Author:     Noah Misch <noah@leadboat.com>
Commit:     Noah Misch <noah@leadboat.com>

    [not for commit] demo pin starvation from autovacuum inplace update
    
    Test with intra-grant-inplace-db.spec.  Changes here:
    
    a. Start intra-grant-inplace-db.spec step vac2 just after some
       autovacuum worker is blocked on the xid of the uncommitted GRANT.
       While blocked, the worker holds a pin on the one pg_database page.
       vac2 needs LockBufferForCleanup(that-page).
    b. Make LockBufferForCleanup() sleep 10ms after WAIT_EVENT_BUFFER_PIN,
       to give the next autovacuum worker time to win the pinning race.

diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 0f02bf6..5268294 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -5344,8 +5344,17 @@ LockBufferForCleanup(Buffer buffer)
 			SetStartupBufferPinWaitBufId(-1);
 		}
 		else
+		{
 			ProcWaitForSignal(WAIT_EVENT_BUFFER_PIN);
 
+			/*
+			 * Sleep to induce starvation.  With heavyweight locks, the
+			 * releaser grants to the next queue member, preventing
+			 * starvation.  Buffer pins have no such mechanism.
+			 */
+			pg_usleep(10000);
+		}
+
 		/*
 		 * Remove flag marking us as waiter. Normally this will not be set
 		 * anymore, but ProcWaitForSignal() can return for other signals as
diff --git a/src/test/isolation/Makefile b/src/test/isolation/Makefile
index ade2256..ca487de 100644
--- a/src/test/isolation/Makefile
+++ b/src/test/isolation/Makefile
@@ -64,6 +64,9 @@ installcheck: all
 check: all
 	$(pg_isolation_regress_check) --schedule=$(srcdir)/isolation_schedule
 
+nmcheck: all temp-install
+	$(pg_isolation_regress_check) intra-grant-inplace-db
+
 # Non-default tests.  It only makes sense to run these if set up to use
 # prepared transactions, via TEMP_CONFIG for the check case, or via the
 # postgresql.conf for the installcheck case.
diff --git a/src/test/isolation/expected/intra-grant-inplace-db.out b/src/test/isolation/expected/intra-grant-inplace-db.out
index a91402c..c019952 100644
--- a/src/test/isolation/expected/intra-grant-inplace-db.out
+++ b/src/test/isolation/expected/intra-grant-inplace-db.out
@@ -1,6 +1,6 @@
 Parsed test spec with 3 sessions
 
-starting permutation: snap3 b1 grant1 vac2 snap3 c1 cmp3
+starting permutation: snap3 b1 grant1 wait vac2 snap3 c1 cmp3
 step snap3: 
 	INSERT INTO frozen_witness
 	SELECT datfrozenxid FROM pg_database WHERE datname = current_catalog;
@@ -9,6 +9,14 @@ step b1: BEGIN;
 step grant1: 
 	GRANT TEMP ON DATABASE isolation_regression TO regress_temp_grantee;
 
+step wait: 
+	DO $$
+	BEGIN
+		WHILE NOT EXISTS(SELECT 1 FROM pg_locks WHERE NOT granted AND transactionid IS NOT NULL) LOOP
+			NULL;
+		END LOOP;
+	END$$;
+
 step vac2: VACUUM (FREEZE); <waiting ...>
 step snap3: 
 	INSERT INTO frozen_witness
diff --git a/src/test/isolation/specs/intra-grant-inplace-db.spec b/src/test/isolation/specs/intra-grant-inplace-db.spec
index 9de40ec..7a9cae4 100644
--- a/src/test/isolation/specs/intra-grant-inplace-db.spec
+++ b/src/test/isolation/specs/intra-grant-inplace-db.spec
@@ -40,6 +40,19 @@ step cmp3	{
 	WHERE datname = current_catalog
 		AND age(datfrozenxid) > (SELECT min(age(x)) FROM frozen_witness);
 }
+# Busy-wait until some autovacuum workers's vac_update_datfrozenxid() blocks
+# in heap_inplace_wait(), waiting for the xid of s1 to end.  A vac2 at that
+# point will block trying to acquire a cleanup lock on the page autovacuum has
+# pinned.  That's because systable_inplace_update_begin() acquires a pin that
+# we hold until systable_inplace_update_finish().
+step wait	{
+	DO $$
+	BEGIN
+		WHILE NOT EXISTS(SELECT 1 FROM pg_locks WHERE NOT granted AND transactionid IS NOT NULL) LOOP
+			NULL;
+		END LOOP;
+	END$$;
+}
 
 
-permutation snap3 b1 grant1 vac2(c1) snap3 c1 cmp3
+permutation snap3 b1 grant1 wait vac2(c1) snap3 c1 cmp3