Record pip in INSTALLER file

[ncoghlan]

When installing from a wheel archive (whether directly, or by building a wheel file from a source format), generate and install an INSTALLER metadata file indicating that "pip" handled the installation.

This initial implementation is hardcoded to use "pip\n" as the identifier.

[domenkozar]

@ncoghlan would it make sense to also record the version number?

[pfmoore]

@domenkozar The PR is implementing the INSTALLER file as defined in PEP 376. See https://www.python.org/dev/peps/pep-0376/#installer which doesn't allow for a version number in the file. (That's not to say it's impossible, as PEP 376 is at best incompletely supported by tools, and could be amended, but interoperability is a tricky subject, and I'd say we should follow the published standard even if it's not widely used in preference to creating our own subtly incompatible variant).

[domenkozar]

@pfmoore understood.

[ncoghlan]

The main eventual use case here is to avoid pip accidentally stepping on system packages when run with root privileges, since you can foul up a Linux system pretty badly doing that. The version of the installer doesn't matter for that purpose, just the name - we'd like pip to be able to give a warning when asked to upgrade or remove something it didn't install.

Longer term, something more like the metadata file in the wheel format would likely be nicer, but the PEP 376 scheme has the advantage of already being defined and partially adopted (pip writes RECORD in order to handle uninstalls).

[szepeviktor]

How is it possible now to distinguish between pip-installed and Debian python packages?

[ncoghlan]

It's unfortunately not reliable to do so yet, as distros haven't set policies for dealing with INSTALLER files (e.g. if the system package is using pip to do the installation during the package build process, INSTALLER may still claim "pip").

There's a current discussion on distutils-sig about that, and once PyPA has articulated how we'd like distros to handle the file, then distros can update their policies accordingly.

[szepeviktor]

Thank you very much for clarifying these.