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.)

1 files changed, 6 insertions, 3 deletions

diff --git a/src/build_log.cc b/src/build_log.cc
index e72a93e..73c2b5d 100644
--- a/src/build_log.cc
+++ b/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;
       }
     }