Allow native symlinks to non-existing targets in 'nativestrict' mode

Windows native symlinks must match the type of their target (file or
directory), otherwise native Windows tools will fail. Creating symlinks in
'nativestrict' mode currently requires the target to exist in order to
check its type.

However, the target of a symlink can change at any time after the symlink
has been created. Thus users of native symlinks must be prepared to deal
with type mismatches anyway. Checking the target type at symlink creation
time is not a good reason to violate the symlink() API specification.

In 'nativestrict' mode, always create native symlinks. Choose the symlink
type according to the target if it exists. Otherwise check the target path
for a trailing '/' as hint to create a directory symlink.

This allows callers to explicitly specify the expected target type, e.g.:

  $ ln -s test/ link-to-test
  $ mkdir test

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/path.cc

@@ -1865,7 +1865,7 @@ symlink_native (const char *oldpath, path_conv &win32_newpath)
   path_conv win32_oldpath;
   PUNICODE_STRING final_oldpath, final_newpath;
   UNICODE_STRING final_oldpath_buf;
-  DWORD flags;
+  DWORD flags = 0;
 
   if (isabspath (oldpath))
     {
@@ -1926,14 +1926,39 @@ symlink_native (const char *oldpath, path_conv &win32_newpath)
 	  wcpcpy (e_old, c_old);
 	}
     }
-  /* If the symlink target doesn't exist, don't create native symlink.
-     Otherwise the directory flag in the symlink is potentially wrong
-     when the target comes into existence, and native tools will fail.
-     This is so screwball. This is no problem on AFS, fortunately. */
-  if (!win32_oldpath.exists () && !win32_oldpath.fs_is_afs ())
+
+  /* The directory flag in the symlink must match the target type,
+     otherwise native tools will fail (fortunately this is no problem
+     on AFS). Do our best to guess the symlink type correctly. */
+  if (win32_oldpath.exists () || win32_oldpath.fs_is_afs ())
     {
-      SetLastError (ERROR_FILE_NOT_FOUND);
-      return -1;
+      /* If the target exists (or on AFS), check the target type. Note
+	 that this may still be wrong if the target is changed after
+	 creating the symlink (e.g. in bulk operations such as rsync,
+	 unpacking archives or VCS checkouts). */
+      if (win32_oldpath.isdir ())
+        flags |= SYMBOLIC_LINK_FLAG_DIRECTORY;
+    }
+  else
+    {
+      if (allow_winsymlinks == WSYM_nativestrict)
+	{
+	  /* In nativestrict mode, if the target does not exist, use
+	     trailing '/' in the target path as hint to create a
+	     directory symlink. */
+	  ssize_t len = strlen(oldpath);
+	  if (len && isdirsep(oldpath[len - 1]))
+            flags |= SYMBOLIC_LINK_FLAG_DIRECTORY;
+	}
+      else
+	{
+	 /* In native mode, if the target does not exist, fall back
+	    to creating a Cygwin symlink file (or in case of MSys:
+	    try to copy the (non-existing) target, which will of
+	    course fail). */
+	  SetLastError (ERROR_FILE_NOT_FOUND);
+	  return -1;
+	}
     }
   /* Don't allow native symlinks to Cygwin special files.  However, the
      caller shoud know because this case shouldn't be covered by the
@@ -1962,7 +1987,6 @@ symlink_native (const char *oldpath, path_conv &win32_newpath)
 	final_oldpath->Buffer[1] = L'\\';
     }
   /* Try to create native symlink. */
-  flags = win32_oldpath.isdir () ? SYMBOLIC_LINK_FLAG_DIRECTORY : 0;
   if (wincap.has_unprivileged_createsymlink ())
     flags |= SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE;
   if (!CreateSymbolicLinkW (final_newpath->Buffer, final_oldpath->Buffer,