runtime: make runtime.GC() trigger GC even if GOGC=off

Currently, the priority of checks in (gcTrigger).test() puts the
gcpercent<0 test above gcTriggerCycle, which is used for runtime.GC().
This is an unintentional change from 1.8 and before, where
runtime.GC() triggered a GC even if GOGC=off.

Fix this by rearranging the priority so the gcTriggerCycle test
executes even if gcpercent < 0.

Fixes #22023.

Change-Id: I109328d7b643b6824eb9d79061a9e775f0149575
Reviewed-on: https://go-review.googlesource.com/65994
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Rick Hudson <[email protected]>

Author: aclements

src/runtime/gc_test.go

@@ -499,3 +499,19 @@ func BenchmarkReadMemStats(b *testing.B) {
 
 	hugeSink = nil
 }
+
+func TestUserForcedGC(t *testing.T) {
+	// Test that runtime.GC() triggers a GC even if GOGC=off.
+	defer debug.SetGCPercent(debug.SetGCPercent(-1))
+
+	var ms1, ms2 runtime.MemStats
+	runtime.ReadMemStats(&ms1)
+	runtime.GC()
+	runtime.ReadMemStats(&ms2)
+	if ms1.NumGC == ms2.NumGC {
+		t.Fatalf("runtime.GC() did not trigger GC")
+	}
+	if ms1.NumForcedGC == ms2.NumForcedGC {
+		t.Fatalf("runtime.GC() was not accounted in NumForcedGC")
+	}
+}

src/runtime/mgc.go

@@ -1158,7 +1158,7 @@ func (t gcTrigger) test() bool {
 	if t.kind == gcTriggerAlways {
 		return true
 	}
-	if gcphase != _GCoff || gcpercent < 0 {
+	if gcphase != _GCoff {
 		return false
 	}
 	switch t.kind {
@@ -1169,6 +1169,9 @@ func (t gcTrigger) test() bool {
 		// own write.
 		return memstats.heap_live >= memstats.gc_trigger
 	case gcTriggerTime:
+		if gcpercent < 0 {
+			return false
+		}
 		lastgc := int64(atomic.Load64(&memstats.last_gc_nanotime))
 		return lastgc != 0 && t.now-lastgc > forcegcperiod
 	case gcTriggerCycle: