Add RegisterDirective

core/dnsserver/directives.go

 package dnsserver
 
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+// RegisterDirective splices name into the list of directives
+// immediately before another directive. .
+// If before is empty,
+// the directive will be appended to the end of the list.
+//
+// It is imperative that directives execute in the proper
+// order, and hard-coding the list of directives guarantees
+// a correct, absolute order every time. This function is
+// convenient when developing a plugin, but it does not
+// guarantee absolute ordering. Multiple plugins registering
+// directives with this function will lead to non-
+// deterministic builds and buggy software.
+//
+// Directive names must be lower-cased and unique. Any errors
+// here are fatal, and even successful calls print a message
+// to stdout as a reminder to use it only in development.
+func RegisterDirective(name, before string) {
+	if name == "" {
+		fmt.Println("[FATAL] Cannot register empty directive name")
+		os.Exit(1)
+	}
+	if strings.ToLower(name) != name {
+		fmt.Printf("[FATAL] %s: directive name must be lowercase\n", name)
+		os.Exit(1)
+	}
+	for _, dir := range directives {
+		if dir == name {
+			fmt.Printf("[FATAL] %s: directive name already exists\n", name)
+			os.Exit(1)
+		}
+	}
+	if before == "" {
+		directives = append(directives, name)
+	} else {
+		var found bool
+		for i, dir := range directives {
+			if dir == before {
+				directives = append(directives[:i], append([]string{name}, directives[i:]...)...)
+				found = true
+				break
+			}
+		}
+		if !found {
+			fmt.Printf("[FATAL] %s: directive not found\n", before)
+			os.Exit(1)
+		}
+	}
+	msg := fmt.Sprintf("Registered directive '%s' ", name)
+	if before == "" {
+		msg += "at end of list"
+	} else {
+		msg += fmt.Sprintf("before '%s'", before)
+	}
+	fmt.Printf("[INFO] %s\n", msg)
+}
+
 // Add here, and in core/coredns.go to use them.
 
 // Directives are registered in the order they should be