* Revert r19650 (implement gc-keep-outputs by looking for derivations

with the same name as the output) and instead use the
  DerivationOutputs table in the database, which is the correct way to
  to do things.

src/libstore/local-store.cc

@@ -297,6 +297,8 @@ void LocalStore::prepareStatements()
         "select time from FailedPaths where path = ?;");
     stmtAddDerivationOutput.create(db,
         "insert or replace into DerivationOutputs (drv, id, path) values (?, ?, ?);");
+    stmtQueryValidDerivers.create(db,
+        "select v.id, v.path from DerivationOutputs d join ValidPaths v on d.drv = v.id where d.path = ?;");
 }
 
 
@@ -599,6 +601,28 @@ Path LocalStore::queryDeriver(const Path & path)
 }
 
 
+PathSet LocalStore::queryValidDerivers(const Path & path)
+{
+    assertStorePath(path);
+
+    SQLiteStmtUse use(stmtQueryValidDerivers);
+    stmtQueryValidDerivers.bind(path);
+
+    PathSet derivers;
+    int r;
+    while ((r = sqlite3_step(stmtQueryValidDerivers)) == SQLITE_ROW) {
+        const char * s = (const char *) sqlite3_column_text(stmtQueryValidDerivers, 1);
+        assert(s);
+        derivers.insert(s);
+    }
+    
+    if (r != SQLITE_DONE)
+        throw SQLiteError(db, format("error getting valid derivers of `%1%'") % path);
+    
+    return derivers;
+}
+
+
 void LocalStore::startSubstituter(const Path & substituter, RunningSubstituter & run)
 {
     if (run.pid != -1) return;