Fix 'expand-file-name' for remote files

This reverts most of commit 14fb657ba82da346d36f05f88da26f1c5498b798
and its followup fixes, and instead fixes the original bugs in a
different manner that doesn't affect any unrelated use cases.  As
part of this, the code which caused 'expand-file-name' to enforce
a trailing slash on expanded directories is removed, as this kind
of semantic processing is outside of 'expand-file-name's scope.
* src/fileio.c (Fexpand_file_name): If expanding default_directory
yields a remote file name, call its handlers.  (Bug#26911)
(Bug#34834)

* doc/lispref/files.texi (File Name Expansion): Remove the
requirement that expanding a directory name yields a directory
name, i.e. that the expansion must end in a slash.

* etc/NEWS: Remove the announcement of the changed behavior of
'expand-file-name' wrt trailing slashes.

* test/src/fileio-tests.el (fileio-tests--HOME-trailing-slash)
(fileio-tests--expand-file-name-trailing-slash): Remove tests.
* test/lisp/net/tramp-tests.el (tramp-test05-expand-file-name): No
need to expect different results in Emacs 28 and later.

1 files changed, 30 insertions, 56 deletions

diff --git a/src/fileio.c b/src/fileio.c
index c91af36fdf6..1e4ca82e5f3 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;
@@ -947,6 +947,22 @@ the root directory.  */)
         )
       {
         default_directory = Fexpand_file_name (default_directory, Qnil);
+
+        /* The above expansion might have produced a remote file name,
+           so give the handlers one last chance to DTRT.  This can
+           happen when both NAME and DEFAULT-DIRECTORY arguments are
+           relative file names, and the buffer's default-directory is
+           remote.  */
+        handler = Ffind_file_name_handler (default_directory,
+                                           Qexpand_file_name);
+        if (!NILP (handler))
+          {
+            handled_name = call3 (handler, Qexpand_file_name,
+                                  name, default_directory);
+            if (STRINGP (handled_name))
+              return handled_name;
+            error ("Invalid handler in `file-name-handler-alist'");
+          }
       }
   }
   multibyte = STRING_MULTIBYTE (name);
@@ -1065,7 +1081,7 @@ the root directory.  */)
 #endif /* WINDOWSNT */
 #endif /* DOS_NT */
 
-  /* If nm is absolute, look for "/./" or "/../" or "//" sequences; if
+  /* If nm is absolute, look for `/./' or `/../' or `//''sequences; if
      none are found, we can probably return right away.  We will avoid
      allocating a new string if name is already fully expanded.  */
   if (
@@ -1183,7 +1199,9 @@ the root directory.  */)
               newdir = SSDATA (hdir);
               newdirlim = newdir + SBYTES (hdir);
             }
+#ifdef DOS_NT
           collapse_newdir = false;
+#endif
         }
       else                      /* ~user/filename */
         {
@@ -1203,7 +1221,9 @@ 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
@@ -1370,15 +1390,12 @@ the root directory.  */)
     }
 #endif /* DOS_NT */
 
-  length = newdirlim - newdir;
-
-#ifdef 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--;
-#endif
 
   /* Now concatenate the directory and name to new space in the stack frame.  */
   tlen = length + file_name_as_directory_slop + (nmlim - nm) + 1;
@@ -1392,16 +1409,12 @@ the root directory.  */)
 #else  /* not DOS_NT */
   target = SAFE_ALLOCA (tlen);
 #endif /* not DOS_NT */
+  *target = 0;
   nbytes = 0;
 
   if (newdir)
     {
-#ifndef DOS_NT
-      bool treat_as_absolute = !collapse_newdir;
-#else
-      bool treat_as_absolute = !nm[0] || IS_DIRECTORY_SEP (nm[0]);
-#endif
-      if (treat_as_absolute)
+      if (nm[0] == 0 || IS_DIRECTORY_SEP (nm[0]))
         {
 #ifdef DOS_NT
           /* If newdir is effectively "C:/", then the drive letter will have
@@ -1413,23 +1426,13 @@ the root directory.  */)
                 && newdir[1] == '\0'))
 #endif
             {
-              /* 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);
+              target[length] = 0;
               nbytes = length;
             }
         }
       else
         nbytes = file_name_as_directory (target, newdir, length, multibyte);
-
-#ifndef DOS_NT
-      /* 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++;
-#endif
     }
 
   memcpy (target + nbytes, nm, nmlim - nm + 1);
@@ -1446,20 +1449,6 @@ the root directory.  */)
           {
             *o++ = *p++;
           }
-#ifndef DOS_NT
-        else if (p[1] == '.' && IS_DIRECTORY_SEP (p[2]))
-          {
-            /* Replace "/./" with "/".  */
-            p += 2;
-          }
-        else if (p[1] == '.' && !p[2])
-          {
-            /* At the end of the file name, replace "/." with "/".
-               The trailing "/" is for symlinks.  */
-            *o++ = *p;
-            p += 2;
-          }
-#else
         else if (p[1] == '.'
                  && (IS_DIRECTORY_SEP (p[2])
                      || p[2] == 0))
@@ -1470,7 +1459,6 @@ the root directory.  */)
               *o++ = *p;
             p += 2;
           }
-#endif
         else if (p[1] == '.' && p[2] == '.'
                  /* `/../' is the "superroot" on certain file systems.
                     Turned off on DOS_NT systems because they have no
@@ -1484,35 +1472,21 @@ the root directory.  */)
 #endif
                  && (IS_DIRECTORY_SEP (p[3]) || p[3] == 0))
           {
-#ifndef DOS_NT
-            while (o != target)
-              {
-                o--;
-                if (IS_DIRECTORY_SEP (*o))
-                  {
-                    /* Keep "/" at the end of the name, for symlinks.  */
-                    o += p[3] == 0;
-
-                    break;
-                  }
-              }
-#else
-# ifdef WINDOWSNT
+#ifdef WINDOWSNT
             char *prev_o = o;
-# endif
+#endif
             while (o != target && (--o, !IS_DIRECTORY_SEP (*o)))
               continue;
-# ifdef WINDOWSNT
+#ifdef WINDOWSNT
             /* Don't go below server level in UNC filenames.  */
             if (o == target + 1 && IS_DIRECTORY_SEP (*o)
                 && IS_DIRECTORY_SEP (*target))
               o = prev_o;
             else
-# endif
+#endif
             /* Keep initial / only if this is the whole name.  */
             if (o == target && IS_ANY_SEP (*o) && p[3] == 0)
               ++o;
-#endif
             p += 3;
           }
         else if (IS_DIRECTORY_SEP (p[1])