Improve cache behavior

* don't block while filling cache
* make cache wait for pending calls

pkg/util/time_cache.go

@@ -40,9 +40,13 @@ type timeCacheEntry struct {
 type timeCache struct {
 	clock    Clock
 	fillFunc func(string) T
-	cache    map[string]timeCacheEntry
-	lock     sync.Mutex
 	ttl      time.Duration
+
+	inFlight     map[string]chan T
+	inFlightLock sync.Mutex
+
+	cache map[string]timeCacheEntry
+	lock  sync.RWMutex
 }
 
 // NewTimeCache returns a cache which calls fill to fill its entries, and
@@ -51,6 +55,7 @@ func NewTimeCache(clock Clock, ttl time.Duration, fill func(key string) T) TimeC
 	return &timeCache{
 		clock:    clock,
 		fillFunc: fill,
+		inFlight: map[string]chan T{},
 		cache:    map[string]timeCacheEntry{},
 		ttl:      ttl,
 	}
@@ -59,17 +64,62 @@ func NewTimeCache(clock Clock, ttl time.Duration, fill func(key string) T) TimeC
 // Get returns the value of key from the cache, if it is present
 // and recent enough; otherwise, it blocks while it gets the value.
 func (c *timeCache) Get(key string) T {
-	c.lock.Lock()
-	defer c.lock.Unlock()
+	if item, ok := c.get(key); ok {
+		return item
+	}
+
+	// We need to fill the cache. Calling the function could be
+	// expensive, so do it while unlocked.
+	wait := c.fillOrWait(key)
+	item := <-wait
+
+	// Put it back in the channel in case there's multiple waiters
+	// (this channel is non-blocking)
+	wait <- item
+	return item
+}
+
+// returns the item and true if it is found and not expired, otherwise nil and false.
+func (c *timeCache) get(key string) (T, bool) {
+	c.lock.RLock()
+	defer c.lock.RUnlock()
 	data, ok := c.cache[key]
 	now := c.clock.Now()
-
 	if !ok || now.Sub(data.lastUpdate) > c.ttl {
-		data = timeCacheEntry{
-			item:       c.fillFunc(key),
-			lastUpdate: now,
-		}
-		c.cache[key] = data
+		return nil, false
 	}
-	return data.item
+	return data.item, true
+}
+
+func (c *timeCache) fillOrWait(key string) chan T {
+	c.inFlightLock.Lock()
+	defer c.inFlightLock.Unlock()
+
+	// Already a call in progress?
+	if current, ok := c.inFlight[key]; ok {
+		return current
+	}
+
+	// We are the first, so we have to make the call.
+	result := make(chan T, 1) // non-blocking
+	c.inFlight[key] = result
+	go func() {
+		// Make potentially slow call
+		data := timeCacheEntry{
+			item:       c.fillFunc(key),
+			lastUpdate: c.clock.Now(),
+		}
+		result <- data.item
+
+		// Store in cache
+		c.lock.Lock()
+		c.cache[key] = data
+		c.lock.Unlock()
+
+		// Remove in flight entry
+		c.inFlightLock.Lock()
+		delete(c.inFlight, key)
+		c.inFlightLock.Unlock()
+	}()
+	return result
 }

pkg/util/time_cache_test.go

@@ -17,8 +17,11 @@ limitations under the License.
 package util
 
 import (
+	"sync"
 	"testing"
 	"time"
+
+	fuzz "github.com/google/gofuzz"
 )
 
 func TestCacheExpire(t *testing.T) {
@@ -61,3 +64,54 @@ func TestCacheNotExpire(t *testing.T) {
 		t.Errorf("Wrong number of calls for foo: wanted %v, got %v", e, a)
 	}
 }
+
+func TestCacheParallel(t *testing.T) {
+	ff := func(key string) T { time.Sleep(time.Second); return key }
+	clock := &FakeClock{time.Now()}
+	c := NewTimeCache(clock, 60*time.Second, ff)
+
+	// Make some keys
+	keys := []string{}
+	fuzz.New().NilChance(0).NumElements(50, 50).Fuzz(&keys)
+
+	// If we have high parallelism, this will take only a second.
+	var wg sync.WaitGroup
+	wg.Add(len(keys))
+	for _, key := range keys {
+		go func(key string) {
+			c.Get(key)
+			wg.Done()
+		}(key)
+	}
+	wg.Wait()
+}
+
+func TestCacheParallelOneCall(t *testing.T) {
+	calls := 0
+	var callLock sync.Mutex
+	ff := func(key string) T {
+		time.Sleep(time.Second)
+		callLock.Lock()
+		defer callLock.Unlock()
+		calls++
+		return key
+	}
+	clock := &FakeClock{time.Now()}
+	c := NewTimeCache(clock, 60*time.Second, ff)
+
+	// If we have high parallelism, this will take only a second.
+	var wg sync.WaitGroup
+	wg.Add(50)
+	for i := 0; i < 50; i++ {
+		go func(key string) {
+			c.Get(key)
+			wg.Done()
+		}("aoeu")
+	}
+	wg.Wait()
+
+	// And if we wait for existing calls, we should have only one call.
+	if e, a := 1, calls; e != a {
+		t.Errorf("Expected %v, got %v", e, a)
+	}
+}