* Add --require-hashes option. This is handy in deployment scripts to force application authors to hash their requirements. It is also a convenient way to get pip to show computed hashes for a virgin, unhashed requirements file. Eventually, additions to `pip freeze` should fill a superset of this use case.
* In --require-hashes mode, at least one hash is required to match for each requirement.
* Option-based requirements (--sha256=...) turn on --require-hashes mode implicitly.
* Internet-derived URL-based hashes are "necessary but not sufficient": they do not satisfy --require-hashes mode when they match, but they are still used to guard against transmission errors.
* Other URL-based requirements (#md5=...) are treated just like flag-based ones, except they don't turn on --require-hashes.
* Complain informatively, with the most devastating errors first so you don't chase your tail all day only to run up against a brick wall at the end. This also means we don't complain that a hash is missing, only for the user to find, after fixing it, that we have no idea how to even compute a hash for that type of requirement.
* Complain about unpinned requirements when hash-checking mode is on, lest they cause the user surprise later.
* Complain about missing hashes.
* Complain about requirement types we don't know how to hash (like VCS ones and local dirs).
* Have InstallRequirement keep its original Link around (original_link) so we can differentiate between URL hashes from requirements files and ones downloaded from the (untrustworthy) internet.
* Remove test_download_hashes, which is obsolete. Similar coverage is provided in test_utils.TestHashes and the various hash cases in test_req.py.
* Deprecates the --download-cache option & removes the download
cache code.
* Removes the in memory page cache on the index
* Uses CacheControl to cache all cacheable HTTP requests to the
filesystem.
* Properly handles CacheControl headers for unconditional
caching.
* Will use ETag and Last-Modified headers to attempt to do a
conditional HTTP request to speed up cache misses and turn
them into cache hits.
* Removes some concurrency unsafe code in the download cache
accesses.
* Uses a Cache-Control request header to limit the maximum
length of time a cache is valid for.
* Adds pip.appdirs to handle platform specific application
directories such as cache, config, data, etc.