spec.py: fix __getitem__ looking outside of dag (#45090)

`Spec.__getitem__` queries dependent edges, which almost always point to
nodes outside the sub-dag considered. It should only ever look at edges
being traversed.

lib/spack/spack/spec.py

@@ -4164,29 +4164,21 @@ def __getitem__(self, name: str):
             csv = query_parameters.pop().strip()
             query_parameters = re.split(r"\s*,\s*", csv)
 
-        # In some cases a package appears multiple times in the same DAG for *distinct*
-        # specs. For example, a build-type dependency may itself depend on a package
-        # the current spec depends on, but their specs may differ. Therefore we iterate
-        # in an order here that prioritizes the build, test and runtime dependencies;
-        # only when we don't find the package do we consider the full DAG.
         order = lambda: itertools.chain(
-            self.traverse(deptype="link"),
-            self.dependencies(deptype=dt.BUILD | dt.RUN | dt.TEST),
-            self.traverse(),  # fall back to a full search
+            self.traverse_edges(deptype=dt.LINK, order="breadth", cover="edges"),
+            self.edges_to_dependencies(depflag=dt.BUILD | dt.RUN | dt.TEST),
+            self.traverse_edges(deptype=dt.ALL, order="breadth", cover="edges"),
         )
 
+        # Consider runtime dependencies and direct build/test deps before transitive dependencies,
+        # and prefer matches closest to the root.
         try:
             child: Spec = next(
-                itertools.chain(
-                    # Regular specs
-                    (x for x in order() if x.name == name),
-                    (
-                        x
-                        for x in order()
-                        if (not x.virtual)
-                        and any(name in edge.virtuals for edge in x.edges_from_dependents())
-                    ),
-                    (x for x in order() if (not x.virtual) and x.package.provides(name)),
+                e.spec
+                for e in itertools.chain(
+                    (e for e in order() if e.spec.name == name or name in e.virtuals),
+                    # for historical reasons
+                    (e for e in order() if e.spec.concrete and e.spec.package.provides(name)),
                 )
             )
         except StopIteration:

lib/spack/spack/test/spec_dag.py

@@ -1105,3 +1105,23 @@ def test_indexing_prefers_direct_or_transitive_link_deps():
 
     # Ensure that the full DAG is still searched
     assert root["a2"]
+
+
+def test_getitem_sticks_to_subdag():
+    """Test that indexing on Spec by virtual does not traverse outside the dag, which happens in
+    the unlikely case someone would rewrite __getitem__ in terms of edges_from_dependents instead
+    of edges_to_dependencies."""
+    x, y, z = Spec("x"), Spec("y"), Spec("z")
+    x.add_dependency_edge(z, depflag=dt.LINK, virtuals=("virtual",))
+    y.add_dependency_edge(z, depflag=dt.LINK, virtuals=())
+    assert x["virtual"].name == "z"
+    with pytest.raises(KeyError):
+        y["virtual"]
+
+
+def test_getitem_finds_transitive_virtual():
+    x, y, z = Spec("x"), Spec("y"), Spec("z")
+    x.add_dependency_edge(z, depflag=dt.LINK, virtuals=())
+    x.add_dependency_edge(y, depflag=dt.LINK, virtuals=())
+    y.add_dependency_edge(z, depflag=dt.LINK, virtuals=("virtual",))
+    assert x["virtual"].name == "z"