docker serve: restore volumes concurrently so one slow remote doesn't block others

When restoring multiple volumes from saved state, each volume's
filesystem setup (including connecting to the remote) was done
sequentially. Now volumes are restored concurrently, so a single
slow or unreachable remote doesn't delay the restoration of other
volumes.

cmd/serve/docker/driver.go

@@ -361,17 +361,33 @@ func (drv *Driver) restoreState(ctx context.Context) error {
 		return nil
 	}
 
-	for _, vol := range state {
-		// Use a timeout so that a slow or unreachable remote does
-		// not block the plugin from starting up.
-		volCtx, cancel := context.WithTimeout(ctx, volTimeout)
-		err := vol.restoreState(volCtx, drv)
-		cancel()
-		if err != nil {
-			fs.Logf(nil, "Failed to restore volume %q: %v", vol.Name, err)
+	// Restore volumes concurrently so one slow remote doesn't
+	// block restoring the others.
+	type result struct {
+		vol *Volume
+		err error
+	}
+	results := make([]result, len(state))
+	var wg sync.WaitGroup
+	for i, vol := range state {
+		wg.Add(1)
+		go func(i int, vol *Volume) {
+			defer wg.Done()
+			// Use a timeout so that a slow or unreachable remote does
+			// not block the plugin from starting up.
+			volCtx, cancel := context.WithTimeout(ctx, volTimeout)
+			defer cancel()
+			results[i] = result{vol: vol, err: vol.restoreState(volCtx, drv)}
+		}(i, vol)
+	}
+	wg.Wait()
+
+	for _, r := range results {
+		if r.err != nil {
+			fs.Logf(nil, "Failed to restore volume %q: %v", r.vol.Name, r.err)
 			continue
 		}
-		drv.volumes[vol.Name] = vol
+		drv.volumes[r.vol.Name] = r.vol
 	}
 	return nil
 }