Remove invalid items from the file cache

[kkaefer]

In https://github.com/mapbox/mapbox-gl-native/pull/6176, we attempted implementing eviction of invalid resources from the cache. I still believe this is a valid feature. Invalid resources could end up in the cache in the following scenarios:

• Server bug leads to invalid response data being sent
• Proxy server/captive portal intercepts request and returns data that isn't expected
• Some other failure occurs during data transit, making the data unreadable

Our current workflow works like this:

• We request data from the server
• Once we get a response, we put it into the cache immediately
• We then pass on the data to the code that processes the data (e.g. decompress the image, parse the vector tile)

This means that we cache the data before we know that it's valid/parseable. Subsequent loads will fetch the invalid data from the cache and fail at parsing again. Instead, we should remove this data from the cache, and attempt to re-fetch the data.

Possible caveats are:

• We need good tests for this, since evicting invalid resources will likely be a rarely exercised code path
• We'll have to make sure we don't enter a loop that in quick succession requests + deletes the same item over and over again. An exponential back off (like we have for failing requests) could work here.

[stale[bot]]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.

[chloekraw]

This is still a priority

[stale[bot]]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.