and set/reset the flag that we hold the relation extension/page lock.
  *
  * It is callers responsibility that this function is called after
- * acquiring/releasing the relation extension lock.
+ * acquiring/releasing the relation extension/page lock.
  *
  * Pass acquired as true if lock is acquired, false otherwise.
  */
@@ -1323,6 +1342,9 @@ CheckAndSetLockHeld(LOCALLOCK *locallock, bool acquired)
 #ifdef USE_ASSERT_CHECKING
 	if (LOCALLOCK_LOCKTAG(*locallock) == LOCKTAG_RELATION_EXTEND)
 		IsRelationExtensionLockHeld = acquired;
+	else if (LOCALLOCK_LOCKTAG(*locallock) == LOCKTAG_PAGE)
+		IsPageLockHeld = acquired;
+
 #endif
 }