First draft of modifying RunnerListener to CallListener to get it closer to the action (and named better).

api/agent/agent.go

@@ -209,6 +209,11 @@ func (a *agent) Submit(callI Call) error {
 	// to make this remove the container asynchronously?
 	defer slot.Close() // notify our slot is free once we're done
 
+	err = a.fireBeforeCall(ctx, call.Model())
+	if err != nil {
+		return fmt.Errorf("BeforeCall: %v", err)
+	}
+
 	// TODO Start is checking the timer now, we could do it here, too.
 	err = call.Start(ctx, a)
 	if err != nil {

api/extenders/listeners.go

@@ -0,0 +1,31 @@
+package extenders
+
+import (
+	"context"
+
+	"github.com/fnproject/fn/api/models"
+)
+
+// AppListener is an interface used to inject custom code at key points in app lifecycle.
+type AppListener interface {
+	// BeforeAppCreate called right before creating App in the database
+	BeforeAppCreate(ctx context.Context, app *models.App) error
+	// AfterAppCreate called after creating App in the database
+	AfterAppCreate(ctx context.Context, app *models.App) error
+	// BeforeAppUpdate called right before updating App in the database
+	BeforeAppUpdate(ctx context.Context, app *models.App) error
+	// AfterAppUpdate called after updating App in the database
+	AfterAppUpdate(ctx context.Context, app *models.App) error
+	// BeforeAppDelete called right before deleting App in the database
+	BeforeAppDelete(ctx context.Context, app *models.App) error
+	// AfterAppDelete called after deleting App in the database
+	AfterAppDelete(ctx context.Context, app *models.App) error
+}
+
+// CallListener enables callbacks around Call events
+type CallListener interface {
+	// BeforeCall called before a function is executed
+	BeforeCall(ctx context.Context, call *models.Call) error
+	// AfterCall called after a function completes
+	AfterCall(ctx context.Context, call *models.Call) error
+}

api/server/runner.go

@@ -86,6 +86,7 @@ func (s *Server) serve(c *gin.Context, appName, path string) {
 		return
 	}
 
+	err = doCall(call)
 	err = s.Agent.Submit(call)
 	if err != nil {
 		// NOTE if they cancel the request then it will stop the call (kind of cool),