injection_points: Enable entry count in its variable-sized stats

This serves as coverage for the tracking of entry count added by
7bd2975fa9 as built-in variable-sized stats kinds have no need for it,
at least not yet.

A new function, called injection_points_stats_count(), is added to the
module.  It is able to return the number of entries.  This has been
useful when doing some benchmarking to check the sanity of the counts.

Author: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/aMPKWR81KT5UXvEr@paquier.xyz

src/test/modules/injection_points/injection_points--1.0.sql

@@ -99,6 +99,16 @@ RETURNS bigint
 AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
 LANGUAGE C STRICT;
 
+--
+-- injection_points_stats_count()
+--
+-- Return the number of entries stored in the pgstats hash table.
+--
+CREATE FUNCTION injection_points_stats_count()
+RETURNS bigint
+AS 'MODULE_PATHNAME', 'injection_points_stats_count'
+LANGUAGE C STRICT;
+
 --
 -- injection_points_stats_drop()
 --

src/test/modules/injection_points/injection_stats.c

@@ -40,6 +40,7 @@ static const PgStat_KindInfo injection_stats = {
 	.name = "injection_points",
 	.fixed_amount = false,		/* Bounded by the number of points */
 	.write_to_file = true,
+	.track_entry_count = true,
 
 	/* Injection points are system-wide */
 	.accessed_across_databases = true,
@@ -196,6 +197,17 @@ injection_points_stats_numcalls(PG_FUNCTION_ARGS)
 	PG_RETURN_INT64(entry->numcalls);
 }
 
+/*
+ * SQL function returning the number of entries allocated for injection
+ * points in the shared hashtable of pgstats.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_count);
+Datum
+injection_points_stats_count(PG_FUNCTION_ARGS)
+{
+	PG_RETURN_INT64(pgstat_get_entry_count(PGSTAT_KIND_INJECTION));
+}
+
 /* Only used by injection_points_stats_drop() */
 static bool
 match_inj_entries(PgStatShared_HashEntry *entry, Datum match_data)

src/test/modules/injection_points/t/001_stats.pl

@@ -36,6 +36,9 @@ $node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
 my $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats calls');
+my $entrycount =
+  $node->safe_psql('postgres', "SELECT injection_points_stats_count();");
+is($entrycount, '1', 'number of entries');
 my $fixedstats = $node->safe_psql('postgres',
 	"SELECT * FROM injection_points_stats_fixed();");
 is($fixedstats, '1|0|2|0|0', 'fixed stats after some calls');
@@ -55,6 +58,9 @@ $node->restart;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '3', 'number of stats after clean restart');
+$entrycount =
+  $node->safe_psql('postgres', "SELECT injection_points_stats_count();");
+is($entrycount, '1', 'number of entries after clean restart');
 $fixedstats = $node->safe_psql('postgres',
 	"SELECT * FROM injection_points_stats_fixed();");
 is($fixedstats, '1|0|2|1|1', 'fixed stats after clean restart');
@@ -65,6 +71,9 @@ $node->start;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '', 'number of stats after crash');
+$entrycount =
+  $node->safe_psql('postgres', "SELECT injection_points_stats_count();");
+is($entrycount, '0', 'number of entries after crash');
 $fixedstats = $node->safe_psql('postgres',
 	"SELECT * FROM injection_points_stats_fixed();");
 is($fixedstats, '0|0|0|0|0', 'fixed stats after crash');
@@ -81,6 +90,9 @@ $node->safe_psql('postgres', "SELECT injection_points_stats_drop();");
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '', 'no stats after drop via SQL function');
+$entrycount =
+  $node->safe_psql('postgres', "SELECT injection_points_stats_count();");
+is($entrycount, '0', 'number of entries after drop via SQL function');
 
 # Stop the server, disable the module, then restart.  The server
 # should be able to come up.