tests/svn/svndump-empty.sh

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

#!/bin/sh
#
# Use this script to generate empty.svndump
#

mkdir temp
cd temp

mkdir project-orig
cd project-orig
mkdir trunk
mkdir branches
mkdir tags
cd ..

svnadmin create svn-repo
svnurl=file://`pwd`/svn-repo
svn import project-orig $svnurl -m "init projA"

svn co $svnurl project
cd project
mkdir trunk/dir
echo a > trunk/dir/a
svn add trunk/dir
svn ci -m adddir

echo b > trunk/b
svn add trunk/b
svn ci -m addb

echo c > c
svn add c
svn ci -m addc
cd ..

# svnsync repo/trunk/dir only so the last two revisions are empty
svnadmin create svn-empty
cat > svn-empty/hooks/pre-revprop-change <<EOF
#!/bin/sh
exit 0
EOF
chmod +x svn-empty/hooks/pre-revprop-change
svnsync init --username svnsync file://`pwd`/svn-empty file://`pwd`/svn-repo/trunk/dir
svnsync sync file://`pwd`/svn-empty
svn log -v file://`pwd`/svn-empty

svnadmin dump svn-empty > ../empty.svndump