Fix recently-introduced expand-file-name bug

The bug was that (expand-file-name "~") returned something
like "/home/eggert/" instead of "/home/eggert".
Problem	reported by Mattias Engdegård (Bug#26911#27).
* src/fileio.c (Fexpand_file_name): When concatenating NEWDIR to
NM, instead of stripping trailing slashes from NEWDIR (which can
turn non-symlinks into symlinks), strip leading slashes from NM.
This also simplifies the code by removing no-longer-needed DOS_NT
special-casing.  Also, remove an unnecessary ‘target[length] = 0;’
as that byte will be overwritten by the next memcpy anyway.
* test/src/fileio-tests.el (fileio-tests--HOME-trailing-slash):
New test.

1 files changed, 13 insertions, 25 deletions

diff --git a/src/fileio.c b/src/fileio.c
index b70dff1c22c..47e5e46a003 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -827,9 +827,9 @@ the root directory.  */)
   ptrdiff_t tlen;
 #ifdef DOS_NT
   int drive = 0;
-  bool collapse_newdir = true;
   bool is_escaped = 0;
 #endif /* DOS_NT */
+  bool collapse_newdir = true;
   ptrdiff_t length, nbytes;
   Lisp_Object handler, result, handled_name;
   bool multibyte;
@@ -1183,9 +1183,7 @@ the root directory.  */)
               newdir = SSDATA (hdir);
               newdirlim = newdir + SBYTES (hdir);
             }
-#ifdef DOS_NT
           collapse_newdir = false;
-#endif
         }
       else                      /* ~user/filename */
         {
@@ -1205,9 +1203,7 @@ the root directory.  */)
 
               while (*++nm && !IS_DIRECTORY_SEP (*nm))
                 continue;
-#ifdef DOS_NT
               collapse_newdir = false;
-#endif
             }
 
           /* If we don't find a user of that name, leave the name
@@ -1374,12 +1370,7 @@ the root directory.  */)
     }
 #endif /* DOS_NT */
 
-  /* Ignore any slash at the end of newdir, unless newdir is
-     just "/" or "//".  */
   length = newdirlim - newdir;
-  while (length > 1 && IS_DIRECTORY_SEP (newdir[length - 1])
-         && ! (length == 2 && IS_DIRECTORY_SEP (newdir[0])))
-    length--;
 
   /* Now concatenate the directory and name to new space in the stack frame.  */
   tlen = length + file_name_as_directory_slop + (nmlim - nm) + 1;
@@ -1398,25 +1389,22 @@ the root directory.  */)
 
   if (newdir)
     {
-      if (IS_DIRECTORY_SEP (nm[0]))
+      if (!collapse_newdir)
         {
-#ifdef DOS_NT
-          /* If newdir is effectively "C:/", then the drive letter will have
-             been stripped and newdir will be "/".  Concatenating with an
-             absolute directory in nm produces "//", which will then be
-             incorrectly treated as a network share.  Ignore newdir in
-             this case (keeping the drive letter).  */
-          if (!(drive && nm[0] && IS_DIRECTORY_SEP (newdir[0])
-                && newdir[1] == '\0'))
-#endif
-            {
-              memcpy (target, newdir, length);
-              target[length] = 0;
-              nbytes = length;
-            }
+          /* With ~ or ~user, leave NEWDIR as-is to avoid transforming
+             it from a symlink (or a regular file!) into a directory.  */
+          memcpy (target, newdir, length);
+          nbytes = length;
         }
       else
         nbytes = file_name_as_directory (target, newdir, length, multibyte);
+
+      /* If TARGET ends in a directory separator, omit leading
+         directory separators from NM so that concatenating a TARGET "/"
+         to an NM "/foo" does not result in the incorrect "//foo".  */
+      if (nbytes && IS_DIRECTORY_SEP (target[nbytes - 1]))
+        while (IS_DIRECTORY_SEP (nm[0]))
+          nm++;
     }
 
   memcpy (target + nbytes, nm, nmlim - nm + 1);