Fix OOM with somepath due to infinite loop.

This only happens when a `from` target is in the cycle, and the infinite loop occurs when we traverse a parent-edge loop. If the `from` target is not in the cycle, a parent-edge loop is never created because the parent-edges are already set.

**Fix:** Prevent parent-edges from being created from `from` targets to another `from` target, the idea here is that parents of a `from` target are not relevant to `somepath`.

**Implications:** In the case where `from` targets are dependencies of each other, the path going through other `from` targets will not be considered since all `from` targets are treated as "root" level targets. For example:

`somepath(//foo:a + //foo:transitive_dep_of_a, //foo:transitive_dep_of_both)` where this code will now only output the path from `//foo:transitive_dep_of_a -> //foo:transitive_dep_of_both` and never `//foo:a -> //foo:transitive_dep_of_a -> //foo:transitive_dep_of_both`.

PiperOrigin-RevId: 447410138

src/main/java/com/google/devtools/build/lib/query2/query/PathLabelVisitor.java

@@ -75,11 +75,10 @@ public Iterable<Target> somePath(
       if (visitor.hasVisited(t)) {
         ArrayDeque<Target> result = new ArrayDeque<>();
         Target at = t;
-        // TODO(ulfjack): This can result in an infinite loop if there's a dependency cycle.
         while (true) {
           result.addFirst(at);
           List<Target> pred = visitor.getParents(at);
-          if (pred == null) {
+          if (pred == null || pred.isEmpty()) {
             break;
           }
           at = pred.get(0);
@@ -279,6 +278,11 @@ private void visit(Target from, Attribute attribute, Target target)
         }
 
         visitAspectsIfRequired(from, attribute, target);
+      } else if (mode == VisitorMode.SOMEPATH) {
+        // Here we make sure that if this is a top-level visitation node (where 'from' is null),
+        // a parent edge cannot be made for this node. This prevents parent-edge cycles from being
+        // formed and hence infinite loops impossible when traversing parent-edges.
+        parentMap.putIfAbsent(target, ImmutableList.of());
       }
 
       if (visited.add(target)) {

src/test/shell/integration/bazel_query_test.sh

@@ -747,6 +747,30 @@ EOF
       "$(cat foo/expected_lexicographical_result)" "$(cat bazel-bin/foo/q)"
 }
 
+function test_graphless_query_resilient_to_cycles() {
+  rm -rf foo
+  mkdir -p foo
+  cat > foo/BUILD <<EOF
+sh_library(name = "a", deps = [":b"])
+sh_library(name = "b", deps = [":c"])
+sh_library(name = "c", deps = [":a"])
+sh_library(name = "d")
+EOF
+
+  for command in \
+      "somepath(//foo:a, //foo:c)" \
+      "somepath(//foo:a, //foo:d)" \
+      "somepath(//foo:c, //foo:d)" \
+      "allpaths(//foo:a, //foo:d)" \
+      "deps(//foo:a)" \
+      "rdeps(//foo:a, //foo:d)" \
+      "same_pkg_direct_rdeps(//foo:b)"
+  do
+    bazel query --experimental_graphless_query=true \
+        "$command" || fail "Expected success"
+  done
+}
+
 function test_lexicographical_output_does_not_affect_order_output_no() {
   rm -rf foo
   mkdir -p foo