feat(tunnel): introduce persistent tunnel state dir (#294)

* feat(tunnel): introduce persistent tunnel state dir

This diff introduces a persistent state directory for tunnels, so that
we can bootstrap them more quickly after the first time.

Part of https://github.com/ooni/probe/issues/985

* fix: make tunnel dir optional

We have many tests where it does not make sense to explicitly
provide a tunnel dir because we're not using tunnels.

This should simplify setting up a session.

* fix(tunnel): repair tests

* final changes

* more cleanups

internal/engine/tunnel/tor.go

@@ -3,18 +3,18 @@ package tunnel
 import (
 	"context"
 	"fmt"
+	"io"
 	"net/url"
-	"path"
+	"path/filepath"
 	"strings"
 	"time"
 
 	"github.com/cretz/bine/tor"
 )
 
-// torProcess is a running tor process
+// torProcess is a running tor process.
 type torProcess interface {
-	// Close kills the running tor process
-	Close() error
+	io.Closer
 }
 
 // torTunnel is the Tor tunnel
@@ -52,6 +52,9 @@ func (tt *torTunnel) Stop() {
 	}
 }
 
+// TODO(bassosimone): the current design is such that we have a bunch of
+// torrc-$number and a growing tor.log file inside of stateDir.
+
 // torStart starts the tor tunnel.
 func torStart(ctx context.Context, config *Config) (Tunnel, error) {
 	select {
@@ -59,14 +62,18 @@ func torStart(ctx context.Context, config *Config) (Tunnel, error) {
 		return nil, ctx.Err() // allows to write unit tests using this code
 	default:
 	}
-	logfile := LogFile(config.Session)
+	if config.TunnelDir == "" {
+		return nil, ErrEmptyTunnelDir
+	}
+	stateDir := filepath.Join(config.TunnelDir, "tor")
+	logfile := filepath.Join(stateDir, "tor.log")
 	extraArgs := append([]string{}, config.TorArgs...)
 	extraArgs = append(extraArgs, "Log")
 	extraArgs = append(extraArgs, "notice stderr")
 	extraArgs = append(extraArgs, "Log")
 	extraArgs = append(extraArgs, fmt.Sprintf(`notice file %s`, logfile))
 	instance, err := config.torStart(ctx, &tor.StartConf{
-		DataDir:   path.Join(config.Session.TempDir(), "tor"),
+		DataDir:   stateDir,
 		ExtraArgs: extraArgs,
 		ExePath:   config.TorBinary,
 		NoHush:    true,
@@ -102,9 +109,3 @@ func torStart(ctx context.Context, config *Config) (Tunnel, error) {
 		proxy:         &url.URL{Scheme: "socks5", Host: proxyAddress},
 	}, nil
 }
-
-// LogFile returns the name of tor logs given a specific session. The file
-// is always located somewhere inside the sess.TempDir() directory.
-func LogFile(sess Session) string {
-	return path.Join(sess.TempDir(), "tor.log")
-}