Patchwork [6,of,8] match: avoid translating glob to matcher multiple times for large sets

# HG changeset patch
# User Boris Feld <boris.feld@octobus.net>
# Date 1542651672 0
#      Mon Nov 19 18:21:12 2018 +0000
# Node ID 03b60ccca50c77a552de85ac3402c5174539f150
# Parent  aeea6de7f1a88e2e710ee01299fe4fe9e1f0d336
# EXP-Topic perf-ignore
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 03b60ccca50c
match: avoid translating glob to matcher multiple times for large sets

For hgignore with many globs, the resulting regexp might not fit under the 20K
length limit. So the patterns need to be broken up in smaller pieces.

Before this change, the logic was re-starting the full process from scratch
for each smaller pieces, including the translation of globs into regexp.
Effectively doing the work over and over.

If the 20K limit is reached, we are likely in a case where there is many such
glob, so exporting them is especially expensive and we should be careful not
to do that work more than once.

To work around this, we now translate glob to regexp once and for all. Then,
we assemble the resulting individual regexp into valid blocks.

This raises a very significant performance win for large `.hgignore file`:

Before: ! wall 0.153153 comb 0.150000 user 0.150000 sys 0.000000 (median of 66)
After:  ! wall 0.059793 comb 0.060000 user 0.060000 sys 0.000000 (median of 100)

On Wed, 21 Nov 2018 19:33:56 +0100, Boris Feld wrote:
> # HG changeset patch
> # User Boris Feld <boris.feld@octobus.net>
> # Date 1542651672 0
> #      Mon Nov 19 18:21:12 2018 +0000
> # Node ID 03b60ccca50c77a552de85ac3402c5174539f150
> # Parent  aeea6de7f1a88e2e710ee01299fe4fe9e1f0d336
> # EXP-Topic perf-ignore
> # Available At https://bitbucket.org/octobus/mercurial-devel/
> #              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 03b60ccca50c
> match: avoid translating glob to matcher multiple times for large sets

> +MAXRESIZE = 20000
> +
> +def _groupregexps(regexps):
> +    return '(?:%s)' % '|'.join(regexps)
> +
>  def _buildregexmatch(kindpats, globsuffix):
>      """Build a match function from a list of kinds and kindpats,
>      return regexp string and a matcher function."""
>      try:
> -        regex = '(?:%s)' % '|'.join([_regex(k, p, globsuffix)
> -                                     for (k, p, s) in kindpats])
> -        if len(regex) > 20000:
> -            raise OverflowError
> -        return regex, _rematcher(regex)
> -    except OverflowError:
> -        # We're using a Python with a tiny regex engine and we
> -        # made it explode, so we'll divide the pattern list in two
> -        # until it works
> -        l = len(kindpats)
> -        if l < 2:
> -            raise
> -        regexa, a = _buildregexmatch(kindpats[:l//2], globsuffix)
> -        regexb, b = _buildregexmatch(kindpats[l//2:], globsuffix)
> -        return regex, lambda s: a(s) or b(s)
> +        allgroups = []
> +        regexps = [_regex(k, p, globsuffix) for (k, p, s) in kindpats]
> +        fullregexp = _groupregexps(regexps)
> +
> +        startidx = 0
> +        groupsize = 3
> +        for idx, r in enumerate(regexps):
> +            piecesize = len(r)
> +            if piecesize > MAXRESIZE:

It leaves an empty group if MAXRESIZE - 3 <= piecesize < MAXRESIZE.

> +                raise OverflowError

Maybe we should make it raise Abort as we no longer catch it. Can you add
a test?

> +            elif (groupsize + 1 + piecesize) > MAXRESIZE:
> +                group = regexps[startidx:idx]
> +                allgroups.append(_groupregexps(group))
> +                startidx = idx
> +                groupsize = 3
> +            groupsize += piecesize

Perhaps, it should be piecesize + 1?
> +
> +        if startidx == 0:
> +            func = _rematcher(fullregexp)
> +        else:
> +            group = regexps[startidx:idx]

It should be regexps[startidx:]. idx is the last index, not the length.

> +            allgroups.append(_groupregexps(group))
> +            allmatchers = [_rematcher(g) for g in allgroups]
> +            func = lambda s: any(m(s) for m in allmatchers)
> +        return fullregexp, func

Just an idea. I think fullregexp could be formatted as (?:(?:..)|(?:..)|..)
to reflect the internal structure. That will help debugging.