0001-Add-adversarial-ConditionalLockBufferForCleanup-gizm.txt

From 3f01281af3ba81b35777cb7d717f76e001fd3e10 Mon Sep 17 00:00:00 2001
From: Peter Geoghegan <pg@bowt.ie>
Date: Sat, 19 Feb 2022 14:07:35 -0800
Subject: [PATCH] Add adversarial ConditionalLockBufferForCleanup() gizmo to
 vacuumlazy.c.

---
 src/backend/access/heap/vacuumlazy.c | 36 +++++++++++++++++++++++++++-
 1 file changed, 35 insertions(+), 1 deletion(-)

diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index 242511a23..31c6b360e 100644
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -50,6 +50,7 @@
 #include "commands/dbcommands.h"
 #include "commands/progress.h"
 #include "commands/vacuum.h"
+#include "common/pg_prng.h"
 #include "executor/instrument.h"
 #include "miscadmin.h"
 #include "optimizer/paths.h"
@@ -748,6 +749,39 @@ heap_vacuum_rel(Relation rel, VacuumParams *params,
 	}
 }
 
+/*
+ * Adversarial gizmo, simulates excessive failure to get cleanup locks
+ */
+static inline bool
+lazy_conditionallockbufferforcleanup(Buffer buffer)
+{
+	/*
+	 * Artificially fail to get a cleanup lock 50% of the time.
+	 *
+	 * XXX: What about temp tables?  We simulate not getting a cleanup lock
+	 * there, but is that choice actually reasonable?
+	 */
+	if (pg_prng_uint32(&pg_global_prng_state) <= (PG_UINT32_MAX / 2))
+		return false;
+
+#if 0
+	/*
+	 * 50% is very very aggressive, while 2% - 5% is still basically
+	 * adversarial but in many ways less annoying.
+	 *
+	 * This version (which injects a failure to get a cleanup lock 2% of the
+	 * time) seems to pass the regression tests, even with my parallel make
+	 * check-world recipe.  Expected query plans don't seem to shift on
+	 * account of unexpected index bloat (nor are there any problems of a
+	 * similar nature) with this variant of the gizmo.
+	 */
+	if (pg_prng_uint32(&pg_global_prng_state) <= (PG_UINT32_MAX / 50))
+		return false;
+#endif
+
+	return ConditionalLockBufferForCleanup(buffer);
+}
+
 /*
  *	lazy_scan_heap() -- workhorse function for VACUUM
  *
@@ -1093,7 +1127,7 @@ lazy_scan_heap(LVRelState *vacrel, int nworkers)
 		 * a cleanup lock right away, we may be able to settle for reduced
 		 * processing using lazy_scan_noprune.
 		 */
-		if (!ConditionalLockBufferForCleanup(buf))
+		if (!lazy_conditionallockbufferforcleanup(buf))
 		{
 			bool		hastup,
 						recordfreespace;
-- 
2.30.2