Use cached offset in ftell when reliable

The cached offset is reliable to use in ftell when the stream handle
is active.  We can consider a stream as being active when there is
unflushed data.  However, even in this case, we can use the cached
offset only when the stream is not being written to in a+ mode,
because this case may have unflushed data and a stale offset; the
previous read could have sent it off somewhere other than the end of
the file.

There were a couple of adjustments necessary to get this to work.
Firstly, fdopen now ceases to use _IO_attach_fd because it sets the
offset cache to the current file position.  This is not correct
because there could be changes to the file descriptor before the
stream handle is activated, which would not get reflected.

A similar offset caching action is done in _IO_fwide, claiming that
wide streams have 'problems' with the file offsets.  There don't seem
to be any obvious problems with not having the offset cache available,
other than that it will have to be queried in a subsequent
read/write/seek.  I have removed this as well.

The testsuite passes successfully with these changes on x86_64.

1 files changed, 6 insertions, 10 deletions

diff --git a/libio/iofdopen.c b/libio/iofdopen.c
index 066ff19000..3f266f7288 100644
--- a/libio/iofdopen.c
+++ b/libio/iofdopen.c
@@ -141,9 +141,6 @@ _IO_new_fdopen (fd, mode)
 #ifdef _IO_MTSAFE_IO
   new_f->fp.file._lock = &new_f->lock;
 #endif
-  /* Set up initially to use the `maybe_mmap' jump tables rather than using
-     __fopen_maybe_mmap to do it, because we need them in place before we
-     call _IO_file_attach or else it will allocate a buffer immediately.  */
   _IO_no_init (&new_f->fp.file, 0, 0, &new_f->wd,
 #ifdef _G_HAVE_MMAP
 	       (use_mmap && (read_write & _IO_NO_WRITES))
@@ -159,13 +156,12 @@ _IO_new_fdopen (fd, mode)
 #if  !_IO_UNIFIED_JUMPTABLES
   new_f->fp.vtable = NULL;
 #endif
-  if (_IO_file_attach ((_IO_FILE *) &new_f->fp, fd) == NULL)
-    {
-      _IO_setb (&new_f->fp.file, NULL, NULL, 0);
-      _IO_un_link (&new_f->fp);
-      free (new_f);
-      return NULL;
-    }
+  /* We only need to record the fd because _IO_file_init will have unset the
+     offset.  It is important to unset the cached offset because the real
+     offset in the file could change between now and when the handle is
+     activated and we would then mislead ftell into believing that we have a
+     valid offset.  */
+  new_f->fp.file._fileno = fd;
   new_f->fp.file._flags &= ~_IO_DELETE_DONT_CLOSE;
 
   _IO_mask_flags (&new_f->fp.file, read_write,