plugin/view: Advanced routing interface and new 'view' plugin (#5538)

* introduce new interface "dnsserver.Viewer", that allows a plugin implementing it to decide if a query should be routed into its server block.
* add new plugin "view", that uses the new interface to enable a user to define expression based conditions that must be met for a query to be routed to its server block.

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/view/setup_test.go

@@ -0,0 +1,38 @@
+package view
+
+import (
+	"testing"
+
+	"github.com/coredns/caddy"
+)
+
+func TestSetup(t *testing.T) {
+	tests := []struct {
+		input     string
+		shouldErr bool
+		progCount int
+	}{
+		{"view example {\n expr name() == 'example.com.'\n}", false, 1},
+		{"view example {\n expr incidr(client_ip(), '10.0.0.0/24')\n}", false, 1},
+		{"view example {\n expr name() == 'example.com.'\n expr name() == 'example2.com.'\n}", false, 2},
+		{"view", true, 0},
+		{"view example {\n expr invalid expression\n}", true, 0},
+	}
+
+	for i, test := range tests {
+		v, err := parse(caddy.NewTestController("dns", test.input))
+
+		if test.shouldErr && err == nil {
+			t.Errorf("Test %d: Expected error but found none for input %s", i, test.input)
+		}
+		if err != nil && !test.shouldErr {
+			t.Errorf("Test %d: Expected no error but found one for input %s. Error was: %v", i, test.input, err)
+		}
+		if test.shouldErr {
+			continue
+		}
+		if test.progCount != len(v.progs) {
+			t.Errorf("Test %d: Expected prog length %d, but got %d for %s.", i, test.progCount, len(v.progs), test.input)
+		}
+	}
+}