Reduce timeout on calls to backends with multiple targets

Allows for failover in case a target times out.

clientutil/balancer.go

@@ -98,28 +98,35 @@ func newBalancedBackend(config *BackendConfig, resolver resolver) (*balancedBack
 // with a JSON-encoded request body. It will attempt to decode the
 // response body as JSON.
 func (b *balancedBackend) Call(ctx context.Context, shard, path string, req, resp interface{}) error {
+	// Serialize the request body.
 	data, err := json.Marshal(req)
 	if err != nil {
 		return err
 	}
 
-	var tg targetGenerator = b.backendTracker
-	if b.sharded {
-		if shard == "" {
-			return fmt.Errorf("call without shard to sharded service %s", b.baseURI.String())
-		}
-		tg = newShardedGenerator(shard, b.baseURI.Host, b.resolver)
+	// Create the target sequence for this call. If there are multiple
+	// targets, reduce the timeout on each individual call accordingly to
+	// accomodate eventual failover.
+	seq, err := b.makeSequence(shard)
+	if err != nil {
+		return err
+	}
+	innerTimeout := 1 * time.Hour
+	if deadline, ok := ctx.Deadline(); ok {
+		innerTimeout = time.Until(deadline) / time.Duration(seq.Len())
 	}
-	seq := newSequence(tg)
-	b.log.Printf("%016x: initialized", seq.ID())
 
+	// Call the backends in the sequence until one succeeds, with an
+	// exponential backoff policy controlled by the outer Context.
 	var httpResp *http.Response
 	err = backoff.Retry(func() error {
 		req, rerr := b.newJSONRequest(path, shard, data)
 		if rerr != nil {
 			return rerr
 		}
-		httpResp, rerr = b.do(ctx, seq, req)
+		innerCtx, cancel := context.WithTimeout(ctx, innerTimeout)
+		httpResp, rerr = b.do(innerCtx, seq, req)
+		cancel()
 		return rerr
 	}, backoff.WithContext(newExponentialBackOff(), ctx))
 	if err != nil {
@@ -127,16 +134,34 @@ func (b *balancedBackend) Call(ctx context.Context, shard, path string, req, res
 	}
 	defer httpResp.Body.Close() // nolint
 
+	// Decode the response.
 	if httpResp.Header.Get("Content-Type") != "application/json" {
 		return errors.New("not a JSON response")
 	}
-
 	if resp == nil {
 		return nil
 	}
 	return json.NewDecoder(httpResp.Body).Decode(resp)
 }
 
+// Initialize a new target sequence.
+func (b *balancedBackend) makeSequence(shard string) (*sequence, error) {
+	var tg targetGenerator = b.backendTracker
+	if b.sharded {
+		if shard == "" {
+			return nil, fmt.Errorf("call without shard to sharded service %s", b.baseURI.String())
+		}
+		tg = newShardedGenerator(shard, b.baseURI.Host, b.resolver)
+	}
+
+	seq := newSequence(tg)
+	if seq.Len() == 0 {
+		return nil, errNoTargets
+	}
+	b.log.Printf("%016x: initialized", seq.ID())
+	return seq, nil
+}
+
 // Return the URI to be used for the request. This is used both in the
 // Host HTTP header and as the TLS server name used to pick a server
 // certificate (if using TLS).
@@ -213,6 +238,8 @@ func newSequence(tg targetGenerator) *sequence {
 
 func (s *sequence) ID() uint64 { return s.id }
 
+func (s *sequence) Len() int { return len(s.targets) }
+
 func (s *sequence) reloadTargets() {
 	targets := s.tg.getTargets()
 	if len(targets) > 0 {