mirror of
https://github.com/chanzuckerberg/cellxgene.git
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78c9d24ed4
* move local_server -> backend/server server-> backend/czi_hosted, pull common code into backend/common update imports, tests and make commands
72 lines
2.2 KiB
Python
72 lines
2.2 KiB
Python
import threading
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from collections.abc import MutableMapping
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class ImmutableKVCache(MutableMapping):
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"""
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Guarantees that the factory will be called for each key once, and
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only once.
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"""
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def __init__(self, factory):
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self.factory = factory # user-provided factory function
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self.lock = threading.Lock() # guards factory_calls
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self.factory_calls = {} # per-key factory condition variables
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self.cache = {} # result cache, indexed by key
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super().__init__()
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def __getitem__(self, key):
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if key in self.cache:
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return self.cache[key]
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# we need to call factory. First grab the main lock and the per-key CV.
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factory_calls = None
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creation_thr = False
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with self.lock:
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if key in self.cache:
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return self.cache[key]
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if key not in self.factory_calls:
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creation_thr = True
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self.factory_calls[key] = {"cv": threading.Condition(), "is_done": False, "error": None}
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factory_calls = self.factory_calls[key]
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# with the CV, create the value (or wait for it to be created)
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cv = factory_calls["cv"]
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with cv:
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if creation_thr:
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try:
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self.cache[key] = self.factory(key)
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except Exception as e:
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factory_calls["error"] = e
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factory_calls["is_done"] = True
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cv.notify_all()
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else:
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""" wait for the value to be available """
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while not factory_calls["is_done"]:
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cv.wait()
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with self.lock:
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if key in self.factory_calls:
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del self.factory_calls[key]
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return self.cache[key]
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def __iter__(self):
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""" weak iter, don't call factory """
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return self.cache.__iter__()
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def __len__(self):
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return self.cache.__len__()
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def __contains__(self, key):
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""" weak contain - don't call factory """
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return self.cache.__contains__(key)
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def __delitem__(self, key):
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del self.cache[key]
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def __setitem__(self, key, value):
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""" unsupported """
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raise NotImplementedError
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