Build modules: a restart ends the steps it left running, a step shows what it is, build_*_step_end ends one (issue 1890)

modules/mesh-build-node/cmd/mesh-build-node/asks.go

@@ -74,3 +74,11 @@ func (m *Module) Puller(steps []StepTool, source buildstep.AskSource) *buildstep
 func (m *Module) SeatTools(p *buildstep.Puller) []stdio.Tool {
 	return buildstep.SeatTools(Seat, "Node", p)
 }
+
+// StepEndTool ends one step left running (issue 1890); the module's steps state is the one it reads and writes.
+const StepEndTool = "build_node_step_end"
+
+// stepScan is the module's steps state as buildstep reads it.
+func (m *Module) stepScan() buildstep.Scan {
+	return buildstep.Scan{Keys: m.ReadSteps.Keys, Get: m.ReadSteps.Get}
+}