renameatu: fix test failure on MSVC

[Bruno Haible]

Hi Paul, Eric,

On MSVC I'm seeing these two test failures:

FAIL: test-renameat
===================

c:\testdir-posix-msvc\gltests\test-rename.h:115: assertion 'errno == ENOENT' 
failed
FAIL test-renameat.exe (exit status: 127)

FAIL: test-renameatu
====================

c:\testdir-posix-msvc\gltests\test-rename.h:115: assertion 'errno == ENOENT' 
failed
FAIL test-renameatu.exe (exit status: 127)

What happens is that renameatu fails with error EISDIR instead of ENOENT.
It happens inside at-func.c. There, the code execute the case 13. Since
file2 = '', it sets file2_alt = 'c:\testdir-posix-msvc\gltests\', which
indeed has the syntax of a directory.

There are two ways to fix this:
  (a) In line 229 (case 13), test for file2[0] == '\0'.
  (b) Test for file2[0] == '\0' a bit earlier, before the savecwd.

I would prefer (b), because
  - The test suite found this bug in case 13, but it is well possible
    that similar bugs are also lurking in one of the other cases.
  - It is pointless to save the current directory, make a trivial syntactic
    check, and restore the current directory afterwards. Syntactic checks
    can be done without changing the current directory.

So, here's the proposed fix.

at-func2.c is also used by linkat(), but since both renameat() and linkat()
require the error ENOENT if file1 is empty or file2 is empty:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/rename.html
https://pubs.opengroup.org/onlinepubs/9699919799/functions/link.html
the change will not cause a bug.


2019-07-02  Bruno Haible  <address@hidden>

        renameatu: Fix test failure on MSVC.
        * lib/at-func2.c (at_func2): Fail with ENOENT if file1 or file2 is the
        empty string.

diff --git a/lib/at-func2.c b/lib/at-func2.c
index eaa256c..bb8792f 100644
--- a/lib/at-func2.c
+++ b/lib/at-func2.c
@@ -176,6 +176,13 @@ at_func2 (int fd1, char const *file1,
         return func (file1, file2); /* Reduced to case 5.  */
     }
 
+  /* Catch invalid arguments before changing directories.  */
+  if (file1[0] == '\0' || file2[0] == '\0')
+    {
+      errno = ENOENT;
+      return -1;
+    }
+
   /* Cases 3, 7, 12, 13, 15a, 15b remain.  With all reductions in
      place, it is time to start changing directories.  */