Re: [GSoC][PATCH v5 3/7] dir-iterator: add flags parameter to dir_iterator_begin

From: Thomas Gummerer <t.gummerer@gmail.com>
Date: Apr 24, 2019, 18:36 UTC
Message-ID: <20190424183622.GB2068@hank.intra.tgummerer.com>
In-Reply-To: <CAHd-oW48+cEqSMDFF3PXuOnzUvptM6q5Ba_Zb8gGhqa1bGNwZQ@mail.gmail.com>

On 04/23, Matheus Tavares Bernardino wrote:

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> On Thu, Apr 11, 2019 at 6:09 PM Thomas Gummerer <t.gummerer@gmail.com> wrote:
> >
> > On 04/10, Matheus Tavares Bernardino wrote:
> > > > > diff --git a/dir-iterator.h b/dir-iterator.h
> > > > > index 970793d07a..93646c3bea 100644
> > > > > --- a/dir-iterator.h
> > > > > +++ b/dir-iterator.h
> > > > > @@ -19,7 +19,7 @@
> > > > >   * A typical iteration looks like this:
> > > > >   *
> > > > >   *     int ok;
> > > > > - *     struct iterator *iter = dir_iterator_begin(path);
> > > > > + *     struct iterator *iter = dir_iterator_begin(path, 0);
> > > >
> > > > Outside of this context, we already mentione errorhandling when
> > > > 'ok != ITER_DONE' in his example.  This still can't happen with the
> > > > way the dir iterator is used here, but it serves as a reminder if
> > > > people are using the DIR_ITERATOR_PEDANTIC flag.  Good.
> > >
> > > This made me think again about the documentation saying that
> > > dir_iterator_abort() and dir_iterator_advance() may return ITER_ERROR,
> > > but the implementation does not containing these possibilities.
> > > (Besides when the pedantic flag is used). Maybe the idea was to make
> > > API-users implement the check for an ITER_ERROR in case dir-iterator
> > > needs to start returning it in the future.
> >
> > Yeah, I think that was the intention.
> >
> > > But do you think such a change in dir-iterator is likely to happen?
> > > Maybe we could just make dir_iterator_abort() be void and remove this
> > > section from documentation. Then, for dir_iterator_advance() users
> > > would only need to check for ITER_ERROR if the pedantic flag was given
> > > at dir-iterator creation...
> >
> > Dunno.  In a world where we have the pedantic flag, I think only
> > returning ITER_ERROR if that flag is given might be what we want to
> > do.  I can't think of a reason why we would want to return ITER_ERROR
> > without the pedantic flag in that case.
> 
> Ok. I began doing the change, but got stuck in a specific decision.
> What I was trying to do is:
> 
> 1) Make dir_iterator_advance() return ITER_ERROR only when the
> pedantic flag is given;
> 2) Make dir_iterator_abort() be void.
> 
> The first change is trivial. But the second is not so easy: Since the
> [only] current API user defines other iterators on top of
> dir-iterator, it would require a somehow big surgery on refs/* to make
> this change. Should I proceed and make the changes at refs/* or should
> I keep dir_iterator_abort() returning int, although it can never fail?

Maybe I'm missing something, but wouldn't this change in refs.c be enough? (Other than actually making dir_iterator_abort not return anything)

	diff --git a/refs/files-backend.c b/refs/files-backend.c
	index 5848f32ef8..81863c3ee0 100644
	--- a/refs/files-backend.c
	+++ b/refs/files-backend.c
	@@ -2125,13 +2125,12 @@ static int files_reflog_iterator_abort(struct ref_iterator *ref_iterator)
	 {
	        struct files_reflog_iterator *iter =
	                (struct files_reflog_iterator *)ref_iterator;
	-       int ok = ITER_DONE;
	 
	        if (iter->dir_iterator)
	-               ok = dir_iterator_abort(iter->dir_iterator);
	+               dir_iterator_abort(iter->dir_iterator);
	 
	        base_ref_iterator_free(ref_iterator);
	-       return ok;
	+       return ITER_DONE;
	 }
	 
	 static struct ref_iterator_vtable files_reflog_iterator_vtable = {

Currently the only thing calling dir_iterator_abort() is files_reflog_iterator_abort() from what I can see, and dir_iterator_abort() always returns ITER_DONE.

That said, I don't know if this is actually worth pursuing. Having it return some value and having the caller check that makes it more future proof, as we won't have to change all the callers in the future if we want to start returning anything other than ITER_DONE. Just leaving it as it is now doesn't actually hurt anybody I think, but may help in the future.

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> There's also a third option: The only operation that may fail during
> dir_iterator_abort() is closedir(). But even on
> dir_iterator_advance(), I'm treating this error as "non-fatal" in the
> sense that it's not caught by the pedantic flag (although a warning is
> emitted). I did it like this because it doesn't seem like a major
> error during dir iteration... But I could change this and make
> DIR_ITERATOR_PEDANTIC return ITER_ERROR upon closedir() errors for
> both dir-iterator advance() and abort() functions. What do you think?

I think this might be the right way to go. We don't really need an error from closedir, but at the same time if we are being pedantic, maybe it should be an error. I don't have a strong opinion here either way, other than I think it should probably keep returning an int.

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> > Though I think I would change the example the other way in that case,
> > and pass DIR_ITERATOR_PEDANTIC to 'dir_iterator_begin()', as it would
> > be easy to forget error handling otherwise, even when it is
> > necessary.  I'd rather err on the side of showing too much error
> > handling, than having people forget it and having users run into some
> > odd edge cases in the wild that the tests don't cover.
> 
> Yes, I agree.
> 
> > > Also CC-ed Michael in case he has some input