Adds a retry capability to lock monitors in the API client.

api/lock.go

@@ -2,6 +2,7 @@ package api
 
 import (
 	"fmt"
+	"strings"
 	"sync"
 	"time"
 )
@@ -22,9 +23,15 @@ const (
 
 	// DefaultLockRetryTime is how long we wait after a failed lock acquisition
 	// before attempting to do the lock again. This is so that once a lock-delay
-	// is in affect, we do not hot loop retrying the acquisition.
+	// is in effect, we do not hot loop retrying the acquisition.
 	DefaultLockRetryTime = 5 * time.Second
 
+	// DefaultMonitorRetryTime is how long we wait after a failed monitor check
+	// of a lock (500 response code). This allows the monitor to ride out brief
+	// periods of unavailability, subject to the MonitorRetries setting in the
+	// lock options which is by default set to 0, disabling this feature.
+	DefaultMonitorRetryTime = 2 * time.Second
+
 	// LockFlagValue is a magic flag we set to indicate a key
 	// is being used for a lock. It is used to detect a potential
 	// conflict with a semaphore.
@@ -62,11 +69,13 @@ type Lock struct {
 
 // LockOptions is used to parameterize the Lock behavior.
 type LockOptions struct {
-	Key         string // Must be set and have write permissions
-	Value       []byte // Optional, value to associate with the lock
-	Session     string // Optional, created if not specified
-	SessionName string // Optional, defaults to DefaultLockSessionName
-	SessionTTL  string // Optional, defaults to DefaultLockSessionTTL
+	Key              string        // Must be set and have write permissions
+	Value            []byte        // Optional, value to associate with the lock
+	Session          string        // Optional, created if not specified
+	SessionName      string        // Optional, defaults to DefaultLockSessionName
+	SessionTTL       string        // Optional, defaults to DefaultLockSessionTTL
+	MonitorRetries   int           // Optional, defaults to 0 which means no retries
+	MonitorRetryTime time.Duration // Optional, defaults to DefaultMonitorRetryTime
 }
 
 // LockKey returns a handle to a lock struct which can be used
@@ -96,6 +105,9 @@ func (c *Client) LockOpts(opts *LockOptions) (*Lock, error) {
 			return nil, fmt.Errorf("invalid SessionTTL: %v", err)
 		}
 	}
+	if opts.MonitorRetryTime == 0 {
+		opts.MonitorRetryTime = DefaultMonitorRetryTime
+	}
 	l := &Lock{
 		c:    c,
 		opts: opts,
@@ -327,8 +339,24 @@ func (l *Lock) monitorLock(session string, stopCh chan struct{}) {
 	kv := l.c.KV()
 	opts := &QueryOptions{RequireConsistent: true}
 WAIT:
+	retries := l.opts.MonitorRetries
+RETRY:
 	pair, meta, err := kv.Get(l.opts.Key, opts)
 	if err != nil {
+		// TODO (slackpad) - Make a real error type here instead of using
+		// a string check.
+		const serverError = "Unexpected response code: 500"
+
+		// If configured we can try to ride out a brief Consul unavailability
+		// by doing retries. Note that we have to attempt the retry in a non-
+		// blocking fashion so that we have a clean place to reset the retry
+		// counter if service is restored.
+		if retries > 0 && strings.Contains(err.Error(), serverError) {
+			time.Sleep(l.opts.MonitorRetryTime)
+			retries--
+			opts.WaitIndex = 0
+			goto RETRY
+		}
 		return
 	}
 	if pair != nil && pair.Session == session {

api/lock_test.go

@@ -2,6 +2,10 @@ package api
 
 import (
 	"log"
+	"net/http"
+	"net/http/httptest"
+	"net/http/httputil"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -369,3 +373,118 @@ func TestLock_ReclaimLock(t *testing.T) {
 		t.Fatalf("should not be leader")
 	}
 }
+
+func TestLock_MonitorRetry(t *testing.T) {
+	t.Parallel()
+	raw, s := makeClient(t)
+	defer s.Stop()
+
+	// Set up a server that always responds with 500 errors.
+	failer := func(w http.ResponseWriter, req *http.Request) {
+		w.WriteHeader(500)
+	}
+	outage := httptest.NewServer(http.HandlerFunc(failer))
+	defer outage.Close()
+
+	// Set up a reverse proxy that will send some requests to the
+	// 500 server and pass everything else through to the real Consul
+	// server.
+	var mutex sync.Mutex
+	errors := 0
+	director := func(req *http.Request) {
+		mutex.Lock()
+		defer mutex.Unlock()
+
+		req.URL.Scheme = "http"
+		if errors > 0 && req.Method == "GET" && strings.Contains(req.URL.Path, "/v1/kv/test/lock") {
+			req.URL.Host = outage.URL[7:] // Strip off "http://".
+			errors--
+		} else {
+			req.URL.Host = raw.config.Address
+		}
+	}
+	proxy := httptest.NewServer(&httputil.ReverseProxy{Director: director})
+	defer proxy.Close()
+
+	// Make another client that points at the proxy instead of the real
+	// Consul server.
+	config := raw.config
+	config.Address = proxy.URL[7:] // Strip off "http://".
+	c, err := NewClient(&config)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Set up a lock with retries enabled.
+	opts := &LockOptions{
+		Key:            "test/lock",
+		SessionTTL:     "60s",
+		MonitorRetries: 3,
+	}
+	lock, err := c.LockOpts(opts)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Make sure the default got set.
+	if lock.opts.MonitorRetryTime != DefaultMonitorRetryTime {
+		t.Fatalf("bad: %d", lock.opts.MonitorRetryTime)
+	}
+
+	// Now set a custom time for the test.
+	opts.MonitorRetryTime = 250 * time.Millisecond
+	lock, err = c.LockOpts(opts)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if lock.opts.MonitorRetryTime != 250*time.Millisecond {
+		t.Fatalf("bad: %d", lock.opts.MonitorRetryTime)
+	}
+
+	// Should get the lock.
+	leaderCh, err := lock.Lock(nil)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if leaderCh == nil {
+		t.Fatalf("not leader")
+	}
+
+	// Poke the key using the raw client to force the monitor to wake up
+	// and check the lock again. This time we will return errors for some
+	// of the responses.
+	mutex.Lock()
+	errors = 2
+	mutex.Unlock()
+	pair, _, err := raw.KV().Get("test/lock", &QueryOptions{})
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if _, err := raw.KV().Put(pair, &WriteOptions{}); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	time.Sleep(5 * opts.MonitorRetryTime)
+
+	// Should still be the leader.
+	select {
+	case <-leaderCh:
+		t.Fatalf("should be leader")
+	default:
+	}
+
+	// Now return an overwhelming number of errors.
+	mutex.Lock()
+	errors = 10
+	mutex.Unlock()
+	if _, err := raw.KV().Put(pair, &WriteOptions{}); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	time.Sleep(5 * opts.MonitorRetryTime)
+
+	// Should lose leadership.
+	select {
+	case <-leaderCh:
+	case <-time.After(time.Second):
+		t.Fatalf("should not be leader")
+	}
+}