Resolver meeting

17 Feb 2020

Participants: Paul, Pradyun, Tzu-Ping

Some initial thoughts:

    * Existing upgrade strategies may not map nicely to new resolver
        * Define new upgrade strategies that match new resolver ideas
        * For new resolver, map existing strategies to "best fit" new ones as aliases
        * Deprecate old strategies when used with new resolver (longer term)
    * Without `--upgrade`, we can just return installed versions as the only candidate in `find_distributions`
        * Will make the solve faster, but might make reporting more difficult
        * Cargo(?) does a two-pass resolution, the second pass only to collect errors. Could be a good idea?
    * Maybe possible to do similar for other options/upgrade strategies?

Two scenarios:
	* I want to upgrade this, do the minimal possible to make it work
		This is usually what people want when they `pip install -U` globally
	*  “Upgrade all” (which pip does not have right now)
		This is usually what people want when they `pip install -U` in a virtualenv

 * pip install -U foo
   * Tells pip “Try what you need to make this work”
   * Implication: If this can’t work, error out
   * pip can suggest `pip install -U foo bar` when it errors out to help the user
   * don't automatically upgrade going "up" the dependency graph.

* Add a new --upgrade-strategy that is even more conservative than only-if-needed that reports (instead of upgrade) if pip needs to upgrade
	* Probably doesn’t need to, it’s the user’s responsibility
	
iterating on upgrade strategies

--no-deps: for people doing resolution externally to pip

Environment metadata issues:
	No information about "requested" extras
          * https://github.com/pypa/packaging-problems/issues/215
          * Let's defer this; it'd be nice to fix this but it'll be very tricky to do so on the time-boxed nature.
	REQUESTED (https://www.python.org/dev/peps/pep-0376/#requested)
		Pradyun: I want to flip the file, to indicate "i was automatically installed"
                  * Marking existing installed packages as "requested".
                  * Trimming installed-as-dependency packages.
	Pradyun: How about we combine them both? "REQUESTED" serves as our manifest