[PATCH 1/2] Move offset to end of file when fdopen is in "a" mode (#16532)

[Rich Felker]

On Tue, Feb 11, 2014 at 03:35:58PM +0530, Siddhesh Poyarekar wrote:
> On Tue, Feb 11, 2014 at 04:54:55AM -0500, Rich Felker wrote:
> > Are you sure this fixes the issue? I think there are still cases where
> > the wrong offset might be seen. For instance it's legal to call lseek
> > on the fd after calling fdopen (the FILE is not the "active handle"
> > until you read/write with it) and then ftell could still report the
> > wrong result.
> 
> It shouldn't make a difference.  The only reason to seek to the end is
> to record the offset of the file end into the FILE object.  If an
> lseek moves it elsewhere, we don't care.  Data will be appended to the
> end of the file in any case since we add an O_APPEND to the fd.

Then what if you write() to the fd after calling fdopen?

> > We had a similar bug in musl (but not the same; it manifested in more
> > cases than the glibc one) and I fixed it by doing the repositioning at
> > ftello time when the stream is append-mode and there's unwritten
> > buffered data:
> > 
> > http://git.musl-libc.org/cgit/musl/commit/?id=3af2edee150484940916eba1984f78c3b965dd05
> 
> This is precisely what my earlier ftell optimization avoided whenever
> it could.  It is still avoidable for append, but not for append-update
> (a+) at least in the initial case.

I think it's a little bit different, at least according to your
earlier description. You said you were avoiding fflush; we don't
fflush but just lseek. In any case, it's not possible to avoid some
call to lseek here unless you can prove the FILE (and not a different
FILE or the underlying fd) is the active handle.

Rich