perf(metrics): implement plugin chain tracking (#7791)

Remove expensive runtime.Caller calls from metrics Recorder.WriteMsg
by tracking the responding plugin through the plugin chain instead.

- Add PluginTracker interface and pluginWriter wrapper in plugin.go
- Modify NextOrFailure to wrap ResponseWriter with plugin name
- Update metrics Recorder to implement PluginTracker
- Remove authoritativePlugin method using filepath inspection

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/metrics/handler.go

@@ -2,7 +2,6 @@ package metrics
 
 import (
 	"context"
-	"path/filepath"
 
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/metrics/vars"
@@ -36,9 +35,9 @@ func (m *Metrics) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 		// see https://github.com/coredns/coredns/blob/master/core/dnsserver/server.go#L318
 		rc = status
 	}
-	plugin := m.authoritativePlugin(rw.Caller)
 	// Pass the original request size to vars.Report
-	vars.Report(WithServer(ctx), state, zone, WithView(ctx), rcode.ToString(rc), plugin,
+	// rw.Plugin is set automatically by the plugin chain via the PluginTracker interface
+	vars.Report(WithServer(ctx), state, zone, WithView(ctx), rcode.ToString(rc), rw.Plugin,
 		rw.Len, rw.Start, vars.WithOriginalReqSize(originalSize))
 
 	return status, err
@@ -46,17 +45,3 @@ func (m *Metrics) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 
 // Name implements the Handler interface.
 func (m *Metrics) Name() string { return "prometheus" }
-
-// authoritativePlugin returns which of made the write, if none is found the empty string is returned.
-func (m *Metrics) authoritativePlugin(caller [3]string) string {
-	// a b and c contain the full path of the caller, the plugin name 2nd last elements
-	// .../coredns/plugin/whoami/whoami.go --> whoami
-	// this is likely FS specific, so use filepath.
-	for _, c := range caller {
-		plug := filepath.Base(filepath.Dir(c))
-		if _, ok := m.plugins[plug]; ok {
-			return plug
-		}
-	}
-	return ""
-}

plugin/metrics/recorder.go

@@ -1,8 +1,6 @@
 package metrics
 
 import (
-	"runtime"
-
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
 
 	"github.com/miekg/dns"
@@ -11,8 +9,9 @@ import (
 // Recorder is a dnstest.Recorder specific to the metrics plugin.
 type Recorder struct {
 	*dnstest.Recorder
-	// CallerN holds the string return value of the call to runtime.Caller(N+1)
-	Caller [3]string
+	// Plugin holds the name of the plugin that wrote the response.
+	// This is set automatically by the plugin chain via the PluginTracker interface.
+	Plugin string
 }
 
 // NewRecorder makes and returns a new Recorder.
@@ -21,8 +20,15 @@ func NewRecorder(w dns.ResponseWriter) *Recorder { return &Recorder{Recorder: dn
 // WriteMsg records the status code and calls the
 // underlying ResponseWriter's WriteMsg method.
 func (r *Recorder) WriteMsg(res *dns.Msg) error {
-	_, r.Caller[0], _, _ = runtime.Caller(1)
-	_, r.Caller[1], _, _ = runtime.Caller(2)
-	_, r.Caller[2], _, _ = runtime.Caller(3)
 	return r.Recorder.WriteMsg(res)
 }
+
+// SetPlugin implements the plugin.PluginTracker interface.
+func (r *Recorder) SetPlugin(name string) {
+	r.Plugin = name
+}
+
+// GetPlugin implements the plugin.PluginTracker interface.
+func (r *Recorder) GetPlugin() string {
+	return r.Plugin
+}

plugin/metrics/recorder_test.go

@@ -23,6 +23,34 @@ func (r *inmemoryWriter) Write(buf []byte) (int, error) {
 	return r.ResponseWriter.Write(buf)
 }
 
+func TestRecorder_PluginTracker(t *testing.T) {
+	tw := inmemoryWriter{ResponseWriter: test.ResponseWriter{}}
+	rec := NewRecorder(&tw)
+
+	// Initially Plugin should be empty
+	if rec.Plugin != "" {
+		t.Errorf("Expected empty Plugin, got %q", rec.Plugin)
+	}
+	if rec.GetPlugin() != "" {
+		t.Errorf("Expected GetPlugin() to return empty string, got %q", rec.GetPlugin())
+	}
+
+	// SetPlugin should set the plugin name
+	rec.SetPlugin("whoami")
+	if rec.Plugin != "whoami" {
+		t.Errorf("Expected Plugin to be 'whoami', got %q", rec.Plugin)
+	}
+	if rec.GetPlugin() != "whoami" {
+		t.Errorf("Expected GetPlugin() to return 'whoami', got %q", rec.GetPlugin())
+	}
+
+	// SetPlugin should overwrite previous value
+	rec.SetPlugin("cache")
+	if rec.Plugin != "cache" {
+		t.Errorf("Expected Plugin to be 'cache', got %q", rec.Plugin)
+	}
+}
+
 func TestRecorder_WriteMsg(t *testing.T) {
 	successResp := dns.Msg{}
 	successResp.Answer = []dns.RR{