tests/test-diffdir.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

  $ hg init
  $ touch a
  $ hg add a
  $ hg ci -m "a"

  $ echo 123 > b
  $ hg add b
  $ hg diff --nodates
  diff -r 3903775176ed b
  --- /dev/null
  +++ b/b
  @@ -0,0 +1,1 @@
  +123

  $ hg diff --nodates -r tip
  diff -r 3903775176ed b
  --- /dev/null
  +++ b/b
  @@ -0,0 +1,1 @@
  +123

  $ echo foo > a
  $ hg diff --nodates
  diff -r 3903775176ed a
  --- a/a
  +++ b/a
  @@ -0,0 +1,1 @@
  +foo
  diff -r 3903775176ed b
  --- /dev/null
  +++ b/b
  @@ -0,0 +1,1 @@
  +123

  $ hg diff -r ""
  hg: parse error: empty query
  [10]
  $ hg diff -r tip -r ""
  hg: parse error: empty query
  [10]

Remove a file that was added via merge. Since the file is not in parent 1,
it should not be in the diff.

  $ hg ci -m 'a=foo' a
  $ hg co -Cq null
  $ echo 123 > b
  $ hg add b
  $ hg ci -m "b"
  created new head
  $ hg merge 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg rm -f a
  $ hg diff --nodates

Rename a file that was added via merge. Since the rename source is not in
parent 1, the diff should be relative to /dev/null

  $ hg co -Cq 2
  $ hg merge 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg mv a a2
  $ hg diff --nodates
  diff -r cf44b38435e5 a2
  --- /dev/null
  +++ b/a2
  @@ -0,0 +1,1 @@
  +foo
  $ hg diff --nodates --git
  diff --git a/a2 b/a2
  new file mode 100644
  --- /dev/null
  +++ b/a2
  @@ -0,0 +1,1 @@
  +foo