Fix expand-file-name symlink-to-dir bug

Problem reported by Yegor Timoshenko (Bug#26911),
and I ran into it myself recently in normal-top-level.
* doc/lispref/files.texi (File Name Expansion), etc/NEWS: Mention this.
* src/fileio.c (Fexpand_file_name): Expand "/a/b/." to "/a/b/" not
"/a/b", to avoid misinterpreting a symlink "/a/b".  Similarly,
expand "/a/b/c/.." to "/a/b/" not "/a/b".
* test/lisp/net/tramp-tests.el (tramp-test05-expand-file-name):
Adjust to match new behavior.
(tramp-test05-expand-file-name-relative): This test now succeeds,
at least on Fedora 31.
* test/src/fileio-tests.el:
(fileio-tests--expand-file-name-trailing-slash)	New test.

1 files changed, 22 insertions, 15 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 37072d9b6bd..b70dff1c22c 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -1065,7 +1065,7 @@ 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 (
@@ -1398,7 +1398,7 @@ the root directory.  */)
 
   if (newdir)
     {
-      if (nm[0] == 0 || IS_DIRECTORY_SEP (nm[0]))
+      if (IS_DIRECTORY_SEP (nm[0]))
         {
 #ifdef DOS_NT
           /* If newdir is effectively "C:/", then the drive letter will have
@@ -1433,14 +1433,16 @@ the root directory.  */)
           {
             *o++ = *p++;
           }
-        else if (p[1] == '.'
-                 && (IS_DIRECTORY_SEP (p[2])
-                     || p[2] == 0))
+        else if (p[1] == '.' && IS_DIRECTORY_SEP (p[2]))
           {
-            /* If "/." is the entire filename, keep the "/".  Otherwise,
-               just delete the whole "/.".  */
-            if (o == target && p[2] == '\0')
-              *o++ = *p;
+            /* 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 if (p[1] == '.' && p[2] == '.'
@@ -1459,18 +1461,23 @@ the root directory.  */)
 #ifdef WINDOWSNT
             char *prev_o = o;
 #endif
-            while (o != target && (--o, !IS_DIRECTORY_SEP (*o)))
-              continue;
+            while (o != target)
+              {
+                o--;
+                if (IS_DIRECTORY_SEP (*o))
+                  {
+                    /* Keep "/" at the end of the name, for symlinks.  */
+                    o += p[3] == 0;
+
+                    break;
+                  }
+              }
 #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
-            /* Keep initial / only if this is the whole name.  */
-            if (o == target && IS_ANY_SEP (*o) && p[3] == 0)
-              ++o;
             p += 3;
           }
         else if (IS_DIRECTORY_SEP (p[1])