Backtrack on inconsistent candidate

[uranusjr]

Currently the resolver only allow one way for the provider to trigger backtracking, by supplying requirements (dependencies) that would empty a criterion’s candidate list. This prevents some oppertunities for applications to reject candidates in later stages. pip, for example, wants to reject a candidate with invalid/inconsistent metadata, but there’s currently no way for it to do this.

Except there is already a hook for that: is_satisfied_by(). This hook is already checked during pinning:

resolvelib/src/resolvelib/resolvers.py

Lines 205 to 221 in 4d6963a

	for candidate in criterion.candidates:
	try:
	criteria = self._get_criteria_to_update(candidate)
	except RequirementsConflicted as e:
	causes.append(e.criterion)
	continue
	# Check the newly-pinned candidate actually works. This should
	# always pass under normal circumstances, but in the case of a
	# faulty provider, we will raise an error to notify the implementer
	# to fix find_matches() and/or is_satisfied_by().
	satisfied = all(
	self._p.is_satisfied_by(r, candidate)
	for r in criterion.iter_requirement()
	)
	if not satisfied:
	raise InconsistentCandidate(candidate, criterion)

The first block catches RequirementsConflicted, which would be used to trigger backtracking. The second calls is_satisfied_by(), but expects it to always return True (otherwise raises InconsistentCandidate). If we modify the block to instead continue (i.e. give up on the candidate) when is_satisfied() returns false, the provider can then use it to validate the candidate and signal any latent issues that cannot be found previously.

This would help issues such as pypa/pip#9203, and provide the companion fix needed for pypa/pip#9199.

[uranusjr]

This may also help pypa/pip#9246.

[pradyunsg]

We ended up implementing this in pip's logic for this stuff.

[pradyunsg]

See pypa/pip#9289 for the PR.