When repairing a derivation, check and repair the entire output closure

If we find a corrupted path in the output closure, we rebuild the
derivation that produced that particular path.

src/libstore/local-store.hh

@@ -206,6 +206,8 @@ public:
        contents. */
     bool pathContentsGood(const Path & path);
 
+    void markContentsGood(const Path & path);
+
 private:
 
     Path schemaPath;
@@ -233,6 +235,9 @@ private:
     SQLiteStmt stmtQueryDerivationOutputs;
     SQLiteStmt stmtQueryPathFromHashPart;
 
+    /* Cache for pathContentsGood(). */
+    std::map<Path, bool> pathContentsGoodCache;
+
     int getSchema();
 
     void openDB(bool create);