Prefer backtracking on dependencies involved in the most recent conflict

news/10479.feature.rst

@@ -0,0 +1 @@
+When backtracking during dependency resolution, prefer the dependencies which are involved in the most recent conflict. This can significantly reduce the amount of backtracking required.

news/resolvelib.vendor.rst

@@ -0,0 +1 @@
+Upgrade resolvelib to 0.8.0

noxfile.py

@@ -174,7 +174,7 @@ def vendoring(session: nox.Session) -> None:
     session.install("vendoring~=1.0.0")
 
     if "--upgrade" not in session.posargs:
-        session.run("vendoring", "sync", ".", "-v")
+        session.run("vendoring", "sync", "-v")
         return
 
     def pinned_requirements(path: Path) -> Iterator[Tuple[str, str]]:

src/pip/_internal/resolution/resolvelib/provider.py

@@ -66,12 +66,13 @@ def __init__(
     def identify(self, requirement_or_candidate: Union[Requirement, Candidate]) -> str:
         return requirement_or_candidate.name
 
-    def get_preference(
+    def get_preference(  # type: ignore
         self,
         identifier: str,
         resolutions: Mapping[str, Candidate],
         candidates: Mapping[str, Iterator[Candidate]],
         information: Mapping[str, Iterable["PreferenceInformation"]],
+        backtrack_causes: Sequence["PreferenceInformation"],
     ) -> "Preference":
         """Produce a sort key for given requirement based on preference.
 
@@ -132,11 +133,17 @@ def get_preference(
         # while we work on "proper" branch pruning techniques.
         delay_this = identifier == "setuptools"
 
+        # Prefer the causes of backtracking on the assumption that the problem
+        # resolving the dependency tree is related to the failures that caused
+        # the backtracking
+        backtrack_cause = self.is_backtrack_cause(identifier, backtrack_causes)
+
         return (
             not requires_python,
             delay_this,
             not direct,
             not pinned,
+            not backtrack_cause,
             inferred_depth,
             requested_order,
             not unfree,
@@ -195,3 +202,14 @@ def is_satisfied_by(self, requirement: Requirement, candidate: Candidate) -> boo
     def get_dependencies(self, candidate: Candidate) -> Sequence[Requirement]:
         with_requires = not self._ignore_dependencies
         return [r for r in candidate.iter_dependencies(with_requires) if r is not None]
+
+    @staticmethod
+    def is_backtrack_cause(
+        identifier: str, backtrack_causes: Sequence["PreferenceInformation"]
+    ) -> bool:
+        for backtrack_cause in backtrack_causes:
+            if identifier == backtrack_cause.requirement.name:
+                return True
+            if backtrack_cause.parent and identifier == backtrack_cause.parent.name:
+                return True
+        return False

src/pip/_vendor/resolvelib/__init__.py

@@ -11,7 +11,7 @@
     "ResolutionTooDeep",
 ]
 
-__version__ = "0.7.1"
+__version__ = "0.8.0"
 
 
 from .providers import AbstractProvider, AbstractResolver

src/pip/_vendor/resolvelib/providers.py

@@ -9,7 +9,14 @@ def identify(self, requirement_or_candidate):
         """
         raise NotImplementedError
 
-    def get_preference(self, identifier, resolutions, candidates, information):
+    def get_preference(
+        self,
+        identifier,
+        resolutions,
+        candidates,
+        information,
+        backtrack_causes,
+    ):
         """Produce a sort key for given requirement based on preference.
 
         The preference is defined as "I think this requirement should be
@@ -25,6 +32,8 @@ def get_preference(self, identifier, resolutions, candidates, information):
             Each value is an iterator of candidates.
         :param information: Mapping of requirement information of each package.
             Each value is an iterator of *requirement information*.
+        :param backtrack_causes: Sequence of requirement information that were
+            the requirements that caused the resolver to most recently backtrack.
 
         A *requirement information* instance is a named tuple with two members:
 

src/pip/_vendor/resolvelib/resolvers.py

@@ -99,7 +99,7 @@ def __init__(self, round_count):
 
 
 # Resolution state in a round.
-State = collections.namedtuple("State", "mapping criteria")
+State = collections.namedtuple("State", "mapping criteria backtrack_causes")
 
 
 class Resolution(object):
@@ -131,6 +131,7 @@ def _push_new_state(self):
         state = State(
             mapping=base.mapping.copy(),
             criteria=base.criteria.copy(),
+            backtrack_causes=base.backtrack_causes[:],
         )
         self._states.append(state)
 
@@ -185,6 +186,7 @@ def _get_preference(self, name):
                 self.state.criteria,
                 operator.attrgetter("information"),
             ),
+            backtrack_causes=self.state.backtrack_causes,
         )
 
     def _is_current_pin_satisfying(self, name, criterion):
@@ -335,7 +337,13 @@ def resolve(self, requirements, max_rounds):
         self._r.starting()
 
         # Initialize the root state.
-        self._states = [State(mapping=collections.OrderedDict(), criteria={})]
+        self._states = [
+            State(
+                mapping=collections.OrderedDict(),
+                criteria={},
+                backtrack_causes=[],
+            )
+        ]
         for r in requirements:
             try:
                 self._add_to_criteria(self.state.criteria, r, parent=None)
@@ -369,11 +377,13 @@ def resolve(self, requirements, max_rounds):
                 # Backtrack if pinning fails. The backtrack process puts us in
                 # an unpinned state, so we can work on it in the next round.
                 success = self._backtrack()
+                self.state.backtrack_causes[:] = [
+                    i for c in failure_causes for i in c.information
+                ]
 
                 # Dead ends everywhere. Give up.
                 if not success:
-                    causes = [i for c in failure_causes for i in c.information]
-                    raise ResolutionImpossible(causes)
+                    raise ResolutionImpossible(self.state.backtrack_causes)
             else:
                 # Pinning was successful. Push a new state to do another pin.
                 self._push_new_state()

src/pip/_vendor/vendor.txt

@@ -14,7 +14,7 @@ requests==2.26.0
     chardet==4.0.0
     idna==3.2
     urllib3==1.26.7
-resolvelib==0.7.1
+resolvelib==0.8.0
 setuptools==44.0.0
 six==1.16.0
 tenacity==8.0.1

tests/unit/resolution_resolvelib/test_provider.py

@@ -19,7 +19,7 @@ def build_requirement_information(
     install_requirement = install_req_from_req_string(name)
     # RequirementInformation is typed as a tuple, but it is a namedtupled.
     # https://github.com/sarugaku/resolvelib/blob/7bc025aa2a4e979597c438ad7b17d2e8a08a364e/src/resolvelib/resolvers.pyi#L20-L22
-    requirement_information: PreferenceInformation = RequirementInformation(
+    requirement_information: "PreferenceInformation" = RequirementInformation(
         requirement=SpecifierRequirement(install_requirement),  # type: ignore[call-arg]
         parent=parent,
     )
@@ -46,6 +46,7 @@ def test_provider_known_depths(factory: Factory) -> None:
         resolutions={},
         candidates={},
         information={root_requirement_name: root_requirement_information},
+        backtrack_causes=[],
     )
     assert provider._known_depths == {root_requirement_name: 1.0}
 
@@ -69,6 +70,7 @@ def test_provider_known_depths(factory: Factory) -> None:
             root_requirement_name: root_requirement_information,
             transative_requirement_name: transative_package_information,
         },
+        backtrack_causes=[],
     )
     assert provider._known_depths == {
         transative_requirement_name: 2.0,