Default behavior is to fall back to an alternative backend
if the uBO-selected one is not available. However there will be
no fall back when the `cacheStorageAPI` is set to one specific
backend by the user.
Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/409
By default `indexedDB` is used in Firefox for purpose of cache storage
backend.
This commit allows to force the use of `browser.storage.local` instead
as cache storage backend. For this to happen, set `cacheStorageAPI` to
`browser.storage.local` in advanced settings.
Additionally, should `indexedDB` not be available for whatever reason,
uBO will automatically fallback to `browser.storage.local`.
The motivation is to address the higher peak memory usage at launch
time with 3rd-gen HNTrie when a selfie was present.
The selfie generation prior to this change was to collect all
filtering data into a single data structure, and then to serialize
that whole structure at once into storage (using JSON.stringify).
However, HNTrie serialization requires that a large UintArray32 be
converted into a plain JS array, which itslef would be indirectly
converted into a JSON string. This was the main reason why peak
memory usage would be higher at launch from selfie, since the JSON
string would need to be wholly unserialized into JS objects, which
themselves would need to be converted into more specialized data
structures (like that Uint32Array one).
The solution to lower peak memory usage at launch is to refactor
selfie generation to allow a more piecemeal approach: each filtering
component is given the ability to serialize itself rather than to be
forced to be embedded in the master selfie. With this approach, the
HNTrie buffer can now serialize to its own storage by converting the
buffer data directly into a string which can be directly sent to
storage. This avoiding expensive intermediate steps such as
converting into a JS array and then to a JSON string.
As part of the refactoring, there was also opportunistic code
upgrade to ES6 and Promise (eventually all of uBO's code will be
proper ES6).
Additionally, the polyfill to bring getBytesInUse() to Firefox has
been revisited to replace the rather expensive previous
implementation with an implementation with virtually no overhead.
Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/399
The advanced setting `cacheStorageAPI` has been added to allow
a user to force the use of IndexedDB as cache storage. Set to
`IndexedDB` to force use of IndexedDB. Default to `unset`.
