Fix pattern matching logic for logs in TAP tests of pgbench

The logic checking for the format of per-thread logs used grep() with
directly "$re", which would cause the test to consider all the logs as
a match without caring about their format at all.  Using "/$re/" makes
grep() perform a regex test, which is what we want here.

While on it, improve some of the tests to be more picky with the
patterns expected and add more comments to describe the tests.

Issue discovered while digging into a separate patch.

Author: Fabien Coelho, Michael Paquier
Discussion: https://postgr.es/m/YNPsPAUoVDCpPOGk@paquier.xyz
Backpatch-through: 11

Author: michaelpq

src/bin/pgbench/t/001_pgbench_with_server.pl

@@ -855,7 +855,12 @@ sub list_files
 	return map { $dir . '/' . $_ } @files;
 }
 
-# check log contents and cleanup
+# Check log contents and clean them up:
+#   $dir: directory holding logs
+#   $prefix: file prefix for per-thread logs
+#   $nb: number of expected files
+#   $min/$max: minimum and maximum number of lines in log files
+#   $re: regular expression each log line has to match
 sub check_pgbench_logs
 {
 	local $Test::Builder::Level = $Test::Builder::Level + 1;
@@ -876,7 +881,7 @@ sub check_pgbench_logs
 			my $clen     = @contents;
 			ok( $min <= $clen && $clen <= $max,
 				"transaction count for $log ($clen)");
-			ok( grep($re, @contents) == $clen,
+			ok( grep(/$re/, @contents) == $clen,
 				"transaction format for $prefix");
 			close $fh or die "$@";
 		};
@@ -887,25 +892,25 @@ sub check_pgbench_logs
 
 my $bdir = $node->basedir;
 
-# with sampling rate
+# Run with sampling rate, 2 clients with 50 transactions each.
 pgbench(
 	"-n -S -t 50 -c 2 --log --sampling-rate=0.5", 0,
 	[ qr{select only}, qr{processed: 100/100} ], [qr{^$}],
 	'pgbench logs', undef,
 	"--log-prefix=$bdir/001_pgbench_log_2");
-
+# The IDs of the clients (1st field) in the logs should be either 0 or 1.
 check_pgbench_logs($bdir, '001_pgbench_log_2', 1, 8, 92,
-	qr{^0 \d{1,2} \d+ \d \d+ \d+$});
+	qr{^[01] \d{1,2} \d+ \d \d+ \d+$});
 
-# check log file in some detail
+# Run with different read-only option pattern, 1 client with 10 transactions.
 pgbench(
-	"-n -b se -t 10 -l", 0,
+	"-n -b select-only -t 10 -l", 0,
 	[ qr{select only}, qr{processed: 10/10} ], [qr{^$}],
 	'pgbench logs contents', undef,
 	"--log-prefix=$bdir/001_pgbench_log_3");
-
+# The ID of a single client (1st field) should match 0.
 check_pgbench_logs($bdir, '001_pgbench_log_3', 1, 10, 10,
-	qr{^\d \d{1,2} \d+ \d \d+ \d+$});
+	qr{^0 \d{1,2} \d+ \d \d+ \d+$});
 
 # done
 $node->stop;