fix: preserve errno through XS scope cleanup

FileCheck.xs

@@ -14,6 +14,8 @@
 #include <XSUB.h>
 #include <embed.h>
 
+#include <errno.h>
+
 #include "FileCheck.h"
 
 /*
@@ -98,9 +100,15 @@ int _overload_ft_ops() {
 
   /* printf ("######## The result is %d /// OPTYPE is %d\n", check_status, optype); */
 
-  PUTBACK;
-  FREETMPS;
-  LEAVE;
+  /* Save errno before scope cleanup — FREETMPS/LEAVE can trigger
+   * DESTROY or other Perl code that clobbers errno set by _check(). */
+  {
+    int saved_errno = errno;
+    PUTBACK;
+    FREETMPS;
+    LEAVE;
+    errno = saved_errno;
+  }
 
   return check_status;
 }
@@ -135,9 +143,13 @@ SV* _overload_ft_ops_sv() {
 
   /* printf ("######## The result is %d /// OPTYPE is %d\n", check_status, optype); */
 
-  PUTBACK;
-  FREETMPS;
-  LEAVE;
+  {
+    int saved_errno = errno;
+    PUTBACK;
+    FREETMPS;
+    LEAVE;
+    errno = saved_errno;
+  }
 
   return status;
 }
@@ -241,9 +253,13 @@ int _overload_ft_stat(Stat_t *stat, int *size) {
 
   }
 
-  PUTBACK;
-  FREETMPS;
-  LEAVE;
+  {
+    int saved_errno = errno;
+    PUTBACK;
+    FREETMPS;
+    LEAVE;
+    errno = saved_errno;
+  }
 
   return check_status;
 }

t/stat-errno-preservation.t

@@ -0,0 +1,123 @@
+#!/usr/bin/perl -w
+
+# Test that errno ($!) is preserved through XS scope cleanup
+# after _check() sets it for failed stat/lstat/file-check operations.
+#
+# The XS functions _overload_ft_ops() and _overload_ft_stat() call
+# FREETMPS/LEAVE after the Perl _check() function returns. Without
+# saving/restoring errno, this cleanup can clobber $! values set by
+# the mock callback or by _check()'s default errno logic.
+
+use strict;
+use warnings;
+
+use Test2::Bundle::Extended;
+use Test2::Tools::Explain;
+use Test2::Plugin::NoWarnings;
+
+use Overload::FileCheck q{:all};
+use Errno ();
+
+my $missing = '/stat/errno/missing';
+my $exists  = '/stat/errno/exists';
+
+# --- Test with mock_all_from_stat (the -from-stat path) ---
+
+subtest 'errno preserved through stat via mock_all_from_stat' => sub {
+    mock_all_from_stat(
+        sub {
+            my ( $type, $file ) = @_;
+            if ( $file eq $exists ) {
+                return stat_as_file();
+            }
+            if ( $file eq $missing ) {
+                $! = Errno::ENOENT();
+                return 0;    # falsy = file not found
+            }
+            return FALLBACK_TO_REAL_OP;
+        }
+    );
+
+    subtest '-e on missing file sets ENOENT' => sub {
+        $! = 0;
+        my $check = -e $missing;
+        ok( !$check, '-e returns false for missing mock' );
+        is( int($!), Errno::ENOENT(), '$! is ENOENT after -e on missing file' );
+    };
+
+    subtest '-f on missing file sets ENOENT' => sub {
+        $! = 0;
+        my $check = -f $missing;
+        ok( !$check, '-f returns false for missing mock' );
+        is( int($!), Errno::ENOENT(), '$! is ENOENT after -f on missing file' );
+    };
+
+    subtest 'stat on missing file preserves errno' => sub {
+        $! = 0;
+        my @st = stat($missing);
+        is( scalar @st, 0, 'stat returns empty list for missing mock' );
+        is( int($!), Errno::ENOENT(), '$! is ENOENT after stat on missing file' );
+    };
+
+    subtest 'lstat on missing file preserves errno' => sub {
+        $! = 0;
+        my @st = lstat($missing);
+        is( scalar @st, 0, 'lstat returns empty list for missing mock' );
+        is( int($!), Errno::ENOENT(), '$! is ENOENT after lstat on missing file' );
+    };
+
+    subtest '-e on existing file does not set errno' => sub {
+        $! = 0;
+        my $check = -e $exists;
+        ok( $check, '-e returns true for existing mock' );
+        is( int($!), 0, '$! is not set after successful -e' );
+    };
+
+    unmock_all_file_checks();
+};
+
+# --- Test with custom errno values ---
+
+subtest 'custom errno preserved through file check' => sub {
+    mock_file_check(
+        '-e' => sub {
+            my $f = shift;
+            if ( $f eq $missing ) {
+                $! = Errno::EACCES();
+                return CHECK_IS_FALSE;
+            }
+            return FALLBACK_TO_REAL_OP;
+        }
+    );
+
+    $! = 0;
+    my $check = -e $missing;
+    ok( !$check, '-e returns false' );
+    is( int($!), Errno::EACCES(), '$! preserves custom EACCES through XS cleanup' );
+
+    unmock_all_file_checks();
+};
+
+# --- Test default errno when callback doesn't set one ---
+
+subtest 'default ENOENT set when callback returns false without setting errno' => sub {
+    mock_all_from_stat(
+        sub {
+            my ( $type, $file ) = @_;
+            if ( $file eq $missing ) {
+                # Don't set $!, let _check() set the default
+                return 0;
+            }
+            return FALLBACK_TO_REAL_OP;
+        }
+    );
+
+    $! = 0;
+    my $check = -e $missing;
+    ok( !$check, '-e returns false' );
+    is( int($!), Errno::ENOENT(), '$! gets default ENOENT when callback omits errno' );
+
+    unmock_all_file_checks();
+};
+
+done_testing;