Improve self-check logic

[techtonik]

What's the problem this feature will solve?

Simplify the code to give less space to bugs.

From #5746 (comment). The self-checking logic is strange:

• It uses one global state file outside of virtualenv for caching pip version checks.
• It uses PackageFinder component where it could just query some URL with latest tag.
• It checks installed version instead of checking itself.

Describe the solution you'd like

• Cache pip version check state local to virtualenv
• Implement latest stable API query in PyPI API for fetching latest and stable versions pypi/warehouse#4663
• Use simple URL query instead of PackageFinder import for version check

[pradyunsg]

What PackageFinder does is it encapsulates the logic needed to find all the available versions of a package (in this case, pip itself) -- among a lot of other things.

Cache pip version check state local to virtualenv

We removed this functionality in #5419. There is only one latest version of pip, so we don't need to check upstream unless it changes, which is where the caching comes in.

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I don't think anything would change on this front unless Warehouse gets a new endpoint for fetching the latest version number of a package -- information that is already exposed in a different manner already.

[techtonik]

Why pip needs to find all available versions of itself? It should only check its own version, exposed through at attribute somewhere. PackageFinder complicates startup time.

[cjerdonek]

Why pip needs to find all available versions of itself? ... PackageFinder complicates startup time.

FWIW, with changes recently merged (summarized e.g. in this comment here), pip now no longer imports the PackageFinder machinery unless it's a command that already needs to create a network session (specifically download, install, list, search, uninstall, and wheel). So startup is now simpler in the general case.

Also (and this part is the way it was before), for the record pip only does the self-check for the following four commands: download, install, list, and wheel (the commands with the "index group" options like --no-index).

[pradyunsg]

Indeed.

I don't think anything would change on this front unless Warehouse gets a new endpoint for fetching the latest version number of a package -- information that is already exposed in a different manner already.

IMO it is unlikely that PyPI/Warehouse/PEP 503 would gain such an endpoint. It adds another endpoint that has to be cached and the only location where this endpoint would be used by pip is for self-checking its own version. None the less, we can revisit this if and when such an endpoint is added to Warehouse.

Since there's nothing actionable for on this today, I'll go ahead and close this. Please feel free to leave a comment if someone disagrees with my assessment though.