tests/test-backwards-remove.t

perf: make perftags clear tags cache correctly

Before this patch, "hg perftags" command doesn't measure performance
of "repo.tags()" correctly, because it doesn't clear tags cache
correctly.

9dca7653b525 replaced repo._tags with repo._tagscache, but didn't
change the code path to clear tags cache in perftags() at that time.
BTW, full history of "tags cache" is:

  - d7df759d0e97 (or 0.6) introduced repo.tagscache as the first "tags cache"
  - 5614a628d173 (or 1.4) replaced repo.tagscache with repo._tags
  - 9dca7653b525 (or 2.0) replaced repo._tags with repo._tagscache
  - 98c867ac1330 (or 2.5) made repo._tagscache filteredpropertycache

To make perftags clear tags cache correctly, and to increase
"historical portability" of perftags, this patch examines existence of
attributes in repo object, and guess appropriate procedure to clear
tags cache.

To avoid examining existence of attributes at each repetition, this
patch makes repocleartagscachefunc() return the function, which
actually clears tags cache.

mozilla-central repo (85 tags on 308365 revs) with each Mercurial
version between before and after this patch.

  ==== ========= =========
  ver  before    after
  ==== ========= =========
  1.9  0.476062  0.466464
       ------- *1 -------
  2.0  0.346309  0.458327
  2.1  0.343106  0.454489
       ------- *2 -------
  2.2  0.069790  0.071263
  2.3  0.067829  0.069340
  2.4  0.068075  0.069573
       ------- *3 -------
  2.5  0.021896  0.022406
  2.6  0.021900  0.022374
  2.7  0.021883  0.022379
  2.8  0.021949  0.022327
  2.9  0.021877  0.022330
  3.0  0.021860  0.022314
  3.1  0.021869  0.022669
  3.2  0.021831  0.022668
  3.3  0.021809  0.022691
  3.4  0.021861  0.022916
  3.5  0.019335  0.020749
  3.6  0.019319  0.020866
  3.7  0.018781  0.020251
       ------- *4 -------
  3.8  0.068262  0.072558
  3.9  0.069682  0.073773
  ==== ========= =========

(*1) repo._tags was replaced with repo._tagscache at this point

     "repo._tags = None" in perftags "before" this patch doesn't clear
     tags cache for Mercurial 2.0 or later. This causes significant
     gap of "before" between 1.9 and 2.0 .

(*2) I'm not sure about significant gap at this point, but release
     note of 2.2 described "a number of significant performance
     improvements for large repositories"

(*3) filtered changelog was cached in repoview as repoview.changelog
     at this point (by 4d92e2d75cff)

     This avoids calculation of filtered changelog at each repetition
     of t().

(*4) calculation of filtered changelog was included into wall time at
     this point (by 332926212ef8), again

     See below for detail about this significant gap:

     https://www.mercurial-scm.org/pipermail/mercurial-devel/2016-April/083410.html

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ ls
  a
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

B should disappear

  $ ls
  a