OncePerServerBlock may now return an error

config/config.go

@@ -68,7 +68,13 @@ func Load(filename string, input io.Reader) (Group, error) {
 					controller := &setup.Controller{
 						Config:    &config,
 						Dispenser: parse.NewDispenserTokens(filename, tokens),
-						OncePerServerBlock: func(f func()) { onces[dir.name].Do(f) },
+						OncePerServerBlock: func(f func() error) error {
+							var err error
+							onces[dir.name].Do(func() {
+								err = f()
+							})
+							return err
+						},
 					}
 
 					midware, err := dir.setup(controller)

config/setup/controller.go

@@ -19,8 +19,11 @@ type Controller struct {
 
 	// OncePerServerBlock is a function that executes f
 	// exactly once per server block, no matter how many
-	// hosts are associated with it.
-	OncePerServerBlock func(f func())
+	// hosts are associated with it. If it is the first
+	// time, the function f is executed immediately
+	// (not deferred) and may return an error which is
+	// returned by OncePerServerBlock.
+	OncePerServerBlock func(f func() error) error
 }
 
 // NewTestController creates a new *Controller for

config/setup/startupshutdown.go

@@ -55,9 +55,8 @@ func registerCallback(c *Controller, list *[]func() error) error {
 		funcs = append(funcs, fn)
 	}
 
-	c.OncePerServerBlock(func() {
+	return c.OncePerServerBlock(func() error {
 		*list = append(*list, funcs...)
-	})
-
 		return nil
+	})
 }