Fix signalling Wait in regulator.enter

In some conditions, regulator.exit may not send a signal to blocked
regulator.enter.

Let's assume we are in the critical section of regulator.exit and r.available
is equal to 0. And there are three more gorotines. One goroutine also executes
regulator.exit and waits for the lock. Rest run regulator.enter and wait for
the signal.

We send the signal, and after releasing the lock, there will be lock
contention:

  1. Wait from regulator.enter
  2. Lock from regulator.exit

If the winner is Lock from regulator.exit, we will not send another signal to
unlock the second Wait.

Signed-off-by: Oleg Bulatov <obulatov@redhat.com>

registry/storage/driver/base/regulator.go

@@ -38,11 +38,7 @@ func (r *regulator) enter() {
 
 func (r *regulator) exit() {
 	r.L.Lock()
-	// We only need to signal to a waiting FS operation if we're already at the
-	// limit of threads used
-	if r.available == 0 {
-		r.Signal()
-	}
+	r.Signal()
 	r.available++
 	r.L.Unlock()
 }

registry/storage/driver/base/regulator_test.go

@@ -0,0 +1,67 @@
+package base
+
+import (
+	"sync"
+	"testing"
+	"time"
+)
+
+func TestRegulatorEnterExit(t *testing.T) {
+	const limit = 500
+
+	r := NewRegulator(nil, limit).(*regulator)
+
+	for try := 0; try < 50; try++ {
+		run := make(chan struct{})
+
+		var firstGroupReady sync.WaitGroup
+		var firstGroupDone sync.WaitGroup
+		firstGroupReady.Add(limit)
+		firstGroupDone.Add(limit)
+		for i := 0; i < limit; i++ {
+			go func() {
+				r.enter()
+				firstGroupReady.Done()
+				<-run
+				r.exit()
+				firstGroupDone.Done()
+			}()
+		}
+		firstGroupReady.Wait()
+
+		// now we exhausted all the limit, let's run a little bit more
+		var secondGroupReady sync.WaitGroup
+		var secondGroupDone sync.WaitGroup
+		for i := 0; i < 50; i++ {
+			secondGroupReady.Add(1)
+			secondGroupDone.Add(1)
+			go func() {
+				secondGroupReady.Done()
+				r.enter()
+				r.exit()
+				secondGroupDone.Done()
+			}()
+		}
+		secondGroupReady.Wait()
+
+		// allow the first group to return resources
+		close(run)
+
+		done := make(chan struct{})
+		go func() {
+			secondGroupDone.Wait()
+			close(done)
+		}()
+		select {
+		case <-done:
+		case <-time.After(5 * time.Second):
+			t.Fatal("some r.enter() are still locked")
+		}
+
+		firstGroupDone.Wait()
+
+		if r.available != limit {
+			t.Fatalf("r.available: got %d, want %d", r.available, limit)
+		}
+	}
+}