spec.py: ensure == is false if true modulo precomputed dag hash (#48889)

lib/spack/spack/spec.py

@@ -3855,6 +3855,13 @@ def _cmp_iter(self):
         for item in self._cmp_node():
             yield item
 
+        # If there is ever a breaking change to hash computation, whether accidental or purposeful,
+        # two specs can be identical modulo DAG hash, depending on what time they were concretized
+        # From the perspective of many operation in Spack (database, build cache, etc) a different
+        # DAG hash means a different spec. Here we ensure that two otherwise identical specs, one
+        # serialized before the hash change and one after, are considered different.
+        yield self.dag_hash() if self.concrete else None
+
         # This needs to be in _cmp_iter so that no specs with different process hashes
         # are considered the same by `__hash__` or `__eq__`.
         #

lib/spack/spack/test/spec_semantics.py

@@ -1989,3 +1989,26 @@ def test_equality_discriminate_on_propagation(lhs, rhs):
 
 def test_comparison_multivalued_variants():
     assert Spec("x=a") < Spec("x=a,b") < Spec("x==a,b") < Spec("x==a,b,c")
+
+
+def test_comparison_after_breaking_hash_change():
+    # We simulate a breaking change in DAG hash computation in Spack. We have two specs that are
+    # entirely equal modulo DAG hash. When deserializing these specs, we don't want them to compare
+    # as equal, because DAG hash is used throughout in Spack to distinguish between specs
+    # (e.g. database, build caches, install dir).
+    s = Spec("example@=1.0")
+    s._mark_concrete(True)
+
+    # compute the dag hash and a change to it
+    dag_hash = s.dag_hash()
+    new_dag_hash = f"{'b' if dag_hash[0] == 'a' else 'a'}{dag_hash[1:]}"
+
+    before_breakage = s.to_dict()
+    after_breakage = s.to_dict()
+    after_breakage["spec"]["nodes"][0]["hash"] = new_dag_hash
+    assert before_breakage != after_breakage
+
+    x = Spec.from_dict(before_breakage)
+    y = Spec.from_dict(after_breakage)
+    assert x != y
+    assert len({x, y}) == 2