engine: Rewrite the core algorithm

The engine core had some unfortunate bugs that were the result of some
early design errors when I wasn't as familiar with channels. I've
finally rewritten most of the bad parts, and I think it's much more
logical and stable now.

This also simplifies the resource API, since more of the work is done
completely in the engine, and hidden from view.

Lastly, this adds a few new metaparameters and associated code.

There are still some open problems left to solve, but hopefully this
brings us one step closer.

engine/resources/exec.go

@@ -157,10 +157,7 @@ func (obj *ExecRes) Watch() error {
 		ioChan = obj.bufioChanScanner(scanner)
 	}
 
-	// notify engine that we're running
-	if err := obj.init.Running(); err != nil {
-		return err // exit if requested
-	}
+	obj.init.Running() // when started, notify engine that we're running
 
 	var send = false // send event?
 	for {
@@ -180,24 +177,16 @@ func (obj *ExecRes) Watch() error {
 			obj.init.Logf("watch output: %s", data.text)
 			if data.text != "" {
 				send = true
-				obj.init.Dirty() // dirty
 			}
 
-		case event, ok := <-obj.init.Events:
-			if !ok {
-				return nil
-			}
-			if err := obj.init.Read(event); err != nil {
-				return err
-			}
+		case <-obj.init.Done: // closed by the engine to signal shutdown
+			return nil
 		}
 
 		// do all our event sending all together to avoid duplicate msgs
 		if send {
 			send = false
-			if err := obj.init.Event(); err != nil {
-				return err // exit if requested
-			}
+			obj.init.Event() // notify engine of an event (this can block)
 		}
 	}
 }