Re: [PATCH v2 6/9] list-objects-filter-options: make filter_spec a strbuf

From: Matthew DeVore <matvore@comcast.net>
Date: Jun 11, 2019, 18:44 UTC
Message-ID: <20190611184426.GB58112@comcast.net>
In-Reply-To: <xmqqtvcwkowx.fsf@gitster-ct.c.googlers.com>

On Tue, Jun 11, 2019 at 10:33:18AM -0700, Junio C Hamano wrote:

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> Matthew DeVore <matvore@comcast.net> writes:
> 
> >> convention, and it may even be a useful one (i.e. it allows you to
> >> calloc() a structure with an embedded strbuf in it and the "if
> >> .buf==NULL, call strbuf_init() lazily" can become an established
> >> pattern), but at the same time it feels a bit brittle.  
> >
> > Is it brittle because a strbuf may be initialized to non-zero memory, and so
> > the "if (buf.buf == NULL)" may evaluate to false, and then go on treating
> > garbage like a valid buffer?
> 
> It is brittle because callers are bound to forget doing "if
> (!x->buf.buf) lazy_init(&x->buf)" at some point, and blindly use an
> uninitialized x->buf.  Making sure x->buf is always initialized

A corallary proposition would be to make this particular strbuf a "struct strbuf *" rather than an inline strbuf. It should then be rather clear to users that it may be null. Then whoever allocates the memory can also do the strbuf_init one-liner. The free'ing logic of list_objects_filter_options then only becomes trivially more complicated than it was before. Does that sound like a good compromise to you?

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> before any caller touches is the only way to solve it, and as you
> said, there are two possible ways to make that happen.  One way that
> does not violate the current API contract is to make sure whoever
> allocates and/or initializes the surrounding struct that embeds a
> strbuf does strbuf_init(&x->buf) before any user sees the struct.

The thing I don't like about that is that the non-zeroness of its initialization percolates upward to whatever the top-level struct is, which means implementation details leak a lot. This seems quite brittle as well, since anyone can forget to initialize some struct in the nested line.

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> 
> Another would be to update strbuf API so that strbuf_init() does not
> even have to use slopbuf.  But that is a much larger change that
> potentially breaks existing users of strbuf API.  When you have a
> strbuf that has been prepared to be usable, the current API contract
> allows its users to expect buf.buf is never NULL, so they assume
> that they can safely write "if (!buf.buf)", so auditing strbuf.c and
> making sure a strbuf with buf==NULL gets lazily initialized is not
> enough.

That's true. I didn't think it matters in the case of filter_spec in particular, since users of list_objects_filter_options are supposed to use an accessor and not touch the strbuf directly, but looking at it like a more general API change, it seems risky.