net/dns: add start of Linux newOSConfigurator tests

Only one test case so far.

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

net/dns/manager_linux.go

@@ -29,7 +29,6 @@ func (kv kv) String() string {
 func NewOSConfigurator(logf logger.Logf, interfaceName string) (ret OSConfigurator, err error) {
 	return newOSConfigurator(logf, interfaceName, newOSConfigEnv{
 		fs:                         directFS{},
-		ReadFile:                   os.ReadFile,
 		resolvOwner:                resolvOwner,
 		resolvedIsActuallyResolver: resolvedIsActuallyResolver,
 		dbusPing:                   dbusPing,
@@ -42,8 +41,7 @@ func NewOSConfigurator(logf logger.Logf, interfaceName string) (ret OSConfigurat
 // newOSConfigEnv are the funcs newOSConfigurator needs, pulled out for testing.
 type newOSConfigEnv struct {
 	fs                         wholeFileFS
-	ReadFile                   func(name string) ([]byte, error)
-	resolvOwner                func(resolvConfContnets []byte) string
+	resolvOwner                func(resolvConfContents []byte) string
 	resolvedIsActuallyResolver func(wholeFileFS) error
 	dbusPing                   func(string, string) error
 	nmIsUsingResolved          func() error
@@ -63,7 +61,7 @@ func newOSConfigurator(logf logger.Logf, interfaceName string, env newOSConfigEn
 		logf("dns: %v", debug)
 	}()
 
-	bs, err := env.ReadFile("/etc/resolv.conf")
+	bs, err := env.fs.ReadFile(resolvConf)
 	if os.IsNotExist(err) {
 		dbg("rc", "missing")
 		return newDirectManager(), nil

net/dns/manager_linux_test.go

@@ -0,0 +1,84 @@
+// Copyright (c) 2021 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 dns
+
+import (
+	"bytes"
+	"fmt"
+	"io/fs"
+	"os"
+	"testing"
+)
+
+func TestLinuxNewOSConfigurator(t *testing.T) {
+	tests := []struct {
+		name    string
+		env     newOSConfigEnv
+		wantLog string
+		want    string // reflect type string
+	}{
+		{
+			name: "no_obvious_resolv.conf_owner",
+			env: newOSConfigEnv{
+				fs: memFS{
+					"/etc/resolv.conf": "nameserver 10.0.0.1\n",
+				},
+				resolvOwner: resolvOwner,
+			},
+			wantLog: "dns: [rc=unknown ret=dns.directManager]\n",
+			want:    "dns.directManager",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var logBuf bytes.Buffer
+			logf := func(format string, a ...interface{}) {
+				fmt.Fprintf(&logBuf, format, a...)
+				logBuf.WriteByte('\n')
+			}
+			osc, err := newOSConfigurator(logf, "unused_if_name0", tt.env)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if got := fmt.Sprintf("%T", osc); got != tt.want {
+				t.Errorf("got %s; want %s", got, tt.want)
+			}
+			if tt.wantLog != string(logBuf.Bytes()) {
+				t.Errorf("log output mismatch:\n got: %q\nwant: %q\n", logBuf.Bytes(), tt.wantLog)
+			}
+		})
+	}
+}
+
+type memFS map[string]interface{} // full path => string for regular files
+
+func (m memFS) Stat(name string) (isRegular bool, err error) {
+	v, ok := m[name]
+	if !ok {
+		return false, fs.ErrNotExist
+	}
+	if _, ok := v.(string); ok {
+		return true, nil
+	}
+	return false, nil
+}
+
+func (m memFS) Rename(oldName, newName string) error { panic("TODO") }
+func (m memFS) Remove(name string) error             { panic("TODO") }
+func (m memFS) ReadFile(name string) ([]byte, error) {
+	v, ok := m[name]
+	if !ok {
+		return nil, fs.ErrNotExist
+	}
+	if s, ok := v.(string); ok {
+		return []byte(s), nil
+	}
+	panic("TODO")
+}
+
+func (fs memFS) WriteFile(name string, contents []byte, perm os.FileMode) error {
+	fs[name] = string(contents)
+	return nil
+}