Re: [PATCH 03/13] rev-list: fallback to non-bitmap traversal when filtering

From: Jeff King <peff@peff.net>
Date: Feb 13, 2020, 18:40 UTC
Message-ID: <20200213184033.GA15046@coredump.intra.peff.net>
In-Reply-To: <xmqq8sl6s63c.fsf@gitster-ct.c.googlers.com>

On Thu, Feb 13, 2020 at 10:19:19AM -0800, Junio C Hamano wrote:

Show 18 quoted lines

> Jeff King <peff@peff.net> writes:
> 
> > The "--use-bitmap-index" option is usually aspirational: if we have
> > bitmaps and the request can be fulfilled more quickly using them we'll
> > do so, but otherwise fall back to a non-bitmap traversal.
> >
> > The exception is object filtering, which explicitly dies if the two
> > options are combined. Let's convert this to the usual fallback behavior.
> >
> > This is a minor convenience for now (since the caller can easily know
> > that --filter and --use-bitmap-index don't combine), but will become
> > much more useful as we start to support _some_ filters with bitmaps, but
> > not others.
> 
> Makes sense.  
> 
> Perhaps the option should have been called allow-bitmap-index or
> something along that line, but it is too late ;-)

Yeah. It's also annoyingly long to type, and makes for long lines in the test scripts. ;)

There are also some weird semantics with the fallback, because the output may differ depending on whether we use bitmaps (see one of the later patches). I wouldn't be opposed to cleaning this up and giving it a new option ("--allow-bitmaps" or something) to keep compatibility, but it's out of scope here.

The existing option (and my suggestion, as well as most of the internal code) are guilty of equating "bitmap" with "object reachability bitmap". There's lots of things one might use bitmaps for, and at some point we might even expose such a thing via rev-list.

Anyway, that concludes my rant. I don't think any of these are urgent to fix, and definitely shouldn't be part of this series.

-Peff