hgweb: garbage collect on every request
There appears to be a cycle in localrepository or hgweb that
is preventing repositories from being garbage collected when
hgwebdir dispatches to hgweb. Every request creates a new
repository instance and then leaks that object and other referenced
objects. A periodic GC to find cycles will eventually collect the
old repositories. But these don't run reliably and rapid requests
to hgwebdir can result in rapidly increasing memory consumption.
With the Firefox repository, repeated requests to raw-file URLs
leak ~100 MB per hgwebdir request (most of this appears to be
cached manifest data structures). WSGI processes quickly grow
to >1 GB RSS.
Breaking the cycles in localrepository is going to be a bit of
Because we know that hgwebdir leaks localrepository instances, let's
put a band aid on the problem in the form of an explicit gc.collect()
on every hgwebdir request.
As the inline comment states, ideally we'd do this in a finally
block for the current request iff it dispatches to hgweb. But
_runwsgi() returns an explicit value. We need the finally to run
after generator exhaustion. So we'd need to refactor _runwsgi()
to "yield" instead of "return." That's too much change for a patch
to stable. So we implement this hack one function above and run
it on every request.
The performance impact of this change should be minimal. Any
impact should be offset by benefits from not having hgwebdir
processes leak memory.