pip: don't crash on cycles, fix them instead

pip does allow cycles and they are common in larger dependency trees. Instead of crashing on them, we should allow the user to deal with them.

As a result of this, it can happen that during the installation of individual dependencies, some dep(s) might be missing, which then can be fixed by including them manually or with a `--no-deps`

pkgs/fetchPipMetadata/src/fetch_pip_metadata/lock_file_from_report.py

@@ -143,14 +143,6 @@ def evaluate_requirements(env, reqs, dependencies, root_name, extras, seen):
     requirements to save space in the file.
     A circuit breaker is included to avoid infinite recursion in nix.
     """
-    if root_name in seen:
-        print(
-            f"fatal: cycle detected: {root_name} ({' '.join(seen)})",
-            file=sys.stderr,  # noqa: E501
-        )
-        sys.exit(1)
-    # we copy "seen", because we want to track cycles per
-    # tree-branch and the original would be visible for all branches.
     seen = seen.copy()
     seen.append(root_name)
 
@@ -160,10 +152,11 @@ def evaluate_requirements(env, reqs, dependencies, root_name, extras, seen):
     for req in reqs[root_name]:
         if (not req.marker) or evaluate_extras(req, extras, env):
             req_name = canonicalize_name(req.name)
-            dependencies[root_name].add(req_name)
-            evaluate_requirements(
-                env, reqs, dependencies, req_name, req.extras, seen
-            )  # noqa: 501
+            if req_name not in seen:
+                dependencies[root_name].add(req_name)
+                evaluate_requirements(
+                    env, reqs, dependencies, req_name, req.extras, seen
+                )  # noqa: 501
     return dependencies
 
 

pkgs/fetchPipMetadata/src/tests/test_evaluate_requirements.py

@@ -68,3 +68,23 @@ def test_platform_mismatch():
         seen=[],
     )
     assert result == dict(root_package=set())
+
+
+def test_cycle():
+    result = l.evaluate_requirements(
+        env={},
+        reqs=dict(
+            root_package={Requirement("foo")},
+            foo={Requirement("bar")},
+            bar={Requirement("foo")},
+        ),
+        dependencies=dict(),
+        root_name="root_package",
+        extras=None,
+        seen=[],
+    )
+    assert result == dict(
+        root_package={"foo"},
+        foo={"bar"},
+        bar=set(),
+    )