GitFileSystemObjectSink: Fix crash during interrupt

src/libfetchers/git-utils.cc

@@ -1094,6 +1094,13 @@ struct GitFileSystemObjectSinkImpl : GitFileSystemObjectSink
     {
     }
 
+    ~GitFileSystemObjectSinkImpl()
+    {
+        // Make sure the worker threads are destroyed before any state
+        // they're referring to.
+        workers.shutdown();
+    }
+
     struct Child;
 
     /// A directory to be written as a Git tree.

src/libutil/include/nix/util/thread-pool.hh

@@ -52,6 +52,12 @@ public:
      */
     void process();
 
+    /**
+     * Shut down all worker threads and wait until they've exited.
+     * Active work items are finished, but any pending work items are discarded.
+     */
+    void shutdown();
+
 private:
 
     size_t maxThreads;
@@ -72,8 +78,6 @@ private:
     std::condition_variable work;
 
     void doWork(bool mainThread);
-
-    void shutdown();
 };
 
 /**