Fixes bug in GCC implementation

This fixes an issue described in #271.
The original implementation did not transition rate controller states
properly.

pkg/gcc/rate_controller.go

@@ -90,7 +90,8 @@ func (c *rateController) onDelayStats(ds DelayStats) {
 		return
 	}
 	c.delayStats = ds
-	c.delayStats.State = c.delayStats.State.transition(ds.Usage)
+	c.delayStats.State = c.lastState.transition(ds.Usage)
+	c.lastState = c.delayStats.State
 
 	if c.delayStats.State == stateHold {
 		return

pkg/gcc/rate_controller_test.go

@@ -76,3 +76,58 @@ func TestRateControllerRun(t *testing.T) {
 		})
 	}
 }
+
+func TestRateController_StateTransition(t *testing.T) {
+	tcs := []struct {
+		name       string
+		delayStats []DelayStats
+		wantStates []state
+	}{
+		{
+			name:       "overuse-normal",
+			delayStats: []DelayStats{DelayStats{Usage: usageOver}, DelayStats{Usage: usageNormal}},
+			wantStates: []state{stateDecrease, stateHold},
+		},
+		{
+			name:       "overuse-underuse",
+			delayStats: []DelayStats{DelayStats{Usage: usageOver}, DelayStats{Usage: usageUnder}},
+			wantStates: []state{stateDecrease, stateHold},
+		},
+		{
+			name:       "normal",
+			delayStats: []DelayStats{DelayStats{Usage: usageNormal}},
+			wantStates: []state{stateIncrease},
+		},
+		{
+			name:       "under-over",
+			delayStats: []DelayStats{DelayStats{Usage: usageUnder}, DelayStats{Usage: usageOver}},
+			wantStates: []state{stateHold, stateDecrease},
+		},
+		{
+			name:       "under-normal",
+			delayStats: []DelayStats{DelayStats{Usage: usageUnder}, DelayStats{Usage: usageNormal}},
+			wantStates: []state{stateHold, stateIncrease},
+		},
+		{
+			name:       "under-under",
+			delayStats: []DelayStats{DelayStats{Usage: usageUnder}, DelayStats{Usage: usageUnder}},
+			wantStates: []state{stateHold, stateHold},
+		},
+	}
+
+	for _, tc := range tcs {
+		t.Run(tc.name, func(t *testing.T) {
+			cb := func(ds DelayStats) {}
+			rc := newRateController(time.Now, 500_000, 100_000, 1_000_000, cb)
+			// Call it once to initialize the rate controller
+			rc.onDelayStats(DelayStats{})
+
+			for i, ds := range tc.delayStats {
+				rc.onDelayStats(ds)
+				if rc.lastState != tc.wantStates[i] {
+					t.Errorf("expected lastState to be %v but got %v", tc.wantStates[i], rc.lastState)
+				}
+			}
+		})
+	}
+}