tests/test-mq-merge.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

Setup extension:

  $ cat <<EOF >> $HGRCPATH
  > [extensions]
  > mq =
  > [mq]
  > git = keep
  > EOF

Test merge with mq changeset as the second parent:

  $ hg init m
  $ cd m
  $ touch a b c
  $ hg add a
  $ hg commit -m a
  $ hg add b
  $ hg qnew -d "0 0" b
  $ hg update 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ hg add c
  $ hg commit -m c
  created new head
  $ hg merge
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg commit -m merge
  abort: cannot commit over an applied mq patch
  [255]
  $ cd ..

Issue529: mq aborts when merging patch deleting files

  $ checkundo()
  > {
  >     if [ -f .hg/store/undo ]; then
  >         echo ".hg/store/undo still exists"
  >     fi
  > }

Commit two dummy files in "init" changeset:

  $ hg init t
  $ cd t
  $ echo a > a
  $ echo b > b
  $ hg ci -Am init
  adding a
  adding b
  $ hg tag -l init

Create a patch removing a:

  $ hg qnew rm_a
  $ hg rm a
  $ hg qrefresh -m "rm a"

Save the patch queue so we can merge it later:

  $ hg qsave -c -e
  copy $TESTTMP/t/.hg/patches to $TESTTMP/t/.hg/patches.1
  $ checkundo

Update b and commit in an "update" changeset:

  $ hg up -C init
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ echo b >> b
  $ hg st
  M b
  $ hg ci -m update
  created new head

# Here, qpush used to abort with :
# The system cannot find the file specified => a
  $ hg manifest
  a
  b

  $ hg qpush -a -m
  merging with queue at: $TESTTMP/t/.hg/patches.1
  applying rm_a
  now at: rm_a

  $ checkundo
  $ hg manifest
  b

Ensure status is correct after merge:

  $ hg qpop -a
  popping rm_a
  popping .hg.patches.merge.marker
  patch queue now empty

  $ cd ..

Classic MQ merge sequence *with an explicit named queue*:

  $ hg init t2
  $ cd t2
  $ echo '[diff]' > .hg/hgrc
  $ echo 'nodates = 1' >> .hg/hgrc
  $ echo a > a
  $ hg ci -Am init
  adding a
  $ echo b > a
  $ hg ci -m changea
  $ hg up -C 0
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg cp a aa
  $ echo c >> a
  $ hg qnew --git -f -e patcha
  $ echo d >> a
  $ hg qnew -d '0 0' -f -e patcha2

Create the reference queue:

  $ hg qsave -c -e -n refqueue
  copy $TESTTMP/t2/.hg/patches to $TESTTMP/t2/.hg/refqueue
  $ hg up -C 1
  1 files updated, 0 files merged, 1 files removed, 0 files unresolved

Merge:

  $ HGMERGE=internal:other hg qpush -a -m -n refqueue
  merging with queue at: $TESTTMP/t2/.hg/refqueue
  applying patcha
  patching file a
  Hunk #1 succeeded at 2 with fuzz 1 (offset 0 lines).
  fuzz found when applying patch, stopping
  patch didn't work out, merging patcha
  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
  0 files updated, 2 files merged, 0 files removed, 0 files unresolved
  applying patcha2
  now at: patcha2

Check patcha is still a git patch:

  $ cat .hg/patches/patcha
  # HG changeset patch
  # Parent  d3873e73d99ef67873dac33fbcc66268d5d2b6f4
  
  diff --git a/a b/a
  --- a/a
  +++ b/a
  @@ -1,1 +1,2 @@
  -b
  +a
  +c
  diff --git a/a b/aa
  copy from a
  copy to aa
  --- a/a
  +++ b/aa
  @@ -1,1 +1,1 @@
  -b
  +a

Check patcha2 is still a regular patch:

  $ cat .hg/patches/patcha2
  # HG changeset patch
  # Date 0 0
  # Parent  ???????????????????????????????????????? (glob)
  
  diff -r ???????????? -r ???????????? a (glob)
  --- a/a
  +++ b/a
  @@ -1,2 +1,3 @@
   a
   c
  +d

  $ cd ..