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Get a write lock to clear the composite caches. Fail gracefully if we can't get the lock.

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This commit is contained in:
Charles Haley 2021-05-05 16:46:05 +01:00
parent 7a07fcbc48
commit 224d3a68fa
1 changed files with 20 additions and 14 deletions
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34
src/calibre/db/cache.py
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@ -674,8 +674,23 @@ class Cache(object):
cover_as_data is True then as mi.cover_data.
'''
# Check if virtual_libraries_for_books rebuilt its cache. If it did then
# we must clear the composite caches so the new data can be taken into
# account. Clearing the caches requires getting a write lock, so it must
# be done outside of the closure of _get_metadata().
vl_cache_was_none = self.vls_for_books_cache is None
with self.safe_read_lock:
mi = self._get_metadata(book_id, get_user_categories=get_user_categories)
if vl_cache_was_none and self.vls_for_books_cache is not None:
try:
with self.write_lock:
self.clear_composite_caches()
except DowngradeLockError:
# We can't clear the composite caches because a read lock is set.
# As a consequence the value of a composite column that calls
# virtual_libraries() might be wrong. Oh well. Log and keep running.
print("Couldn't get write lock after vls_for_books_cache was loaded")
traceback.print_exc()
if get_cover:
if cover_as_data:
@ -2218,10 +2233,11 @@ class Cache(object):
if self.vls_for_books_cache_is_loading:
# We get here if resolving the books in a VL triggers another VL
# calculation. This can be 'real' recursion, in which case the
# eventual answer might be wrong. It can also be a
# search using a location of 'all' that causes a composite that
# references virtual libraries to be evaluated. If the composite
# isn't used in a VL then the eventual answer will be correct.
# eventual answer will be wrong. It can also be a search using
# a location of 'all' that causes evaluation of a composite that
# references virtual libraries. If the composite isn't used in a
# VL then the eventual answer will be correct because get_metadata
# will clear the caches.
return c
self.vls_for_books_cache_is_loading = True
libraries = self._pref('virtual_libraries', {})
@ -2234,16 +2250,6 @@ class Cache(object):
if book:
c[book].append(_('[Error in Virtual library {0}: {1}]').format(lib, str(e)))
self.vls_for_books_cache = {b:tuple(sorted(libs, key=sort_key)) for b, libs in c.items()}
# Clear the proxy_metadata composite caches so that any template
# evaluation returned during recursion avoidance (above) will be
# recomputed with correct data.
# Note that clear_composite_caches() normally uses a write lock. That
# won't work here because the call chain has already obtained a read
# lock that we can't upgrade. It is safe to call clear_composite_caches
# using the read lock because multiple threads will merely clear the
# cache multiple times, perhaps wasting a bit of time recomputing
# values. The GIL prevents corruption during the clearing process.
self._clear_composite_caches()
if not book_ids:
book_ids = self._all_book_ids()
# book_ids is usually 1 long. The loop will be faster than a comprehension