From c7d7866b5c57c27d1988692d597edd53f102dba5 Mon Sep 17 00:00:00 2001
From: Graham Ollis <gollis@fastly.com>
Date: Tue, 29 Sep 2026 15:34:11 -0600
Subject: [PATCH] _diff: insulate line-reading from caller's $/

_diff reads the diff subprocess's output line-by-line via <$stdout>,
which implicitly relies on the caller not having changed $/. If a
caller left $/ set to undef (slurp mode) or another non-default value
- for example via an improperly scoped `local $/;` earlier in the same
process - reading an already-at-EOF pipe returns an empty string once
instead of undef immediately. That phantom empty "line" was treated as
a real line of diff output, producing a false failure for two JSON
documents that are actually the same.

Found while integrating this module into a large test suite where an
unrelated script had left $/ globally altered.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
---
 Changes               |  4 ++++
 lib/Test/JSON/Diff.pm |  7 +++++++
 t/test_json_diff.t    | 20 ++++++++++++++++++++
 3 files changed, 31 insertions(+)

diff --git a/Changes b/Changes
index 7d5872c..89983d7 100644
--- a/Changes
+++ b/Changes
@@ -1,6 +1,10 @@
 Revision history for {{$dist->name}}
 
 {{$NEXT}}
+  - fix _diff to no longer be affected by a caller's ambient $/ (input
+    record separator); a caller that left $/ set to undef (or any other
+    non-default value) could previously cause a false failure report for
+    two JSON documents that are actually the same
 
 0.01      2026-09-29 13:05:04 -0600
   - initial version
diff --git a/lib/Test/JSON/Diff.pm b/lib/Test/JSON/Diff.pm
index 07c1fb8..b637fe4 100644
--- a/lib/Test/JSON/Diff.pm
+++ b/lib/Test/JSON/Diff.pm
@@ -201,6 +201,13 @@ sub _run_to_files ($cmd, $in_path, $out_path, $err_path) {
 # returns an empty list if the files are the same, otherwise up to
 # $max_lines lines of unified diff, followed by '...' if clipped.
 sub _diff ($diff, $context, $max_lines, $expected, $actual, $err_path) {
+    # reading $diff's output is line based below, so make sure that's true
+    # regardless of what the caller has done to $/ -- in particular, if $/
+    # is set to undef (slurp mode), reading an already-at-EOF pipe returns
+    # an empty string once instead of undef immediately, which is read as a
+    # single (phantom) line of diff output, producing a false failure.
+    local $/ = "\n";
+
     my $err = $err_path->openw_raw;
     my $pid = open3(my $stdin, my $stdout, '>&' . fileno($err),
         $diff, "-U$context", '--label', 'expected', '--label', 'actual', $expected, $actual);
diff --git a/t/test_json_diff.t b/t/test_json_diff.t
index 534ef15..2b1949e 100644
--- a/t/test_json_diff.t
+++ b/t/test_json_diff.t
@@ -163,6 +163,26 @@ subtest 'usage errors' => sub {
         qr/max_lines must be a positive integer/, 'bad max_lines';
 };
 
+subtest 'insulated from caller $/' => sub {
+    # a caller that has left $/ in slurp mode (or any non-default value)
+    # shouldn't affect _diff's own line-based reading of the diff subprocess's
+    # output -- in particular, with $/ undef, reading an already-at-EOF pipe
+    # returns an empty string once instead of undef immediately, which used
+    # to be misread as a single (phantom) line of diff output, producing a
+    # false failure for two documents that are actually the same.
+    foreach my $sep ( undef, '', "\x00" ) {
+        local $/ = $sep;
+        my $sep_name = defined $sep ? ( length $sep ? "chr(" . ord($sep) . ")" : "''" ) : 'undef';
+
+        my ($ret) = run_check( '{"a":1,"b":2}', '{"b":2,"a":1}', "same despite \$/ = $sep_name" );
+        is $ret, T(), "still detects equal JSON when caller left \$/ = $sep_name";
+
+        my ( undef, undef, $diag ) = run_check( '[1,2]', '[2,1]', "different despite \$/ = $sep_name" );
+        like $diag, qr/^--- expected\n\+\+\+ actual\n\@\@/,
+          "still produces a real diagnostic for an actual difference when \$/ = $sep_name";
+    }
+};
+
 subtest 'missing tools' => sub {
     my $jq = File::Which::which('jq');