Build self-consistent graphs for dupe edges with multiple outputs.

Fixes #867, both the crashes and "[stuck]" issues.

The problem was that a duplicate edge would modify the in_edge of the
outputs of the new build rule, but the edge corresponding to the old
build rule would still think that the in_edge points to itself.
`old_edge->outputs_[0]->in_edge()` would not return `old_edge`, which
confused the scan logic.

As fix, let `State::AddOut()` reject changing in_edge if it's already
set.  This changes behavior in a minor way: Previously, if there were
multiple edges for a single output, the last edge would be kept.  Now,
the first edge is kept.  This only had mostly-well-defined semantics if
all duplicate edges are the same (which is the only case I've seen in
practice), and for that case the behavior doesn't change.

For testing, add a VerifyGraph() function and call that every time any
test graph is parsed.  That's a bit more code than just copying the test
cases from the bug into build_test.cc, but it also yields better test
coverage overall.

1 files changed, 13 insertions, 0 deletions

diff --git a/src/manifest_parser_test.cc b/src/manifest_parser_test.cc
index e13d728..7e38fc6 100644
--- a/src/manifest_parser_test.cc
+++ b/src/manifest_parser_test.cc
@@ -28,6 +28,7 @@ struct ParserTest : public testing::Test,
     string err;
     EXPECT_TRUE(parser.ParseTest(input, &err));
     ASSERT_EQ("", err);
+    VerifyGraph(state);
   }
 
   virtual bool ReadFile(const string& path, string* content, string* err) {
@@ -340,6 +341,18 @@ TEST_F(ParserTest, CanonicalizePathsBackslashes) {
 }
 #endif
 
+TEST_F(ParserTest, DuplicateEdgeWithMultipleOutputs) {
+  ASSERT_NO_FATAL_FAILURE(AssertParse(
+"rule cat\n"
+"  command = cat $in > $out\n"
+"build out1 out2: cat in1\n"
+"build out1: cat in2\n"
+"build final: cat out1\n"
+));
+  // AssertParse() checks that the generated build graph is self-consistent.
+  // That's all the checking that this test needs.
+}
+
 TEST_F(ParserTest, ReservedWords) {
   ASSERT_NO_FATAL_FAILURE(AssertParse(
 "rule build\n"