Add NSID plugin support for CoreDNS (#1273)

* Add NSID plugin support for CoreDNS

This fix adds NSID plugin support for CoreDNS, as was proposed
in 1256.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add test cases for NSID plugin

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Generate code for NSID plugin

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Use hostname as the default (as with bind), and remove unneeded copy

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add README.md

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/nsid/setup_test.go

@@ -0,0 +1,58 @@
+package nsid
+
+import (
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestSetupNsid(t *testing.T) {
+	defaultNsid, err := os.Hostname()
+	if err != nil {
+		defaultNsid = "localhost"
+	}
+	tests := []struct {
+		input              string
+		shouldErr          bool
+		expectedData       string
+		expectedErrContent string // substring from the expected error. Empty for positive cases.
+	}{
+		{
+			`nsid`, false, defaultNsid, "",
+		},
+		{
+			`nsid "ps0"`, false, "ps0", "",
+		},
+		{
+			`nsid "worker1"`, false, "worker1", "",
+		},
+		{
+			`nsid "tf 2"`, false, "tf 2", "",
+		},
+	}
+
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", test.input)
+		nsid, err := nsidParse(c)
+
+		if test.shouldErr && err == nil {
+			t.Errorf("Test %d: Expected error but found %s for input %s", i, err, test.input)
+		}
+
+		if err != nil {
+			if !test.shouldErr {
+				t.Errorf("Test %d: Expected no error but found one for input %s. Error was: %v", i, test.input, err)
+			}
+
+			if !strings.Contains(err.Error(), test.expectedErrContent) {
+				t.Errorf("Test %d: Expected error to contain: %v, found error: %v, input: %s", i, test.expectedErrContent, err, test.input)
+			}
+		}
+
+		if !test.shouldErr && nsid != test.expectedData {
+			t.Errorf("Nsid not correctly set for input %s. Expected: %s, actual: %s", test.input, test.expectedData, nsid)
+		}
+	}
+}