Enable debug globally if enabled in any server config (#4007)

* Enable debug globally if enabled in any server config

It was currently enabled only if the plugin debug
was enabled in the last server config of the Corefile.

Signed-off-by: Olivier Lemasle <o.lemasle@gmail.com>

* Add test and update debug's README

Signed-off-by: Olivier Lemasle <o.lemasle@gmail.com>

3 files changed, 36 insertions, 4 deletions

diff --git a/core/dnsserver/server.go b/core/dnsserver/server.go
index b5bc14a63..915bd73d3 100644
--- a/core/dnsserver/server.go
+++ b/core/dnsserver/server.go
@@ -66,10 +66,6 @@ func NewServer(addr string, group []*Config) (*Server, error) {
 		if site.Debug {
 			s.debug = true
 			log.D.Set()
-		} else {
-			// When reloading we need to explicitly disable debug logging if it is now disabled.
-			s.debug = false
-			log.D.Clear()
 		}
 		// set the config per zone
 		s.zones[site.Zone] = site
@@ -97,6 +93,11 @@ func NewServer(addr string, group []*Config) (*Server, error) {
 		site.pluginChain = stack
 	}
 
+	if !s.debug {
+		// When reloading we need to explicitly disable debug logging if it is now disabled.
+		log.D.Clear()
+	}
+
 	return s, nil
 }
 
diff --git a/core/dnsserver/server_test.go b/core/dnsserver/server_test.go
index c3bed02bc..d7289474b 100644
--- a/core/dnsserver/server_test.go
+++ b/core/dnsserver/server_test.go
@@ -5,6 +5,7 @@ import (
 	"testing"
 
 	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/pkg/log"
 	"github.com/coredns/coredns/plugin/test"
 
 	"github.com/miekg/dns"
@@ -48,6 +49,33 @@ func TestNewServer(t *testing.T) {
 	}
 }
 
+func TestDebug(t *testing.T) {
+	configNoDebug, configDebug := testConfig("dns", testPlugin{}), testConfig("dns", testPlugin{})
+	configDebug.Debug = true
+
+	s1, err := NewServer("127.0.0.1:53", []*Config{configDebug, configNoDebug})
+	if err != nil {
+		t.Errorf("Expected no error for NewServer, got %s", err)
+	}
+	if !s1.debug {
+		t.Errorf("Expected debug mode enabled for server s1")
+	}
+	if !log.D.Value() {
+		t.Errorf("Expected debug logging enabled")
+	}
+
+	s2, err := NewServer("127.0.0.1:53", []*Config{configNoDebug})
+	if err != nil {
+		t.Errorf("Expected no error for NewServer, got %s", err)
+	}
+	if s2.debug {
+		t.Errorf("Expected debug mode disabled for server s2")
+	}
+	if log.D.Value() {
+		t.Errorf("Expected debug logging disabled")
+	}
+}
+
 func BenchmarkCoreServeDNS(b *testing.B) {
 	s, err := NewServer("127.0.0.1:53", []*Config{testConfig("dns", testPlugin{})})
 	if err != nil {
diff --git a/plugin/debug/README.md b/plugin/debug/README.md
index a6234866d..c52ee7299 100644
--- a/plugin/debug/README.md
+++ b/plugin/debug/README.md
@@ -12,6 +12,9 @@ will be printed to standard output.
 
 Note that the *errors* plugin (if loaded) will also set a `recover`, negating this setting.
 
+Enabling this plugin is process-wide: enabling *debug* in at least one server block enables
+debug mode globally.
+
 ## Syntax
 
 ~~~ txt