dstufft is nervous about blowing a single-char option on something that will usually be copied and pasted anyway. We can always put it back later if it proves to be a pain.
* 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* Remove test_download_hashes, which is obsolete.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.
We purposely keep it off the CLI for now. optparse isn't really geared to expose interspersed args and options, so a more heavy-handed approach will be necessary to support things like `pip install SomePackage --sha256=abcdef... OtherPackage --sha256=012345...`.
This adds constraints files. Like requirements files constraints files
control what version of a package is installed, but unlike
requirements files this doesn't itself choose to install the package.
This allows things that aren't explicitly desired to be constrained if
and only if they are installed.
