Do not skip path delimiters after multibyte characters in dln_find on Windows

The dln_find.c object built on its own receives UTF-8 command names from process.c, but scanned them with CharNext in the ANSI code page. With a DBCS code page, a backslash after a multibyte character was missed, and `exec("あ\\foo", "x")` ran `あ\foo` found in PATH instead of the one under the current directory. The copy that win32.c includes keeps using its code page.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dln_find.c

@@ -124,8 +124,9 @@ dln_find_1(const char *fname, const char *path, char *fbuf, size_t size,
         return NULL;
     }
 #ifdef DOSISH
-# ifndef CharNext
-# define CharNext(p) ((p)+1)
+# ifndef DLN_CHAR_NEXT
+/* fname is UTF-8 unless win32.c includes this file with its code page */
+# define DLN_CHAR_NEXT(p) ((p)+1)
 # endif
 # ifdef DOSISH_DRIVE_LETTER
     if ((unsigned char)((p[0] | 0x20) - 'a') < 26u  && p[1] == ':') {
@@ -151,7 +152,7 @@ dln_find_1(const char *fname, const char *path, char *fbuf, size_t size,
             p++;
             break;
           default:
-            p = CharNext(p);
+            p = DLN_CHAR_NEXT(p);
         }
     }
     if (ext) {

test/ruby/test_process.rb

@@ -1166,6 +1166,21 @@ class TestProcess < Test::Unit::TestCase
     }
   end
 
+  def test_exec_relative_path_after_multibyte_dir
+    return unless windows?
+    with_tmpchdir {|d|
+      dir = "\u{3042}"
+      Dir.mkdir(dir)
+      File.write("#{dir}/foo.cmd", "@echo cwd\n")
+      Dir.mkdir("path")
+      Dir.mkdir("path/#{dir}")
+      File.write("path/#{dir}/foo.cmd", "@echo path\n")
+      env = {"PATH"=>"#{d}/path;#{ENV["PATH"]}"}
+      r = IO.popen([env, RUBY, "-e", "exec(*ARGV)", "#{dir}\\foo", "x"], &:read)
+      assert_equal("cwd", r.chomp)
+    }
+  end
+
   def test_system_wordsplit
     with_tmpchdir {|d|
       File.write("script", <<-'End')

win32/win32.c

@@ -81,8 +81,7 @@ static char *w32_getenv(const char *name, UINT cp);
 #define DLN_FIND_EXTRA_ARG ,cp
 #define rb_w32_stati128(path, st) w32_stati128(path, st, cp, FALSE)
 #define getenv(name) w32_getenv(name, cp) /* Necessarily For dln.c */
-#undef CharNext
-#define CharNext(p) CharNextExA(cp, (p), 0)
+#define DLN_CHAR_NEXT(p) CharNextExA(cp, (p), 0)
 #define dln_find_exe_r rb_w32_udln_find_exe_r
 #define dln_find_file_r rb_w32_udln_find_file_r
 #include "dln.h"