Fix cache handling bug with validation callback #454
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The validation_callback invocation in CacheDownloader forced redownloads even on a cache hit, in order to pass the response content to a parser. After experimenting with several solutions, the easiest is to abstract out `validation_callback` into an arbitrary `response_ok` call which gets passed into the request fetching loop. `response_ok` is then a wrapper over the validation callback which can intentionally answer "yes" eagerly if a response evaluates to a cache hit -- a response which is a '200 Ok' and contains a Last-Modified header which scans as a hit should be treated as 'ok' regardless of its content. In addition to the code changes, a regression test is included, which is demonstrated to fail on the current release.
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The validation_callback invocation in CacheDownloader forced redownloads even on a cache hit, in order to pass the response content to a parser.
After experimenting with several solutions, the easiest is to abstract out
validation_callbackinto an arbitraryresponse_okcall which gets passed into the request fetching loop.response_okis then a wrapper over the validation callback which can intentionally answer "yes" eagerly if a response evaluates to a cache hit -- a response which is a '200 Ok' and contains a Last-Modified header which scans as a hit should be treated as 'ok' regardless of its content.In addition to the code changes, a regression test is included, which is demonstrated to fail on the current release.
resolves #453