Fix bug: (directory-file-name "///") returned "//"

* src/fileio.c (directory_file_name): For "///" and longer,
return "/", not "//", as per POSIX.
* test/src/fileio-tests.el (fileio-tests--directory-file-name)
(fileio-tests--file-name-as-directory): New tests.

1 files changed, 8 insertions, 7 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 0a52982291d..9df3b1bedae 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -566,15 +566,16 @@ is already present.  */)
 static ptrdiff_t
 directory_file_name (char *dst, char *src, ptrdiff_t srclen, bool multibyte)
 {
-  /* Process as Unix format: just remove any final slash.
-     But leave "/" and "//" unchanged.  */
-  while (srclen > 1
+  /* In Unix-like systems, just remove any final slashes.  However, if
+     they are all slashes, leave "/" and "//" alone, and treat "///"
+     and longer as if they were "/".  */
+  if (! (srclen == 2 && IS_DIRECTORY_SEP (src[0])))
+    while (srclen > 1
 #ifdef DOS_NT
-         && !IS_ANY_SEP (src[srclen - 2])
+           && !IS_ANY_SEP (src[srclen - 2])
 #endif
-         && IS_DIRECTORY_SEP (src[srclen - 1])
-         && ! (srclen == 2 && IS_DIRECTORY_SEP (src[0])))
-    srclen--;
+           && IS_DIRECTORY_SEP (src[srclen - 1]))
+      srclen--;
 
   memcpy (dst, src, srclen);
   dst[srclen] = 0;