plugin/cancel: add context cancelation plugin (#2711)

* plugin/cancel: add context cancelation plugin

Per review comments on #2704, move this into a plugin that gets called.
Add the most minimal plugin, tests and documenation.

Signed-off-by: Miek Gieben <miek@miek.nl>

* plugin/cache: add timeout option

review feedback: add option to set custom timeout.

Signed-off-by: Miek Gieben <miek@miek.nl>

* spelling

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/cancel/cancel.go

@@ -0,0 +1,71 @@
+// Package cancel implements a plugin adds a canceling context to each request.
+package cancel
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/mholt/caddy"
+	"github.com/miekg/dns"
+)
+
+func init() {
+	caddy.RegisterPlugin("cancel", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	ca := Cancel{timeout: 5001 * time.Millisecond}
+
+	for c.Next() {
+		args := c.RemainingArgs()
+		switch len(args) {
+		case 0:
+			break
+		case 1:
+			dur, err := time.ParseDuration(args[0])
+			if err != nil {
+				return plugin.Error("cancel", fmt.Errorf("invalid duration: %q", args[0]))
+			}
+			if dur <= 0 {
+				return plugin.Error("cancel", fmt.Errorf("invalid negative duration: %q", args[0]))
+			}
+			ca.timeout = dur
+		default:
+			return plugin.Error("cancel", c.ArgErr())
+		}
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		ca.Next = next
+		return ca
+	})
+
+	return nil
+}
+
+// Cancel is a plugin that adds a canceling context to each request's context.
+type Cancel struct {
+	timeout time.Duration
+	Next    plugin.Handler
+}
+
+// ServeDNS implements the plugin.Handler interface.
+func (c Cancel) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	ctx, cancel := context.WithTimeout(ctx, c.timeout)
+
+	code, err := plugin.NextOrFailure(c.Name(), c.Next, ctx, w, r)
+
+	cancel()
+
+	return code, err
+}
+
+// Name implements the Handler interface.
+func (c Cancel) Name() string { return "cancel" }