interp: Fix panic when trigger before any context has been pushed

Make sure there is a top cancel function before calling it.
Fixes panic caused when interrupting decode before interp context has been pushed.

Also cleanup confusing naming a bit.

Thanks @pldin601 for reporting

internal/ctxstack/ctxstack.go

@@ -13,22 +13,25 @@ import (
 // Stack is a context stack
 type Stack struct {
 	cancelFns []func()
-	closeCh   chan struct{}
+	stopCh    chan struct{}
 }
 
 // New context stack
 func New(triggerCh func(stopCh chan struct{})) *Stack {
 	stopCh := make(chan struct{})
-	s := &Stack{closeCh: stopCh}
+	s := &Stack{stopCh: stopCh}
 
 	go func() {
 		for {
 			triggerCh(stopCh)
 			select {
 			case <-stopCh:
-				// stop if closed
+				// stop if stopCh closed
 			default:
-				s.cancelFns[len(s.cancelFns)-1]()
+				// ignore if triggered before any context pushed
+				if len(s.cancelFns) > 0 {
+					s.cancelFns[len(s.cancelFns)-1]()
+				}
 				continue
 			}
 			break
@@ -43,13 +46,14 @@ func (s *Stack) Stop() {
 	for i := len(s.cancelFns) - 1; i >= 0; i-- {
 		s.cancelFns[i]()
 	}
-	close(s.closeCh)
+	close(s.stopCh)
 }
 
 // Push creates, pushes and returns new context. Cancel pops it.
 func (s *Stack) Push(parent context.Context) (context.Context, func()) {
 	stackCtx, stackCtxCancel := context.WithCancel(parent)
 	stackIdx := len(s.cancelFns)
+
 	s.cancelFns = append(s.cancelFns, stackCtxCancel)
 	cancelled := false
 

internal/ctxstack/ctxstacl_test.go

@@ -0,0 +1,35 @@
+package ctxstack_test
+
+import (
+	"testing"
+
+	"github.com/wader/fq/internal/ctxstack"
+)
+
+func TestCancelBeforePush(t *testing.T) {
+	// TODO: nicer way to test trigger before any push
+	waitTriggerFn := make(chan struct{})
+	triggerCh := make(chan struct{})
+	waitCh := make(chan struct{})
+	hasTriggeredOnce := false
+
+	ctxstack.New(func(stopCh chan struct{}) {
+		if hasTriggeredOnce {
+			close(stopCh)
+			close(waitCh)
+			return
+		}
+
+		close(waitTriggerFn)
+		<-triggerCh
+
+		hasTriggeredOnce = true
+	})
+
+	// wait for trigger func to be called
+	<-waitTriggerFn
+	// make trigger func return and cancel
+	close(triggerCh)
+
+	<-waitCh
+}