lib/events: Hack to make test coverage stable from run to run

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3484

lib/events/events.go

@@ -9,6 +9,7 @@ package events
 
 import (
 	"errors"
+	"runtime"
 	stdsync "sync"
 	"time"
 
@@ -47,6 +48,8 @@ const (
 	AllEvents = (1 << iota) - 1
 )
 
+var runningTests = false
+
 func (t EventType) String() string {
 	switch t {
 	case Ping:
@@ -186,6 +189,13 @@ func (l *Logger) Subscribe(mask EventType) *Subscription {
 	// We need to create the timeout timer in the stopped, non-fired state so
 	// that Subscription.Poll() can safely reset it and select on the timeout
 	// channel. This ensures the timer is stopped and the channel drained.
+	if runningTests {
+		// Make the behavior stable when running tests to avoid randomly
+		// varying test coverage. This ensures, in practice if not in
+		// theory, that the timer fires and we take the true branch of the
+		// next if.
+		runtime.Gosched()
+	}
 	if !s.timeout.Stop() {
 		<-s.timeout.C
 	}
@@ -231,6 +241,14 @@ func (s *Subscription) Poll(timeout time.Duration) (Event, error) {
 		if !ok {
 			return e, ErrClosed
 		}
+		if runningTests {
+			// Make the behavior stable when running tests to avoid randomly
+			// varying test coverage. This ensures, in practice if not in
+			// theory, that the timer fires and we take the true branch of
+			// the next if.
+			s.timeout.Reset(0)
+			runtime.Gosched()
+		}
 		if !s.timeout.Stop() {
 			// The timeout must be stopped and possibly drained to be ready
 			// for reuse in the next call.