Cache AlreadyInstalledCandidate

Since the "Requirement already satisfied" message is printed during
candidate preparation, instantiating the candidate multiple times result
in excessive logging during intensive backtracking. By caching the
already-installed candidates, each package is only prepared, and thus
only logged once.

Author: uranusjr

news/9117.bugfix.rst

@@ -0,0 +1,2 @@
+New resolver: The "Requirement already satisfied" log is not printed only once
+for each package during resolution.

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

@@ -96,6 +96,8 @@ def __init__(
 
         self._link_candidate_cache = {}  # type: Cache[LinkCandidate]
         self._editable_candidate_cache = {}  # type: Cache[EditableCandidate]
+        self._installed_candidate_cache = {
+        }  # type: Dict[str, AlreadyInstalledCandidate]
 
         if not ignore_installed:
             self._installed_dists = {
@@ -117,7 +119,11 @@ def _make_candidate_from_dist(
         template,  # type: InstallRequirement
     ):
         # type: (...) -> Candidate
-        base = AlreadyInstalledCandidate(dist, template, factory=self)
+        if dist.key in self._installed_candidate_cache:
+            base = self._installed_candidate_cache[dist.project_name]
+        else:
+            base = AlreadyInstalledCandidate(dist, template, factory=self)
+            self._installed_candidate_cache[dist.key] = base
         if extras:
             return ExtrasCandidate(base, extras)
         return base