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