This is a bit of a blunt hammer, but this hooks an action to clear the
HTTP disk cache into the existing Clear Cache action. Upon invocation,
it stops all existing cache entries from making further progress, and
then deletes the entire cache index and all cache files.
In the future, we will of course want more fine-grained control over
cache deletion, e.g. via an about:history page.
This adds a disk cache for HTTP responses received from the network. For
now, we take a rather conservative approach to caching. We don't cache a
response until we're 100% sure it is cacheable (there are heuristics we
can implement in the future based on the absence of specific headers).
The cache is broken into 2 categories of files:
1. An index file. This is a SQL database containing metadata about each
cache entry (URL, timestamps, etc.).
2. Cache files. Each cached response is in its own file. The file is an
amalgamation of all info needed to reconstruct an HTTP response. This
includes the status code, headers, body, etc.
A cache entry is created once we receive the headers for a response. The
index, however, is not updated at this point. We stream the body into
the cache entry as it is received. Once we've successfully cached the
entire body, we create an index entry in the database. If any of these
steps failed along the way, the cache entry is removed and the index is
left untouched.
Subsequent requests are checked for cache hits from the index. If a hit
is found, we read just enough of the cache entry to inform WebContent of
the status code and headers. The body of the response is piped to WC via
syscalls, such that the transfer happens entirely in the kernel; no need
to allocate the memory for the body in userspace (WC still allocates a
buffer to hold the data, of course). If an error occurs while piping the
body, we currently error out the request. There is a FIXME to switch to
a network request.
Cache hits are also validated for freshness before they are used. If a
response has expired, we remove it and its index entry, and proceed with
a network request.
