From eb548fc9a3180aebeff088099f5ce42381f97bf2 Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Tue, 1 Jun 2021 03:19:50 +0800
Subject: [PATCH] Rework resolution ordering to consider "depth"

---
 news/9455.feature.rst                         |  2 +
 .../resolution/resolvelib/provider.py         | 70 ++++++++-----------
 2 files changed, 32 insertions(+), 40 deletions(-)
 create mode 100644 news/9455.feature.rst

diff --git a/news/9455.feature.rst b/news/9455.feature.rst
new file mode 100644
index 00000000000..4008a65c89a
--- /dev/null
+++ b/news/9455.feature.rst
@@ -0,0 +1,2 @@
+New resolver: The order of dependencies resolution has been tweaked to traverse
+the dependency graph in a more depth-first approach.
diff --git a/src/pip/_internal/resolution/resolvelib/provider.py b/src/pip/_internal/resolution/resolvelib/provider.py
index 0be58fd3ba8..c5c65a5320c 100644
--- a/src/pip/_internal/resolution/resolvelib/provider.py
+++ b/src/pip/_internal/resolution/resolvelib/provider.py
@@ -59,6 +59,7 @@ def __init__(
         self._ignore_dependencies = ignore_dependencies
         self._upgrade_strategy = upgrade_strategy
         self._user_requested = user_requested
+        self._known_depths = {key: 1.0 for key in user_requested}
 
     def identify(self, requirement_or_candidate):
         # type: (Union[Requirement, Candidate]) -> str
@@ -78,47 +79,36 @@ def get_preference(
 
         Currently pip considers the followings in order:
 
-        * Prefer if any of the known requirements points to an explicit URL.
-        * If equal, prefer if any requirements contain ``===`` and ``==``.
-        * If equal, prefer if requirements include version constraints, e.g.
-          ``>=`` and ``<``.
-        * If equal, prefer user-specified (non-transitive) requirements, and
-          order user-specified requirements by the order they are specified.
+        * Prefer if any of the known requirements is "direct", e.g. points to an
+          explicit URL.
+        * If equal, prefer if any requirement is "pinned", i.e. contains
+          operator ``===`` or ``==``.
+        * If equal, calculate an approximate "depth" and resolve requirements
+          closer to the user-specified requirements first.
+        * Order user-specified requirements by the order they are specified.
+        * If equal, prefers "non-free" requirements, i.e. contains at least one
+          operator, such as ``>=`` or ``<``.
         * If equal, order alphabetically for consistency (helps debuggability).
         """
+        lookups = (r.get_candidate_lookup() for r, _ in information[identifier])
+        candidate, ireqs = zip(*lookups)
+        operators = [
+            specifier.operator
+            for specifier_set in (ireq.specifier for ireq in ireqs if ireq)
+            for specifier in specifier_set
+        ]
+
+        direct = candidate is not None
+        pinned = any(op[:2] == "==" for op in operators)
+        free = bool(operators)
+
+        parent_depths = (
+            self._known_depths.get(parent.name, float("inf")) if parent else 0.0
+            for _, parent in information[identifier]
+        )
+        inferred_depth = min(d for d in parent_depths if d is not None) + 1.0
+        depth = self._known_depths[identifier] = inferred_depth
 
-        def _get_restrictive_rating(requirements):
-            # type: (Iterable[Requirement]) -> int
-            """Rate how restrictive a set of requirements are.
-
-            ``Requirement.get_candidate_lookup()`` returns a 2-tuple for
-            lookup. The first element is ``Optional[Candidate]`` and the
-            second ``Optional[InstallRequirement]``.
-
-            * If the requirement is an explicit one, the explicitly-required
-              candidate is returned as the first element.
-            * If the requirement is based on a PEP 508 specifier, the backing
-              ``InstallRequirement`` is returned as the second element.
-
-            We use the first element to check whether there is an explicit
-            requirement, and the second for equality operator.
-            """
-            lookups = (r.get_candidate_lookup() for r in requirements)
-            cands, ireqs = zip(*lookups)
-            if any(cand is not None for cand in cands):
-                return 0
-            spec_sets = (ireq.specifier for ireq in ireqs if ireq)
-            operators = [
-                specifier.operator for spec_set in spec_sets for specifier in spec_set
-            ]
-            if any(op in ("==", "===") for op in operators):
-                return 1
-            if operators:
-                return 2
-            # A "bare" requirement without any version requirements.
-            return 3
-
-        rating = _get_restrictive_rating(r for r, _ in information[identifier])
         order = self._user_requested.get(identifier, float("inf"))
 
         # HACK: Setuptools have a very long and solid backward compatibility
@@ -129,9 +119,9 @@ def _get_restrictive_rating(requirements):
         # delaying Setuptools helps reduce branches the resolver has to check.
         # This serves as a temporary fix for issues like "apache-airlfow[all]"
         # while we work on "proper" branch pruning techniques.
-        delay_this = identifier == "setuptools"
+        delayed = identifier == "setuptools"
 
-        return (delay_this, rating, order, identifier)
+        return (delayed, not direct, not pinned, depth, order, free, identifier)
 
     def find_matches(
         self,