Metrics2 (#1588)

* plugin/metrics: still need nil check for shutdown

the second prometheus statement will trigger: (on control-C)

[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x94f45a]

goroutine 25 [running]:
github.com/coredns/coredns/plugin/metrics.(*Metrics).OnShutdown(0xc420252000, 0x0, 0x0)
	/home/miek/g/src/github.com/coredns/coredns/plugin/metrics/metrics.go:107 +0x2a
github.com/coredns/coredns/plugin/metrics.(*Metrics).OnShutdown-fm(0x0, 0x0)
	/home/miek/g/src/github.com/coredns/coredns/plugin/metrics/setup.go:39 +0x2a
github.com/mholt/caddy.(*Instance).ShutdownCallbacks(0xc4202c81e0, 0x0, 0x0, 0x0)
	/home/miek/g/src/github.com/mholt/caddy/caddy.go:164 +0xb3
github.com/mholt/caddy.allShutdownCallbacks(0x1743935, 0x8, 0x14a1b40)
	/home/miek/g/src/github.com/mholt/caddy/sigtrap.go:95 +0x10d
github.com/mholt/caddy.executeShutdownCallbacks.func1()
	/home/miek/g/src/github.com/mholt/caddy/sigtrap.go:75 +0x8f
sync.(*Once).Do(0x2256b80, 0xc42036df88)
	/home/miek/upstream/go/src/sync/once.go:44 +0xbe
github.com/mholt/caddy.executeShutdownCallbacks(0x174033f, 0x6, 0x0)
	/home/miek/g/src/github.com/mholt/caddy/sigtrap.go:71 +0x73
github.com/mholt/caddy.trapSignalsCrossPlatform.func1.1()
	/home/miek/g/src/github.com/mholt/caddy/sigtrap.go:61 +0x36
created by github.com/mholt/caddy.trapSignalsCrossPlatform.func1
	/home/miek

* comments on why

1 files changed, 8 insertions, 1 deletions

diff --git a/plugin/metrics/metrics.go b/plugin/metrics/metrics.go
index 0c24ffd15..a49984b3d 100644
--- a/plugin/metrics/metrics.go
+++ b/plugin/metrics/metrics.go
@@ -104,7 +104,14 @@ func (m *Metrics) OnStartup() error {
 }
 
 // OnShutdown tears down the metrics listener on shutdown and restart.
-func (m *Metrics) OnShutdown() error { return m.ln.Close() }
+func (m *Metrics) OnShutdown() error {
+	// We allow prometheus statements in multiple Server Blocks, but only the first
+	// will open the listener, for the rest they are all nil; guard against that.
+	if m.ln != nil {
+		return m.ln.Close()
+	}
+	return nil
+}
 
 func keys(m map[string]bool) []string {
 	sx := []string{}