* Added a command `nix-store --clear-failed-paths <PATHS>' to clear

the "failed" status of the given store paths.  The special value `*'
  clears all failed paths.

src/libstore/local-store.cc

@@ -329,6 +329,8 @@ void LocalStore::openDB(bool create)
         "select time from FailedPaths where path = ?;");
     stmtQueryFailedPaths.create(db,
         "select path from FailedPaths;");
+    stmtClearFailedPath.create(db,
+        "delete from FailedPaths where ?1 = '*' or path = ?1;");
     stmtAddDerivationOutput.create(db,
         "insert or replace into DerivationOutputs (drv, id, path) values (?, ?, ?);");
     stmtQueryValidDerivers.create(db,
@@ -529,6 +531,21 @@ PathSet LocalStore::queryFailedPaths()
 }
 
 
+void LocalStore::clearFailedPaths(const PathSet & paths)
+{
+    SQLiteTxn txn(db);
+
+    foreach (PathSet::const_iterator, i, paths) {
+        SQLiteStmtUse use(stmtClearFailedPath);
+        stmtClearFailedPath.bind(*i);
+        if (sqlite3_step(stmtClearFailedPath) != SQLITE_DONE)
+            throw SQLiteError(db, format("clearing failed path `%1%' in database") % *i);
+    }
+
+    txn.commit();
+}
+
+
 Hash parseHashField(const Path & path, const string & s)
 {
     string::size_type colon = s.find(':');