Merge pull request #3375 from dstufft/bundling-policy

Update our policy on bundling

pip/_vendor/README.rst

@@ -2,11 +2,84 @@ Policy
 ======
 
 Vendored libraries should not be modified except as required to actually
-successfully vendor them.
+successfully vendor them. Only released copies of libraries from PyPI may be
+vendored and the vendored version must be reflected in
+``pip/_vendor/vendor.txt``. In addition, any modifications must be noted in
+``pip/_vendor/README.rst``.
+
+
+Rationale
+---------
+
+Historically pip has not had any dependencies except for setuptools itself,
+choosing instead to implement any functionality it needed to prevent needing
+a dependency. However, starting with pip 1.5 we begun to replace code that was
+implemented inside of pip with reusable libraries from PyPI. This brought the
+typical benefits of reusing libraries instead of reinventing the wheel like
+higher quality and more battle tested code, centralization of bug fixes
+(particularly security sensitive ones), and better/more features for less work.
+
+However, there is several issues with having dependencies in the traditional
+way (via ``install_requires``) for pip. These issues are:
+
+* Fragility. When pip depends on another library to function then if for
+  whatever reason that library either isn't installed or an incompatible
+  version is installed then pip ceases to function. This is of course true for
+  all Python applications, however for every application *except* for pip the
+  way you fix it is by re-running pip. Obviously when pip can't run you can't
+  use pip to fix pip so you're left having to manually resolve dependencies and
+  installing them by hand.
+
+* Making other libraries uninstallable. One of pip's current dependencies is
+  the requests library, for which pip requires a fairly recent version to run.
+  If pip dependended on requests in the traditional manner then we'd end up
+  needing to either maintain compatibility with every version of requests that
+  has ever existed (and will ever exist) or some subset of the versions of
+  requests available will simply become uninstallable depending on what version
+  of pip you're using. This is again a problem that is technically true for all
+  Python applications, however the nature of pip is that you're likely to have
+  pip installed in every single environment since it is installed by default
+  in Python, in pyvenv, and in virtualenv.
+
+* Security. On the surface this is oxymoronic since traditionally vendoring
+  tends to make it harder to update a dependent library for security updates
+  and that holds true for pip. However given the *other* reasons that exist for
+  pip to avoid dependencies the alternative (and what was done historically) is
+  for pip to reinvent the wheel itself. This led to pip having implemented
+  its own HTTPS verification routines to work around the lack of ssl
+  validation in the Python standard library which ended up having similar bugs
+  to validation routine in requests/urllib3 but which had to be discovered and
+  fixed independently. By reusing the libraries, even though we're vendoring,
+  we make it easier to keep pip secure by relying on the great work of our
+  dependencies *and* making it easier to actually fix security issues by simply
+  pulling in a newer version of the dependencies.
+
+Many downstream redistributors have policies against this kind of bundling and
+instead opt to patch the software they distribute to debundle it and make it
+rely on the global versions of the software that they already have packaged
+(which may have its own patches applied to it). We (the pip team) would prefer
+it if pip was *not* debundled in this manner due to the above reasons and
+instead we would prefer it if pip would be left intact as it is now. The one
+exception to this, is it is acceptable to remove the
+``pip/_vendor/requests/cacert.pem`` file provided you ensure that the
+``ssl.get_default_verify_paths().cafile`` API returns the correct CA bundle for
+your system. This will ensure that pip will use your system provided CA bundle
+instead of the copy bundled with pip.
+
+In the longer term, if someone has a solution to the above problems, other than
+the bundling method we currently use, that doesn't add additional problems that
+are unreasonable then we would be happy to consider, and possibly switch to
+said method.
+
+
+_markerlib and pkg_resources
+----------------------------
+
+_markerlib and pkg_resources has been pulled in from setuptools 19.4
 
 
 Modifications
-=============
+-------------
 
 * html5lib has been modified to import six from pip._vendor
 * pkg_resources has been modified to import _markerlib from pip._vendor
@@ -14,26 +87,26 @@ Modifications
 * CacheControl has been modified to import it's dependencies from pip._vendor
 
 
-_markerlib and pkg_resources
-============================
+Debundling
+----------
 
-_markerlib and pkg_resources has been pulled in from setuptools 19.4
+As mentioned in the rationale we, the pip team, would prefer it if pip was not
+debundled (other than optionally ``pip/_vendor/requests/cacert.pem``) and that
+pip was left intact. However, if you insist on doing so we have a
+semi-supported method that we do test in our CI but which requires a bit of
+extra work on your end to make it still solve the problems from above.
 
+1. Delete everything in ``pip/_vendor/`` **except** for
+   ``pip/_vendor/__init__.py``.
 
-Note to Downstream Distributors
-===============================
+2. Generate wheels for each of pip's dependencies (and any of their
+   dependencies) using your patched copies of these libraries. These must be
+   placed somewhere on the filesystem that pip can access, by default we will
+   assume you've placed them in ``pip/_vendor``.
 
-Libraries are vendored/bundled inside of this directory in order to prevent
-end users from needing to manually install packages if they accidently remove
-something that pip depends on.
+3. Modify ``pip/_vendor/__init__.py`` so that the ``DEBUNDLED`` variable is
+   ``True``.
 
-All bundled packages exist in the ``pip._vendor`` namespace, and the versions
-(fetched from PyPI) that we use are located in ``vendor.txt``. If you wish
-to debundle these you can do so by either deleting everything in
-``pip/_vendor`` **except** for ``pip/_vendor/__init__.py`` or by running
-``PIP_NO_VENDOR_FOR_DOWNSTREAM=1 setup.py install``. No other changes should
-be required as the ``pip/_vendor/__init__.py`` file will alias the "real"
-names (such as ``import six``) to the bundled names (such as
-``import pip._vendor.six``) automatically. Alternatively if you delete the
-entire ``pip._vendor`` you will need to adjust imports that import from those
-locations.
+4. *(Optional)* If you've placed the wheels in a location other than
+   ``pip/_vendor/`` then modify ``pip/_vendor/__init__.py`` so that the
+   ``WHEEL_DIR`` variable points to the location you've placed them.