Add pg_buffercache_evict() function for testing.

When testing buffer pool logic, it is useful to be able to evict
arbitrary blocks.  This function can be used in SQL queries over the
pg_buffercache view to set up a wide range of buffer pool states.  Of
course, buffer mappings might change concurrently so you might evict a
block other than the one you had in mind, and another session might
bring it back in at any time.  That's OK for the intended purpose of
setting up developer testing scenarios, and more complicated interlocking
schemes to give stronger guararantees about that would likely be less
flexible for actual testing work anyway.  Superuser-only.

Author: Palak Chaturvedi <chaturvedipalak1911@gmail.com>
Author: Thomas Munro <thomas.munro@gmail.com> (docs, small tweaks)
Reviewed-by: Nitin Jadhav <nitinjadhavpostgres@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Cary Huang <cary.huang@highgo.ca>
Reviewed-by: Cédric Villemain <cedric.villemain+pgsql@abcsql.com>
Reviewed-by: Jim Nasby <jim.nasby@gmail.com>
Reviewed-by: Maxim Orlov <orlovmg@gmail.com>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/CALfch19pW48ZwWzUoRSpsaV9hqt0UPyaBPC4bOZ4W+c7FF566A@mail.gmail.com

5 files changed, 29 insertions, 2 deletions

diff --git a/contrib/pg_buffercache/Makefile b/contrib/pg_buffercache/Makefile
index d6b58d4da94..eae65ead9e5 100644
--- a/contrib/pg_buffercache/Makefile
+++ b/contrib/pg_buffercache/Makefile
@@ -8,7 +8,7 @@ OBJS = \
 EXTENSION = pg_buffercache
 DATA = pg_buffercache--1.2.sql pg_buffercache--1.2--1.3.sql \
 	pg_buffercache--1.1--1.2.sql pg_buffercache--1.0--1.1.sql \
-	pg_buffercache--1.3--1.4.sql
+	pg_buffercache--1.3--1.4.sql pg_buffercache--1.4--1.5.sql
 PGFILEDESC = "pg_buffercache - monitoring of shared buffer cache in real-time"
 
 REGRESS = pg_buffercache
diff --git a/contrib/pg_buffercache/meson.build b/contrib/pg_buffercache/meson.build
index c86e33cc958..1ca3452918b 100644
--- a/contrib/pg_buffercache/meson.build
+++ b/contrib/pg_buffercache/meson.build
@@ -22,6 +22,7 @@ install_data(
   'pg_buffercache--1.2--1.3.sql',
   'pg_buffercache--1.2.sql',
   'pg_buffercache--1.3--1.4.sql',
+  'pg_buffercache--1.4--1.5.sql',
   'pg_buffercache.control',
   kwargs: contrib_data_args,
 )
diff --git a/contrib/pg_buffercache/pg_buffercache--1.4--1.5.sql b/contrib/pg_buffercache/pg_buffercache--1.4--1.5.sql
new file mode 100644
index 00000000000..0fb18ff786d
--- /dev/null
+++ b/contrib/pg_buffercache/pg_buffercache--1.4--1.5.sql
@@ -0,0 +1,6 @@
+\echo Use "ALTER EXTENSION pg_buffercache UPDATE TO '1.5'" to load this file. \quit
+
+CREATE FUNCTION pg_buffercache_evict(IN int)
+RETURNS bool
+AS 'MODULE_PATHNAME', 'pg_buffercache_evict'
+LANGUAGE C PARALLEL SAFE VOLATILE STRICT;
diff --git a/contrib/pg_buffercache/pg_buffercache.control b/contrib/pg_buffercache/pg_buffercache.control
index a82ae5f9bb5..5ee875f77dd 100644
--- a/contrib/pg_buffercache/pg_buffercache.control
+++ b/contrib/pg_buffercache/pg_buffercache.control
@@ -1,5 +1,5 @@
 # pg_buffercache extension
 comment = 'examine the shared buffer cache'
-default_version = '1.4'
+default_version = '1.5'
 module_pathname = '$libdir/pg_buffercache'
 relocatable = true
diff --git a/contrib/pg_buffercache/pg_buffercache_pages.c b/contrib/pg_buffercache/pg_buffercache_pages.c
index 33167323653..3ae0a018e10 100644
--- a/contrib/pg_buffercache/pg_buffercache_pages.c
+++ b/contrib/pg_buffercache/pg_buffercache_pages.c
@@ -63,6 +63,7 @@ typedef struct
 PG_FUNCTION_INFO_V1(pg_buffercache_pages);
 PG_FUNCTION_INFO_V1(pg_buffercache_summary);
 PG_FUNCTION_INFO_V1(pg_buffercache_usage_counts);
+PG_FUNCTION_INFO_V1(pg_buffercache_evict);
 
 Datum
 pg_buffercache_pages(PG_FUNCTION_ARGS)
@@ -347,3 +348,22 @@ pg_buffercache_usage_counts(PG_FUNCTION_ARGS)
 
 	return (Datum) 0;
 }
+
+/*
+ * Try to evict a shared buffer.
+ */
+Datum
+pg_buffercache_evict(PG_FUNCTION_ARGS)
+{
+	Buffer		buf = PG_GETARG_INT32(0);
+
+	if (!superuser())
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 errmsg("must be superuser to use pg_buffercache_evict function")));
+
+	if (buf < 1 || buf > NBuffers)
+		elog(ERROR, "bad buffer ID: %d", buf);
+
+	PG_RETURN_BOOL(EvictUnpinnedBuffer(buf));
+}