runtime/debug: make SetGCPercent(-1) wait for concurrent GC

Currently, SetGCPercent(-1) disables GC, but doesn't wait for any
currently running concurrent GC to finish, so GC can still be running
when it returns. This is a change in behavior from Go 1.8, probably
defies user expectations, and can break various runtime tests that
depend on SetGCPercent(-1) to disable garbage collection in order to
prevent preemption deadlocks.

Fix this by making SetGCPercent(-1) block until any concurrently
running GC cycle finishes.

Fixes #22443.

Change-Id: I904133a34acf97a7942ef4531ace0647b13930ef
Reviewed-on: https://go-review.googlesource.com/79195
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/mgc.go

@@ -230,6 +230,24 @@ func setGCPercent(in int32) (out int32) {
 	// Update pacing in response to gcpercent change.
 	gcSetTriggerRatio(memstats.triggerRatio)
 	unlock(&mheap_.lock)
+
+	// If we just disabled GC, wait for any concurrent GC to
+	// finish so we always return with no GC running.
+	if in < 0 {
+		// Disable phase transitions.
+		lock(&work.sweepWaiters.lock)
+		if gcphase == _GCmark {
+			// GC is active. Wait until we reach sweeping.
+			gp := getg()
+			gp.schedlink = work.sweepWaiters.head
+			work.sweepWaiters.head.set(gp)
+			goparkunlock(&work.sweepWaiters.lock, "wait for GC cycle", traceEvGoBlock, 1)
+		} else {
+			// GC isn't active.
+			unlock(&work.sweepWaiters.lock)
+		}
+	}
+
 	return out
 }