[gcc r17-3819] libcpp: Use -1 for closed header-unit fd [PR c++/127097]

[Jason Merrill]

https://gcc.gnu.org/g:e1c8dac7ca8dde4d99776685dee43e05cd3ea6d2

commit r17-3819-ge1c8dac7ca8dde4d99776685dee43e05cd3ea6d2
Author: Alexey Klimkin <alexey.klimkin@intel.com>
Date:   Tue Aug 25 20:36:48 2026 -0700

    libcpp: Use -1 for closed header-unit fd [PR c++/127097]
    
    _cpp_find_header_unit closes the source descriptor after resolving a
    header unit, but records zero instead of the established -1 sentinel.
    A later textual include through an alternate pathname therefore skips
    reopening the source and reads stdin.  Descriptor zero is also a valid
    result from open when stdin was previously closed.
    
    Close every valid descriptor and restore -1.  Add a module regression
    that imports a header unit and includes the same file through an
    alternate spelling.
    
    Assisted-by: Claude Opus 5 (Anthropic)
    
    libcpp/
            PR c++/127097
            * files.cc (_cpp_find_header_unit): Close descriptor zero and set
            the closed descriptor sentinel to -1.
    
    gcc/testsuite/
            PR c++/127097
            * g++.dg/modules/pragma-once-hu-1_a.H: New test.
            * g++.dg/modules/pragma-once-hu-1_b.C: New test.
    
    Signed-off-by: Alexey Klimkin <alexey.klimkin@intel.com>

Diff:
---
 gcc/testsuite/g++.dg/modules/pragma-once-hu-1_b.C | 12 ++++++++++++
 libcpp/files.cc                                   |  4 ++--
 gcc/testsuite/g++.dg/modules/pragma-once-hu-1_a.H | 12 ++++++++++++
 3 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/gcc/testsuite/g++.dg/modules/pragma-once-hu-1_b.C b/gcc/testsuite/g++.dg/modules/pragma-once-hu-1_b.C
new file mode 100644
index 000000000000..b2871d9030a0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/pragma-once-hu-1_b.C
@@ -0,0 +1,12 @@
+// PR c++/127097
+// { dg-additional-options "-fmodules" }
+
+import "pragma-once-hu-1_a.H";
+// Reuse the tracked sys directory for an alternate spelling without symlinks.
+#include "sys/../pragma-once-hu-1_a.H"
+
+int
+main ()
+{
+  return pragma_once_hu_f () != 1;
+}
diff --git a/libcpp/files.cc b/libcpp/files.cc
index 917b7799be97..b0ecb00de24c 100644
--- a/libcpp/files.cc
+++ b/libcpp/files.cc
@@ -1227,11 +1227,11 @@ _cpp_find_header_unit (cpp_reader *pfile, const char *name, bool angle,
 {
   if (_cpp_file *file = test_header_unit (pfile, name, angle, loc))
     {
-      if (file->fd > 0)
+      if (file->fd != -1)
 	{
 	  /* Don't leave it open.  */
 	  close (file->fd);
-	  file->fd = 0;
+	  file->fd = -1;
 	}
 
       file->header_unit = +1;
diff --git a/gcc/testsuite/g++.dg/modules/pragma-once-hu-1_a.H b/gcc/testsuite/g++.dg/modules/pragma-once-hu-1_a.H
new file mode 100644
index 000000000000..549f677a7753
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/pragma-once-hu-1_a.H
@@ -0,0 +1,12 @@
+// PR c++/127097
+// { dg-module-do run }
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi {} }
+
+#pragma once
+
+inline int
+pragma_once_hu_f ()
+{
+  return 1;
+}