middleware.cfg to configure middleware directives (#496)

* Use go generate to build middleware setup based on middleware.cfg
Init default config

* generated files

* Move gen to an isolated area

* rename files

* PR review updates

* undo readme

gen/directives_generate.go

@@ -0,0 +1,116 @@
+package main
+
+import (
+	"bufio"
+	"io/ioutil"
+	"os"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+var middlewarePath = "github.com/miekg/coredns/middleware/"
+var header = "// generated by directives_generate.go; DO NOT EDIT\n"
+
+func main() {
+	mwFile := os.Args[1]
+
+	mi := make(map[string]string, 0)
+	md := make(map[int]string, 0)
+
+	if file, err := os.Open(mwFile); err == nil {
+		defer file.Close()
+
+		scanner := bufio.NewScanner(file)
+		for scanner.Scan() {
+			line := scanner.Text()
+			if !strings.HasPrefix(line, `//`) && !strings.HasPrefix(line, "#") {
+				items := strings.Split(line, ":")
+				if len(items) == 3 {
+					if priority, err := strconv.Atoi(items[0]); err == nil {
+						md[priority] = items[1]
+					}
+
+					if items[2] != "" {
+						if strings.Contains(items[2], "/") {
+							mi[items[1]] = items[2]
+						} else {
+							mi[items[1]] = middlewarePath + items[2]
+						}
+					}
+
+				}
+			}
+		}
+
+		var orders []int
+		for k := range md {
+			orders = append(orders, k)
+		}
+		sort.Ints(orders)
+
+		if os.Getenv("GOPACKAGE") == "core" {
+			genImports("zmiddleware.go", mi)
+		}
+		if os.Getenv("GOPACKAGE") == "dnsserver" {
+			genDirectives("zdirectives.go", md)
+		}
+
+	} else {
+		os.Exit(1)
+	}
+	os.Exit(0)
+
+}
+
+func genImports(file string, mi map[string]string) {
+	outs := header + "package " + os.Getenv("GOPACKAGE") + "\n\n" + "import ("
+
+	if len(mi) > 0 {
+		outs += "\n"
+	}
+
+	for _, v := range mi {
+		outs += "		_ \"" + v + "\"\n"
+	}
+	outs += ")\n"
+
+	err := ioutil.WriteFile(file, []byte(outs), 0644)
+	if err != nil {
+		os.Exit(1)
+	}
+
+}
+
+func genDirectives(file string, md map[int]string) {
+
+	outs := header + "package " + os.Getenv("GOPACKAGE") + "\n\n"
+	outs += `
+// Directives are registered in the order they should be
+// executed.
+//
+// Ordering is VERY important. Every middleware will
+// feel the effects of all other middleware below
+// (after) them during a request, but they must not
+// care what middleware above them are doing.
+
+var directives = []string{
+`
+
+	var orders []int
+	for k := range md {
+		orders = append(orders, k)
+	}
+	sort.Ints(orders)
+
+	for _, k := range orders {
+		outs += "		\"" + md[k] + "\",\n"
+	}
+
+	outs += "}\n"
+
+	err := ioutil.WriteFile(file, []byte(outs), 0644)
+	if err != nil {
+		os.Exit(1)
+	}
+}