Ensure that expand-file-name returns an absolute file name

* src/fileio.c (Fexpand_file_name): Don't directly use the current
buffer's default-directory if it is relative.  Instead replace it
by its expansion relative to invocation-directory.  (Bug#36502)
* test/src/fileio-tests.el
(fileio-tests--relative-default-directory): New test.

1 files changed, 16 insertions, 1 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 505e4ec33bf..8f23a305a52 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -804,7 +804,22 @@ the root directory.  */)
 
   /* Use the buffer's default-directory if DEFAULT_DIRECTORY is omitted.  */
   if (NILP (default_directory))
-    default_directory = BVAR (current_buffer, directory);
+    {
+      Lisp_Object dir = BVAR (current_buffer, directory);
+      /* The buffer's default-directory should be absolute.  If it
+         isn't, try to expand it relative to invocation-directory.
+         But we have to be careful to avoid an infinite loop, because
+         the code in emacs.c that sets Vinvocation_directory might
+         call Fexpand_file_name.  */
+      if (STRINGP (dir))
+        {
+          if (!NILP (Ffile_name_absolute_p (dir)))
+            default_directory = dir;
+          else if (STRINGP (Vinvocation_directory)
+                   && !NILP (Ffile_name_absolute_p (Vinvocation_directory)))
+            default_directory = Fexpand_file_name (dir, Vinvocation_directory);
+        }
+    }
   if (! STRINGP (default_directory))
     {
 #ifdef DOS_NT