Re: [PATCH v2 3/3] mingw: replace isatty() hack

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Hannes,

On Thu, 22 Dec 2016, Johannes Sixt wrote:

> Am 22.12.2016 um 18:09 schrieb Johannes Schindelin:
> > +static HANDLE swap_osfhnd(int fd, HANDLE new_handle)
> > +{
> > +	/*
> > +	 * Create a copy of the original handle associated with fd
> > +	 * because the original will get closed when we dup2().
> > +	 */
> > +	HANDLE handle = (HANDLE)_get_osfhandle(fd);
> > +	HANDLE duplicate = duplicate_handle(handle);
> >
> > +	/* Create a temp fd associated with the already open "new_handle". */
> > +	int new_fd = _open_osfhandle((intptr_t)new_handle, O_BINARY);
> >
> > +	assert((fd == 1) || (fd == 2));
> >
> > +	/*
> > +	 * Use stock dup2() to re-bind fd to the new handle.  Note that
> > +	 * this will implicitly close(1) and close both fd=1 and the
> > +	 * originally associated handle.  It will open a new fd=1 and
> > +	 * call DuplicateHandle() on the handle associated with new_fd.
> > +	 * It is because of this implicit close() that we created the
> > +	 * copy of the original.
> > +	 *
> > +	 * Note that the OS can recycle HANDLE (numbers) just like it
> > +	 * recycles fd (numbers), so we must update the cached value
> > +	 * of "console".  You can use GetFileType() to see that
> > +	 * handle and _get_osfhandle(fd) may have the same number
> > +	 * value, but they refer to different actual files now.
> 
> Certainly, the OS does not recycle handle values that are in use (open). Then
> I do not quite get the point of this paragraph. See...
> 
> > +	 *
> > +	 * Note that dup2() when given target := {0,1,2} will also
> > +	 * call SetStdHandle(), so we don't need to worry about that.
> > +	 */
> > +	dup2(new_fd, fd);
> > +	if (console == handle)
> > +		console = duplicate;
> 
> ... This is where "the cached value of console is updated", right? If console
> == handle, then this is not because a handle value was recycled, but because
> fd *was* console. Since the old value of console has been closed by the
> dup2(), we must refer to the back-up value in the future. Am I missing
> something?

You are correct, we must update `console` because `handle` is no longer
the handle we want.

The comment above only meant to reinforce that we have to forget about the
previous handle, too, as we might access something completely different
than a console otherwise.

Would you have a suggestion how to rephrase the comment to make it less
confusing?

Ciao,
Dscho