Fix spurious rebuilds for phony targets with validations

src/graph.cc

@@ -284,8 +284,11 @@ bool DependencyScan::RecomputeOutputDirty(const Edge* edge,
                                           Node* output) {
   if (edge->is_phony()) {
     // Phony edges don't write any output.  Outputs are only dirty if
-    // there are no inputs and we're missing the output.
-    if (edge->inputs_.empty() && !output->exists()) {
+    // there are no inputs or validations and we're missing the output.
+    // If a phony target has inputs or validations, or the output exists,
+    // they are used for dirty calculation instead of this fallback.
+    if (edge->inputs_.empty() && edge->validations_.empty() &&
+        !output->exists()) {
       explanations_.Record(
           output, "output %s of phony edge with no inputs doesn't exist",
           output->path().c_str());

src/graph_test.cc

@@ -1140,4 +1140,19 @@ TEST_F(GraphTest, EdgeQueuePriority) {
   EXPECT_TRUE(queue.empty());
 }
 
+TEST_F(GraphTest, PhonyOutputWithValidation) {
+  ASSERT_NO_FATAL_FAILURE(AssertParse(&state_,
+                                      "build valid: phony\n"
+                                      "build out: phony |@ valid\n"));
+  fs_.Create("valid", "");
+
+  string err;
+  std::vector<Node*> validation_nodes;
+  EXPECT_TRUE(scan_.RecomputeDirty(GetNode("out"), &validation_nodes, &err));
+  ASSERT_EQ("", err);
 
+  // Phony output with validation should not be dirty even if output is missing.
+  EXPECT_FALSE(GetNode("out")->dirty());
+  ASSERT_EQ(1u, validation_nodes.size());
+  EXPECT_EQ("valid", validation_nodes[0]->path());
+}