Respect the base's constraint for extra-ed package

Author: uranusjr

news/10233.bugfix.rst

@@ -0,0 +1,3 @@
+New resolver: When a package is specified with extras in constraints, and with
+extras in non-constraint requirements, the resolver now correctly identifies the
+constraint's existence and avoids backtracking.

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

@@ -143,6 +143,21 @@ def get_preference(
             identifier,
         )
 
+    def _get_constraint(self, identifier: str) -> Constraint:
+        if identifier in self._constraints:
+            return self._constraints[identifier]
+
+        # HACK: Theoratically we should check whether this identifier is a valid
+        # "NAME[EXTRAS]" format, and parse out the name part with packaging or
+        # some regular expression. But since pip's resolver only spits out
+        # three kinds of identifiers: normalized PEP 503 names, normalized names
+        # plus extras, and Requires-Python, we can cheat a bit here.
+        name, open_bracket, _ = identifier.partition("[")
+        if open_bracket and name in self._constraints:
+            return self._constraints[name]
+
+        return Constraint.empty()
+
     def find_matches(
         self,
         identifier: str,
@@ -169,7 +184,7 @@ def _eligible_for_upgrade(name: str) -> bool:
         return self._factory.find_candidates(
             identifier=identifier,
             requirements=requirements,
-            constraint=self._constraints.get(identifier, Constraint.empty()),
+            constraint=self._get_constraint(identifier),
             prefers_installed=(not _eligible_for_upgrade(identifier)),
             incompatibilities=incompatibilities,
         )

tests/functional/test_new_resolver.py

@@ -2009,3 +2009,21 @@ def test_new_resolver_file_url_normalize(script, format_dep, format_input):
         format_input(lib_a), lib_b,
     )
     script.assert_installed(lib_a="1", lib_b="1")
+
+
+def test_new_resolver_dont_backtrack_on_extra_if_base_constrained(script):
+    create_basic_wheel_for_package(script, "dep", "1.0")
+    create_basic_wheel_for_package(script, "pkg", "1.0", extras={"ext": ["dep"]})
+    create_basic_wheel_for_package(script, "pkg", "2.0", extras={"ext": ["dep"]})
+    constraints_file = script.scratch_path / "constraints.txt"
+    constraints_file.write_text("pkg==1.0")
+
+    result = script.pip(
+        "install",
+        "--no-cache-dir", "--no-index",
+        "--find-links", script.scratch_path,
+        "--constraint", constraints_file,
+        "pkg[ext]",
+    )
+    assert "pkg-2.0" not in result.stdout, "Should not try 2.0 due to constraint"
+    script.assert_installed(pkg="1.0", dep="1.0")