fix(tunnel/tor): keep tunneldir clean (#300)

* fix(tunnel/tor): keep tunneldir clean

This diff ensures that we don't keep the log file growing and
we also remove the temporary files created by the library we
are currently using for running tor from golang.

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

* fix(session.go): tell use we're using a tunnel

internal/engine/tunnel/tor_test.go

@@ -3,7 +3,12 @@ package tunnel
 import (
 	"context"
 	"errors"
+	"fmt"
+	"io/ioutil"
 	"net/url"
+	"os"
+	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/cretz/bine/control"
@@ -226,3 +231,53 @@ func TestTorUnsupportedProxy(t *testing.T) {
 		t.Fatal("expected nil tunnel here")
 	}
 }
+
+func TestMaybeCleanupTunnelDir(t *testing.T) {
+	fakeTunDir := filepath.Join("testdata", "fake-tun-dir")
+	if err := os.RemoveAll(fakeTunDir); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.MkdirAll(fakeTunDir, 0700); err != nil {
+		t.Fatal(err)
+	}
+	fakeData := []byte("deadbeef\n")
+	logfile := filepath.Join(fakeTunDir, "tor.log")
+	if err := ioutil.WriteFile(logfile, fakeData, 0600); err != nil {
+		t.Fatal(err)
+	}
+	for idx := 0; idx < 3; idx++ {
+		filename := filepath.Join(fakeTunDir, fmt.Sprintf("torrc-%d", idx))
+		if err := ioutil.WriteFile(filename, fakeData, 0600); err != nil {
+			t.Fatal(err)
+		}
+		filename = filepath.Join(fakeTunDir, fmt.Sprintf("control-port-%d", idx))
+		if err := ioutil.WriteFile(filename, fakeData, 0600); err != nil {
+			t.Fatal(err)
+		}
+		filename = filepath.Join(fakeTunDir, fmt.Sprintf("antani-%d", idx))
+		if err := ioutil.WriteFile(filename, fakeData, 0600); err != nil {
+			t.Fatal(err)
+		}
+	}
+	files, err := filepath.Glob(filepath.Join(fakeTunDir, "*"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(files) != 10 {
+		t.Fatal("unexpected number of files")
+	}
+	maybeCleanupTunnelDir(fakeTunDir, logfile)
+	files, err = filepath.Glob(filepath.Join(fakeTunDir, "*"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(files) != 3 {
+		t.Fatal("unexpected number of files")
+	}
+	expectPrefix := filepath.Join(fakeTunDir, "antani-")
+	for _, file := range files {
+		if !strings.HasPrefix(file, expectPrefix) {
+			t.Fatal("unexpected file name: ", file)
+		}
+	}
+}