Re: [RFC PATCH 0/3] implement composite filters

[Jonathan Tan <jonathantanmy@google.com>]

> Here is a first stab at composite filters. It does not actually support omits,
> but the biggest difficulties of the implementation are already addressed. So I
> decided to send out an early version to give interested people an idea of just
> what is needed to implement it, and then give them a chance to change my mind
> (Jonathan T. especially was concerned about code complexity).

Thanks - seeing these patches reduces my concerns significantly. A
composite filter has LHS and RHS filters, either of which can be
composite themselves, so we support compositing arbitrary numbers of
filters. I see that the approach is to run LHS and RHS in lockstep, with
our own handling of the result flags:

- LOFR_MARK_SEEN is tracked for LHS and RHS separately. To support an
  arbitrary number of filters, we don't use object flags to track this,
  so we use oidsets instead. I don't think that the extra memory usage
  will be a problem (we already allocate more for all the struct
  object). If this is an issue in the future, we can switch to using
  object flags for the first N filters, and oidsets thereafter.

- LOFR_SKIP_TREE is simulated if only one filter wants to skip the tree.

- I haven't fully figured out LOFR_DO_SHOW yet. It seems to me that if
  an object appears twice in the walk, and the LHS says LOFR_DO_SHOW on
  the first occurrence, if the RHS says LOFR_DO_SHOW on the second
  occurrence, the object will be shown twice. But perhaps this isn't a
  problem - as it is, I think that a filter can call LOFR_DO_SHOW
  multiple times on the same object anyway. In any case, if this turns
  out to be a problem, it seems surmountable to me.