Fix regression in expand-file-name with drive-relative HOME

* src/fileio.c (get_homedir) [DOS_NT]: Expand drive-relative
$HOME to begin with "X:/".

* test/src/fileio-tests.el (fileio-tests--relative-HOME): Add
testing of drive-relative value of $HOME on MS-Windows and
MS-DOS.

1 files changed, 28 insertions, 0 deletions

diff --git a/src/fileio.c b/src/fileio.c
index d9795715f9e..687f6ec7452 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -1692,6 +1692,34 @@ get_homedir (void)
       if (!home)
         return "";
     }
+#ifdef DOS_NT
+  /* If home is a drive-relative directory, expand it.  */
+  if (IS_DRIVE (*home)
+      && IS_DEVICE_SEP (home[1])
+      && !IS_DIRECTORY_SEP (home[2]))
+    {
+# ifdef WINDOWSNT
+      static char hdir[MAX_UTF8_PATH];
+# else
+      static char hdir[MAXPATHLEN];
+# endif
+      if (!getdefdir (c_toupper (*home) - 'A' + 1, hdir))
+        {
+          hdir[0] = c_toupper (*home);
+          hdir[1] = ':';
+          hdir[2] = '/';
+          hdir[3] = '\0';
+        }
+      if (home[2])
+        {
+          size_t homelen = strlen (hdir);
+          if (!IS_DIRECTORY_SEP (hdir[homelen - 1]))
+            strcat (hdir, "/");
+          strcat (hdir, home + 2);
+        }
+      home = hdir;
+    }
+#endif
   if (IS_ABSOLUTE_FILE_NAME (home))
     return home;
   if (!emacs_wd)