make it more explicit that a bad build log causes us to rebuild

(Committing this on top of b56fe80 since they're related,
but I may end up reverting both.)

Author: evmar

src/build_log.cc

@@ -235,10 +235,13 @@ bool BuildLog::Load(const string& path, string* err) {
       sscanf(line_start, kFileSignature, &log_version);
 
       if (log_version < kOldestSupportedVersion) {
-        *err = "unable to extract version from build log, perhaps due to "
-          "being too old; you must clobber your build output and rebuild";
+        *err = ("build log version invalid, perhaps due to being too old; "
+                "starting over");
         fclose(file);
-        return false;
+        unlink(path.c_str());
+        // Don't report this as a failure.  An empty build log will cause
+        // us to rebuild the outputs anyway.
+        return true;
       }
     }
 

src/build_log_test.cc

@@ -154,7 +154,7 @@ TEST_F(BuildLogTest, ObsoleteOldVersion) {
 
   string err;
   BuildLog log;
-  EXPECT_FALSE(log.Load(kTestFilename, &err));
+  EXPECT_TRUE(log.Load(kTestFilename, &err));
   ASSERT_NE(err.find("version"), string::npos);
 }
 

src/ninja.cc

@@ -759,6 +759,11 @@ int NinjaMain(int argc, char** argv) {
     Error("loading build log %s: %s", log_path.c_str(), err.c_str());
     return 1;
   }
+  if (!err.empty()) {
+    // Hack: Load() can return a warning via err by returning true.
+    Warning("%s", err.c_str());
+    err.clear();
+  }
 
   if (!build_log.OpenForWrite(log_path, &err)) {
     Error("opening build log: %s", err.c_str());