bug#75207: Fwd: bug#75207: 29.4; Path conversion from native codepage to UTF-8 fails when Windows is set by default to UTF-8

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 75207@debbugs.gnu.org
> Date: Fri, 03 Jan 2025 17:25:31 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > I debugged a bit and it looks like w32_ansi_code_page is set to 1252 at some point.
> 
> AFAICT, that happens when we load the pdumper file.
> 
> > M-: w32-multibyte-code-page -> 0
> > M-: locale-coding-system -> cp65001
> > M-: file-name-coding-system -> nil
> > M-: default-file-name-coding-system -> cp65001
> 
> OK, I think this confirms my hypothesis.  I'll try to come up with a
> patch, probably tomorrow.

The patch is below, and it is for the master branch of the Emacs Git
repository.

> > > If I send you a C-level patch, are you able to build Emacs after patching it,
> > > preferably the master branch of our Git repository?
> > 
> > Sure.
> 
> OK, but you'll need to build Emacs with a different system codepage to
> see the effects of the fix.

This still stands: to fully test the patch, please change your system
codepage after building Emacs and then start Emacs and see if
everything works as expected.

diff --git a/src/emacs.c b/src/emacs.c
index c1e0c9f..896f219 100644
--- a/src/emacs.c
+++ b/src/emacs.c
@@ -1419,7 +1419,18 @@ android_emacs_init (int argc, char **argv, char *dump_file)
 
 #ifdef HAVE_PDUMPER
   if (attempt_load_pdump)
-    initial_emacs_executable = load_pdump (argc, argv, dump_file);
+    {
+      initial_emacs_executable = load_pdump (argc, argv, dump_file);
+#ifdef WINDOWSNT
+  /* Reinitialize the codepage for file names, needed to decode
+     non-ASCII file names during startup.  This is needed because
+     loading the pdumper file above assigns to those variables values
+     from the dump stage, which might be incorrect, if dumping was done
+     on a different system.  */
+      if (dumped_with_pdumper_p ())
+	w32_init_file_name_codepage ();
+#endif
+    }
 #else
   ptrdiff_t bufsize;
   initial_emacs_executable = find_emacs_executable (argv[0], &bufsize);
diff --git a/src/w32.c b/src/w32.c
index a493991..deeca03 100644
--- a/src/w32.c
+++ b/src/w32.c
@@ -1685,6 +1685,19 @@ w32_init_file_name_codepage (void)
 {
   file_name_codepage = CP_ACP;
   w32_ansi_code_page = CP_ACP;
+#ifdef HAVE_PDUMPER
+  /* If we were dumped with pdumper, this function will be called after
+     loading the pdumper file, and needs to reset the following
+     variables that come from the dump stage, which could be on a
+     different system with different default codepages.  Then, the
+     correct value of w32-ansi-code-page will be assigned by
+     globals_of_w32fns, which is called from 'main'.  Until that call
+     happens, w32-ansi-code-page will have the value of CP_ACP, which
+     stands for the default ANSI codepage.  The other variables will be
+     computed by codepage_for_filenames below.  */
+  Vdefault_file_name_coding_system = Qnil;
+  Vfile_name_coding_system = Qnil;
+#endif
 }
 
 /* Produce a Windows ANSI codepage suitable for encoding file names.