runtime/debug: expand SetGCPercent test

The current SetGCPercent test is, shall we say, minimal.

Expand it to check that the GC target is actually computed and updated
correctly.

For #19076.

Change-Id: I6e9b2ee0ef369f22f72e43b58d89e9f1e1b73b1b
Reviewed-on: https://go-review.googlesource.com/39834
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/debug/garbage_test.go

@@ -104,15 +104,74 @@ func TestFreeOSMemory(t *testing.T) {
 	}
 }
 
+var (
+	setGCPercentBallast interface{}
+	setGCPercentSink    interface{}
+)
+
 func TestSetGCPercent(t *testing.T) {
 	// Test that the variable is being set and returned correctly.
-	// Assume the percentage itself is implemented fine during GC,
-	// which is harder to test.
 	old := SetGCPercent(123)
 	new := SetGCPercent(old)
 	if new != 123 {
 		t.Errorf("SetGCPercent(123); SetGCPercent(x) = %d, want 123", new)
 	}
+
+	// Test that the percentage is implemented correctly.
+	defer func() {
+		SetGCPercent(old)
+		setGCPercentBallast, setGCPercentSink = nil, nil
+	}()
+	SetGCPercent(100)
+	runtime.GC()
+	// Create 100 MB of live heap as a baseline.
+	const baseline = 100 << 20
+	var ms runtime.MemStats
+	runtime.ReadMemStats(&ms)
+	setGCPercentBallast = make([]byte, baseline-ms.Alloc)
+	runtime.GC()
+	runtime.ReadMemStats(&ms)
+	if abs64(baseline-int64(ms.Alloc)) > 10<<20 {
+		t.Fatalf("failed to set up baseline live heap; got %d MB, want %d MB", ms.Alloc>>20, baseline>>20)
+	}
+	// NextGC should be ~200 MB.
+	const thresh = 10 << 20
+	if want := int64(2 * baseline); abs64(want-int64(ms.NextGC)) > thresh {
+		t.Errorf("NextGC = %d MB, want %d±%d MB", ms.NextGC>>20, want>>20, thresh>>20)
+	}
+	// Create some garbage, but not enough to trigger another GC.
+	for i := 0; i < int(1.2*baseline); i += 1 << 10 {
+		setGCPercentSink = make([]byte, 1<<10)
+	}
+	setGCPercentSink = nil
+	// Adjust GOGC to 50. NextGC should be ~150 MB.
+	SetGCPercent(50)
+	runtime.ReadMemStats(&ms)
+	if want := int64(1.5 * baseline); abs64(want-int64(ms.NextGC)) > thresh {
+		t.Errorf("NextGC = %d MB, want %d±%d MB", ms.NextGC>>20, want>>20, thresh>>20)
+	}
+
+	// Trigger a GC and get back to 100 MB live with GOGC=100.
+	SetGCPercent(100)
+	runtime.GC()
+	// Raise live to 120 MB.
+	setGCPercentSink = make([]byte, int(0.2*baseline))
+	// Lower GOGC to 10. This must force a GC.
+	runtime.ReadMemStats(&ms)
+	ngc1 := ms.NumGC
+	SetGCPercent(10)
+	runtime.ReadMemStats(&ms)
+	ngc2 := ms.NumGC
+	if ngc1 == ngc2 {
+		t.Errorf("expected GC to run but it did not")
+	}
+}
+
+func abs64(a int64) int64 {
+	if a < 0 {
+		return -a
+	}
+	return a
 }
 
 func TestSetMaxThreadsOvf(t *testing.T) {