net/dns/resolvconffile: unify three /etc/resolv.conf parsers into new package

Change-Id: I2120893ca802d12f1bd0407d49077d3672627d33
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/resolvconffile/resolvconffile_test.go

@@ -0,0 +1,79 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package resolvconffile
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+
+	"inet.af/netaddr"
+	"tailscale.com/util/dnsname"
+)
+
+func TestParse(t *testing.T) {
+	tests := []struct {
+		in      string
+		want    *Config
+		wantErr bool
+	}{
+		{in: `nameserver 192.168.0.100`,
+			want: &Config{
+				Nameservers: []netaddr.IP{
+					netaddr.MustParseIP("192.168.0.100"),
+				},
+			},
+		},
+		{in: `nameserver 192.168.0.100 # comment`,
+			want: &Config{
+				Nameservers: []netaddr.IP{
+					netaddr.MustParseIP("192.168.0.100"),
+				},
+			},
+		},
+		{in: `nameserver 192.168.0.100#`,
+			want: &Config{
+				Nameservers: []netaddr.IP{
+					netaddr.MustParseIP("192.168.0.100"),
+				},
+			},
+		},
+		{in: `nameserver #192.168.0.100`, wantErr: true},
+		{in: `nameserver`, wantErr: true},
+		{in: `# nameserver 192.168.0.100`, want: &Config{}},
+		{in: `nameserver192.168.0.100`, wantErr: true},
+
+		{in: `search tailsacle.com`,
+			want: &Config{
+				SearchDomains: []dnsname.FQDN{"tailsacle.com."},
+			},
+		},
+		{in: `search tailsacle.com # typo`,
+			want: &Config{
+				SearchDomains: []dnsname.FQDN{"tailsacle.com."},
+			},
+		},
+		{in: `searchtailsacle.com`, wantErr: true},
+		{in: `search`, wantErr: true},
+	}
+
+	for _, tt := range tests {
+		cfg, err := Parse(strings.NewReader(tt.in))
+		if tt.wantErr {
+			if err != nil {
+				continue
+			}
+			t.Errorf("missing error for %q", tt.in)
+			continue
+		}
+		if err != nil {
+			t.Errorf("unexpected error for %q: %v", tt.in, err)
+			continue
+		}
+		if !reflect.DeepEqual(cfg, tt.want) {
+			t.Errorf("got: %v\nwant: %v\n", cfg, tt.want)
+		}
+	}
+}