parallel fetching can cause cache conflicts about peru HOT 4 CLOSED

The plugin cache lock and the parallelism semaphore could probably move down into plugins.py. Then RemoteModule.reup wouldn't need any additional locks, and RemoteModule.get_tree could just concern itself with the module lock.

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One issue with this scheme: the git plugin needs to be able to fetch subrepos. If everything lives in keyed dir created by peru, where is the plugin supposed to put the subrepo?

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Note that the previous way of doing this (where the git plugin could predict where caches for other git modules were supposed to live, and write to them) had the benefit of sharing caches on disk, but it still didn't allow fetching submodules in parallel, which is the really important thing. And of course it was unsafe once parallelism was introduced.

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https://phabricator.buildinspace.com/D70
https://phabricator.buildinspace.com/D71

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