From e644bb95ce68ee171e65e6d49c0b609d0ccfbf12 Mon Sep 17 00:00:00 2001
From: Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: Thu, 4 Dec 2025 14:45:45 +0300
Subject: [PATCH v6 1/2] Add read_head_tail() helper to Utils.pm

This function reads lines from a file, from both head and tail
directions, with the size reported tuned depending on a new environment
variable named 'PG_TEST_FILE_READ_LINES', that can be used to override
the default of 50 lines.

Suggested-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CAN55FZ1D6KXvjSs7YGsDeadqCxNF3UUhjRAfforzzP0k-cE%3DbA%40mail.gmail.com
---
 src/test/perl/PostgreSQL/Test/Utils.pm | 50 ++++++++++++++++++++++++++
 doc/src/sgml/regress.sgml              |  8 +++++
 2 files changed, 58 insertions(+)

diff --git a/src/test/perl/PostgreSQL/Test/Utils.pm b/src/test/perl/PostgreSQL/Test/Utils.pm
index 85d36a3171e2..0332d28916eb 100644
--- a/src/test/perl/PostgreSQL/Test/Utils.pm
+++ b/src/test/perl/PostgreSQL/Test/Utils.pm
@@ -68,6 +68,7 @@ our @EXPORT = qw(
   slurp_file
   append_to_file
   string_replace_file
+  read_head_tail
   check_mode_recursive
   chmod_recursive
   check_pg_config
@@ -590,6 +591,55 @@ sub string_replace_file
 
 =pod
 
+=item read_head_tail(filename)
+
+Return lines from the head and the tail of a file.  If the file is smaller
+than the number of lines requested, all its contents are returned in @head,
+leaving @tail empty.
+
+If the PG_TEST_FILE_READ_LINES environment variable is set, use it instead
+of the default of 50 lines.
+
+=cut
+
+sub read_head_tail
+{
+	my $filename = shift;
+	my (@head, @tail);
+	my $line_count = 50;
+
+	# Use PG_TEST_FILE_READ_LINES if set.
+	if (defined $ENV{PG_TEST_FILE_READ_LINES})
+	{
+		$line_count = $ENV{PG_TEST_FILE_READ_LINES};
+	}
+
+	return ([], []) if $line_count <= 0;
+
+	open my $fh, '<', $filename or die "couldn't open file: $filename\n";
+	my @lines = <$fh>;
+	close $fh;
+
+	chomp @lines;
+
+	my $total = scalar @lines;
+
+	# If the file is small enough, return all lines in @head.
+	if (2 * $line_count >= $total)
+	{
+		@head = @lines;
+		@tail = ();
+		return (\@head, \@tail);
+	}
+
+	@head = @lines[ 0 .. $line_count - 1 ];
+	@tail = @lines[ $total - $line_count .. $total - 1 ];
+
+	return (\@head, \@tail);
+}
+
+=pod
+
 =item check_mode_recursive(dir, expected_dir_mode, expected_file_mode, ignore_list)
 
 Check that all file/dir modes in a directory match the expected values,
diff --git a/doc/src/sgml/regress.sgml b/doc/src/sgml/regress.sgml
index fd1e142d5591..d80dd46c5fdb 100644
--- a/doc/src/sgml/regress.sgml
+++ b/doc/src/sgml/regress.sgml
@@ -917,6 +917,14 @@ PG_TEST_NOCLEAN=1 make -C src/bin/pg_dump check
     <varname>PG_TEST_TIMEOUT_DEFAULT</varname> to a higher number will change
     the default to avoid this.
    </para>
+
+   <para>
+    For certain tests, the environment variable
+    <envar>PG_TEST_FILE_READ_LINES</envar> can be set to limit the number of
+    lines read from large output files (head and tail). This is useful when
+    the test output contains a lot of unnecessary content, allowing the test
+    framework to read only a limited number of lines for its reports.
+   </para>
   </sect2>
 
   </sect1>
-- 
2.51.0