Cygwin: execve: fix setting O_APPEND file offset for native child

dtable::set_file_pointers_for_exec is called from
child_info_spawn::worker to move the file position of O_APPEND
files to EOF if the child is a native child.

However, this only works correctly for the first O_APPEND
file descriptor:

- set_file_pointers_for_exec calls SetFilePointer.  The higher
  4 bytes of the desired file offset are given to SetFilePointer
  as pointer to a DWORD value.  On return, SetFilePointer returns
  the higher 4 bytes of the new file position in this DWORD.

- So for the second and subsequent descriptors the higher 4 byte
  of the file position depend on what the actual file position
  of the previous file has been set to:

- If the file is > 2 Gigs, the high offset will not be 0 anymore.

- If the desciptor points to a non-seekable file (i.e., a pipe
  or socket), SetFilePosition returns an error and sets the high
  position to -1.

Fix this by calling SetFilePointerEx instead, which does not
modify the incoming position value.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/dtable.cc

@@ -857,12 +857,13 @@ dtable::set_file_pointers_for_exec ()
 {
 /* This is not POSIX-compliant so the function is only called for
    non-Cygwin processes. */
-  LONG off_high = 0;
+  LARGE_INTEGER eof = { QuadPart: 0 };
+
   lock ();
   fhandler_base *fh;
   for (size_t i = 0; i < size; i++)
     if ((fh = fds[i]) != NULL && fh->get_flags () & O_APPEND)
-      SetFilePointer (fh->get_handle (), 0, &off_high, FILE_END);
+      SetFilePointerEx (fh->get_handle (), eof, NULL, FILE_END);
   unlock ();
 }
 

winsup/cygwin/release/3.0

@@ -100,3 +100,6 @@ Bug Fixes
 
 - Fix exception handling in pthreads.
   Addresses: https://cygwin.com/ml/cygwin/2019-01/msg00149.html
+
+- Fix O_APPEND handling on files when calling non-Cygwin applications
+  Addresses: https://cygwin.com/ml/cygwin/2019-02/msg00081.html