Add a high level test of weak-weak edges.

src/python/pants/engine/internals/scheduler_test.py

@@ -5,7 +5,7 @@
 from contextlib import contextmanager
 from dataclasses import dataclass
 from textwrap import dedent
-from typing import Any
+from typing import Any, FrozenSet
 
 from pants.engine.internals.scheduler import ExecutionError
 from pants.engine.rules import RootRule, rule
@@ -116,6 +116,20 @@ async def error_msg_test_rule(union_wrapper: UnionWrapper) -> UnionX:
     raise AssertionError("The statement above this one should have failed!")
 
 
+class BooleanDeps(FrozenSet[bool]):
+    pass
+
+
+@rule
+async def boolean_cycle(key: bool) -> BooleanDeps:
+    """A rule with exactly two instances (bool == two keys), which depend on one another weakly."""
+    deps = {key}
+    dep = await Get(BooleanDeps, bool, not key, weak=True)
+    if dep is not None:
+        deps.update(dep)
+    return BooleanDeps(deps)
+
+
 class TypeCheckFailWrapper:
     """This object wraps another object which will be used to demonstrate a type check failure when
     the engine processes an `await Get(...)` statement."""
@@ -176,6 +190,8 @@ def rules(cls):
             RootRule(UnionB),
             select_union_b,
             a_union_test,
+            RootRule(bool),
+            boolean_cycle,
         ]
 
     def test_use_params(self):
@@ -215,6 +231,10 @@ def test_consumed_types(self):
             self.scheduler.scheduler.rule_graph_consumed_types([A, C], str)
         )
 
+    def test_weak_gets(self):
+        assert {True, False} == set(self.request_single_product(BooleanDeps, True))
+        assert {True, False} == set(self.request_single_product(BooleanDeps, False))
+
     @contextmanager
     def _assert_execution_error(self, expected_msg):
         with assert_execution_error(self, expected_msg):