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[WIP] pack: Clear all non-current entries after pack #162

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[WIP] pack: Clear all non-current entries after pack #162

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pack: Clear all non-current entries after pack
navytux Jul 29, 2020
69c5609
fixup! pack: Clear all non-current entries after pack
navytux Jul 31, 2020
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11 changes: 10 additions & 1 deletion src/ZEO/ClientStorage.py
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Original file line number Diff line number Diff line change
Expand Up @@ -559,7 +559,16 @@ def pack(self, t=None, referencesf=None, wait=1, days=0):
if t is None:
t = time.time()
t = t - (days * 86400)
return self._call('pack', t, wait)
ret = self._call('pack', t, wait)
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# remove all non-current entries from the cache.
# This way we make sure that loadBefore with before < packtime, won't
# return data from the cache, instead of returning "no data" if requested object
# has current revision >= packtime.
# By clearing all noncurrent entries we might remove more data from the
# cache than is strictly necessary, but since access to noncurrent data
# is seldom, that should not cause problems in practice.
self._cache.clearAllNonCurrent()
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Personally, I would like to see a specific direct test case for this.

While I know that there was a generic ZODB test failure, it's not obvious that this is the fix for it. What I mean is, the symptom (a failure to raise a particular exception) and the fix (making sure there are no non-current items in the cache) aren't necessarily clearly linked. I could imagine that a slight refactoring of the ZODB test that introduces additional clients or changes the timing or whatever might let it pass without this line of code being present.

I would suggest that a direct test for this would (a) establish non-current items in the cache and verify their existence in cache.noncurrent; (b) pack; (c) verify that cache.noncurrent is empty or equivalent.

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I agree, especially in the light of the bug you point out that the cache is not invalidated on other clients.

return ret

def store(self, oid, serial, data, version, txn):
"""Storage API: store data for an object."""
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15 changes: 15 additions & 0 deletions src/ZEO/cache.py
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Expand Up @@ -246,6 +246,21 @@ def clear(self):
self.f.truncate()
self._initfile(ZEC_HEADER_SIZE)

# clearAllNonCurrent removes all non-current entries from the cache.
def clearAllNonCurrent(self):
with self._lock:
f = self.f
for oid, tidofs in self.noncurrent.items():
for (tid, ofs) in tidofs.items():
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f.seek(ofs)
status = f.read(1)
assert status == b'a', (ofs, f.tell(), oid, tid)
f.seek(ofs)
f.write(b'f')
self._len -= 1

self.noncurrent.clear()

##
# Scan the current contents of the cache file, calling `install`
# for each object found in the cache. This method should only
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28 changes: 28 additions & 0 deletions src/ZEO/tests/test_cache.py
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Expand Up @@ -253,6 +253,34 @@ def test_clear_zeo_cache(self):
self.assertEqual(cache.load(n3), None)
self.assertEqual(cache.loadBefore(n3, n2), None)

def testClearAllNonCurrent(self):
cache = self.cache
cache.store(p64(1), n1, n2, b'1@1')
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One space following a comma, please. Please don't introduce extra horizontal whitespace for the purposes of vertical alignment. It's very fiddly and leads to unnecessary multi-line changes in the future.

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@navytux navytux Jul 31, 2020 •
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argh, ok. Even though personally I adhere to the oppsite, let's not argue on this and correct it here to be without vertical alignment. For the reference git diff -w and git gui blame help to go through this whitespace-only changes to the origin of where a line was originally modified.

cache.store(p64(1), n2, n3, b'1@2')
cache.store(p64(1), n3, None, b'1')
cache.store(p64(2), n2, n3, b'2@2')
cache.store(p64(2), n3, None, b'2')

eq = self.assertEqual
eq(len(cache), 5)
eq(cache.load(p64(1)), (b'1', n3))
eq(cache.loadBefore(p64(1), n3), (b'1@2', n2, n3))
eq(cache.loadBefore(p64(1), n2), (b'1@1', n1, n2))
eq(cache.loadBefore(p64(1), n1), None)
eq(cache.load(p64(2)), (b'2', n3))
eq(cache.loadBefore(p64(2), n3), (b'2@2', n2, n3))
eq(cache.loadBefore(p64(2), n2), None)

cache.clearAllNonCurrent()
eq(len(cache), 2)
eq(cache.load(p64(1)), (b'1', n3))
eq(cache.loadBefore(p64(1), n3), None)
eq(cache.loadBefore(p64(1), n2), None)
eq(cache.loadBefore(p64(1), n1), None)
eq(cache.load(p64(2)), (b'2', n3))
eq(cache.loadBefore(p64(2), n3), None)
eq(cache.loadBefore(p64(2), n2), None)

def testChangingCacheSize(self):
# start with a small cache
data = b'x'
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