middleware/debug: add (#735)

* middleware/debug: add

Add a debug "middleware" that disables the recover() and just lets
CoreDNS crash; very useful for testing.

Fixes ##563

* fix test

* Feedback: check the value of Debug

middleware/debug/README.md

@@ -0,0 +1,20 @@
+# debug
+
+*debug* disables the automatic recovery upon a CoreDNS crash so that you'll get a
+nice stack trace.
+Note that the *errors* middleware (if loaded) will also set a `recover` negating this setting.
+The main use of *debug* is to help testing.
+
+## Syntax
+
+~~~ txt
+debug
+~~~
+
+## Examples
+
+Disable CoreDNS' ability to recover from crashes:
+
+~~~ txt
+debug
+~~~

middleware/debug/debug.go

@@ -0,0 +1,28 @@
+package debug
+
+import (
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/middleware"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("debug", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	config := dnsserver.GetConfig(c)
+
+	for c.Next() {
+		if c.NextArg() {
+			return middleware.Error("debug", c.ArgErr())
+		}
+		config.Debug = true
+	}
+
+	return nil
+}

middleware/debug/debug_test.go

@@ -0,0 +1,49 @@
+package debug
+
+import (
+	"io/ioutil"
+	"log"
+	"testing"
+
+	"github.com/coredns/coredns/core/dnsserver"
+
+	"github.com/mholt/caddy"
+)
+
+func TestDebug(t *testing.T) {
+	log.SetOutput(ioutil.Discard)
+
+	tests := []struct {
+		input         string
+		shouldErr     bool
+		expectedDebug bool
+	}{
+		// positive
+		{
+			`debug`, false, true,
+		},
+		// negative
+		{
+			`debug off`, true, false,
+		},
+	}
+
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", test.input)
+		err := setup(c)
+		cfg := dnsserver.GetConfig(c)
+
+		if test.shouldErr && err == nil {
+			t.Fatalf("Test %d: Expected error but found %s for input %s", i, err, test.input)
+		}
+
+		if err != nil {
+			if !test.shouldErr {
+				t.Fatalf("Test %d: Expected no error but found one for input %s. Error was: %v", i, test.input, err)
+			}
+		}
+		if cfg.Debug != test.expectedDebug {
+			t.Fatalf("Test %d: Expected debug to be: %t, but got: %t, input: %s", i, test.expectedDebug, test.input)
+		}
+	}
+}