tests/test-narrow-rebase.t

hgwebdir: avoid systematic full garbage collection

Forcing a systematic full garbage collection upon each request
can serioulsy harm performance. This is reported as
https://bz.mercurial-scm.org/show_bug.cgi?id=6075

With this change we're performing the full collection according
to a new setting, `experimental.web.full-garbage-collection-rate`.
The default value is 1, which doesn't change the behavior and will
allow us to test on real use cases. If the value is 0, no full garbage
collection occurs.

Regardless of the value of the setting, a partial garbage collection
still occurs upon each request (not attempting to collect objects from
the oldest generation). This should be enough to take care of
reference cycles that have been created by the last request
(assessment of this requires changing the setting, not to be 1).

In my experience chasing memory leaks in Mercurial servers,
the full collection never reclaimed any memory, but this is with
Python 3 and biased towards small repositories.

On the other hand, as explained in the Python developer docs [1],
frequent full collections are very harmful in terms of performance if
lots of objects survive the collection, and hence stay in the
oldest generation. Note that `gc.collect()` is indeed trying to
collect the oldest generation [2]. This happens usually in two cases:

- unwanted lingering objects (i.e., an actual memory leak that
  the GC cannot do anything about). Sadly, we have lots of those
  these days.
- desireable long-term objects, typically in caches (not inner caches
  carried by repositories, which should be collected with them). This
  is a subject of interest for the Heptapod project.

In short, the flat rate that this change still permits is
probably a bad idea in most cases, and the default value can
be tweaked later on (or even be set to 0) according to experiments
in the wild.

The test is inspired from test-hgwebdir-paths.py

[1] https://devguide.python.org/garbage_collector/#collecting-the-oldest-generation
[2] https://docs.python.org/3/library/gc.html#gc.collect

Differential Revision: https://phab.mercurial-scm.org/D11204

#testcases continuecommand continueflag
#if continueflag
  $ cat >> $HGRCPATH <<EOF
  > [alias]
  > continue = rebase --continue
  > EOF
#endif

  $ . "$TESTDIR/narrow-library.sh"

create full repo

  $ hg init master
  $ cd master

  $ mkdir inside
  $ echo inside1 > inside/f1
  $ echo inside2 > inside/f2
  $ mkdir outside
  $ echo outside1 > outside/f1
  $ echo outside2 > outside/f2
  $ hg ci -Aqm 'initial'

  $ echo modified > inside/f1
  $ hg ci -qm 'modify inside/f1'

  $ hg update -q 0
  $ echo modified2 > inside/f2
  $ hg ci -qm 'modify inside/f2'

  $ hg update -q 0
  $ echo modified > outside/f1
  $ hg ci -qm 'modify outside/f1'

  $ hg update -q 0
  $ echo modified2 > outside/f1
  $ hg ci -qm 'conflicting outside/f1'

  $ cd ..

  $ hg clone --narrow ssh://user@dummy/master narrow --include inside
  requesting all changes
  adding changesets
  adding manifests
  adding file changes
  added 5 changesets with 4 changes to 2 files (+3 heads)
  new changesets *:* (glob)
  updating to branch default
  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ cd narrow
  $ cat >> $HGRCPATH <<EOF
  > [extensions]
  > rebase=
  > EOF

  $ hg update -q 0

Can rebase onto commit where no files outside narrow spec are involved

  $ hg update -q 0
  $ echo modified > inside/f2
  $ hg ci -qm 'modify inside/f2'
  $ hg rebase -d 'desc("modify inside/f1")'
  rebasing 5:c2f36d04e05d tip "modify inside/f2"
  saved backup bundle to $TESTTMP/narrow/.hg/strip-backup/*-rebase.hg (glob)

Can rebase onto conflicting changes inside narrow spec

  $ hg update -q 0
  $ echo conflicting > inside/f1
  $ hg ci -qm 'conflicting inside/f1'
  $ hg rebase -d 'desc("modify inside/f1")' 2>&1 | egrep -v '(warning:|incomplete!)'
  rebasing 6:cdce97fbf653 tip "conflicting inside/f1"
  merging inside/f1
  unresolved conflicts (see 'hg resolve', then 'hg rebase --continue')
  $ echo modified3 > inside/f1
  $ hg resolve -m 2>&1 | grep -v continue:
  (no more unresolved files)
  $ hg continue
  rebasing 6:cdce97fbf653 tip "conflicting inside/f1"
  saved backup bundle to $TESTTMP/narrow/.hg/strip-backup/*-rebase.hg (glob)

Can rebase onto non-conflicting changes outside narrow spec

  $ hg update -q 0
  $ echo modified > inside/f2
  $ hg ci -qm 'modify inside/f2'
  $ hg rebase -d 'desc("modify outside/f1")'
  rebasing 7:c2f36d04e05d tip "modify inside/f2"
  saved backup bundle to $TESTTMP/narrow/.hg/strip-backup/*-rebase.hg (glob)

Rebase interrupts on conflicting changes outside narrow spec

  $ hg update -q 'desc("conflicting outside/f1")'
  $ hg phase -f -d .
  $ hg rebase -d 'desc("modify outside/f1")'
  rebasing 4:707c035aadb6 "conflicting outside/f1"
  abort: conflict in file 'outside/f1' is outside narrow clone
  [255]