Prevent a race allowing the submission of an invalid filter

Prior to this change, it's possible to type an invalid filter expression
and submit it - using enter - in the brief interval before termshark
determines it's invalid and sets the filter field red.

The fix is to delay the submission of a filter on enter until the filter
expression is determined valid. If the filter changes in the meantime,
then the enter is forgotten. If enter is still pending when the
expression becomes green, then it's submitted.

widgets/filter/filter.go

@@ -68,6 +68,7 @@ type Widget struct {
 	quitchan             chan struct{}
 	readytorunchan       chan struct{}
 	temporarilyDisabled  *bool // set to true right after submitting a new filter, so the menu disappears
+	enterPending         bool  // set to true if the user has hit enter; process if the filter goes to valid before another change. For slow validity processing.
 	*gowid.Callbacks
 	gowid.IsSelectable
 }
@@ -86,7 +87,14 @@ type Options struct {
 }
 
 func New(opt Options) *Widget {
+	res := &Widget{}
+
 	ed := edit.New()
+	ed.OnTextSet(gowid.WidgetCallback{"cb", func(app gowid.IApp, w gowid.IWidget) {
+		// every time the filter changes, drop any pending enter - we don't want to
+		// apply a filter to a stale value
+		res.enterPending = false
+	}})
 
 	fixed := gowid.RenderFixed{}
 	filterList := list.New(list.NewSimpleListWalker([]gowid.IWidget{}))
@@ -121,8 +129,7 @@ func New(opt Options) *Widget {
 
 	cb := gowid.NewCallbacks()
 
-	temporarilyDisabled := false
-	onelineEd := appkeys.New(ed, handleEnter(cb, &temporarilyDisabled), appkeys.Options{
+	onelineEd := appkeys.New(ed, handleEnter(cb, res), appkeys.Options{
 		ApplyBefore: true,
 	})
 
@@ -148,7 +155,7 @@ func New(opt Options) *Widget {
 	readytorunchan := make(chan struct{})
 	filterchangedchan := make(chan *filtStruct)
 
-	res := &Widget{
+	*res = Widget{
 		wrapped:              wrapped,
 		opts:                 opt,
 		ed:                   ed,
@@ -166,7 +173,7 @@ func New(opt Options) *Widget {
 		runthisfilterchan:    runthisfilterchan,
 		quitchan:             quitchan,
 		readytorunchan:       readytorunchan,
-		temporarilyDisabled:  &temporarilyDisabled,
+		temporarilyDisabled:  new(bool),
 		Callbacks:            cb,
 	}
 
@@ -175,6 +182,14 @@ func New(opt Options) *Widget {
 			app.Run(gowid.RunFunction(func(app gowid.IApp) {
 				res.validitySite.SetSubWidget(res.valid, app)
 				gowid.RunWidgetCallbacks(res.Callbacks, ValidCB{}, app, res)
+
+				if res.enterPending {
+					var dummy gowid.IWidget
+					gowid.RunWidgetCallbacks(cb, SubmitCB{}, app, dummy)
+					*res.temporarilyDisabled = true
+					res.enterPending = false
+				}
+
 			}))
 		},
 	}
@@ -184,6 +199,7 @@ func New(opt Options) *Widget {
 			app.Run(gowid.RunFunction(func(app gowid.IApp) {
 				res.validitySite.SetSubWidget(res.invalid, app)
 				gowid.RunWidgetCallbacks(res.Callbacks, InvalidCB{}, app, res)
+				res.enterPending = false
 			}))
 		},
 	}
@@ -193,6 +209,7 @@ func New(opt Options) *Widget {
 			app.Run(gowid.RunFunction(func(app gowid.IApp) {
 				res.validitySite.SetSubWidget(res.intermediate, app)
 				gowid.RunWidgetCallbacks(res.Callbacks, IntermediateCB{}, app, res)
+				res.enterPending = false
 			}))
 		},
 	}
@@ -337,18 +354,26 @@ func (f *Validator) Validate(filter string) {
 	}
 }
 
+type iFilterEnter interface {
+	setDisabled()
+	setEnterPending()
+	isValid() bool
+}
+
 // if the filter is valid when enter is pressed, submit the SubmitCB callback. Those
 // registered will be able to respond e.g. start handling the valid filter value.
-func handleEnter(cb *gowid.Callbacks, temporarilyDisabled *bool) appkeys.KeyInputFn {
+func handleEnter(cb *gowid.Callbacks, fe iFilterEnter) appkeys.KeyInputFn {
 	return func(evk *tcell.EventKey, app gowid.IApp) bool {
 		handled := false
 		switch evk.Key() {
 		case tcell.KeyEnter:
-
-			var dummy gowid.IWidget
-			gowid.RunWidgetCallbacks(cb, SubmitCB{}, app, dummy)
-			*temporarilyDisabled = true
-
+			if fe.isValid() {
+				var dummy gowid.IWidget
+				gowid.RunWidgetCallbacks(cb, SubmitCB{}, app, dummy)
+				fe.setDisabled()
+			} else {
+				fe.setEnterPending() // remember in case the filter goes valid shortly
+			}
 			handled = true
 		}
 		return handled
@@ -451,6 +476,19 @@ func makeCompletions(comp termshark.IPrefixCompleter, txt string, max int, app g
 	comp.Completions(txt, cb)
 }
 
+func (w *Widget) setDisabled() {
+	*w.temporarilyDisabled = true
+}
+
+func (w *Widget) setEnterPending() {
+	w.enterPending = true
+}
+
+// isCurrentlyValid returns true if the current state of the filter is valid (green)
+func (w *Widget) isValid() bool {
+	return w.validitySite.SubWidget() == w.valid
+}
+
 // Start an asynchronous routine to update the drop-down menu with completion
 // options. Runs on a small delay so it can be cancelled and restarted if the
 // user is typing quickly.