fix: avoid submitting when tor binary is missing

This diff modifies tunnel, torsf, and vanilla such that:

1. we return a special error when tor is not found on PATH

2. we use such an error to avoid submitting

By doing that, we save some useless submissions.

Part of ooni/probe#2406 because
I wrote this diff while investigating it.

internal/experiment/torsf/torsf.go

@@ -25,7 +25,7 @@ import (
 // We may want to have a single implementation for both nettests in the future.
 
 // testVersion is the experiment version.
-const testVersion = "0.3.0"
+const testVersion = "0.4.0"
 
 // Config contains the experiment config.
 type Config struct {
@@ -82,6 +82,9 @@ type TestKeys struct {
 
 	// TransportName is always set to "snowflake" for this experiment.
 	TransportName string `json:"transport_name"`
+
+	// cannotFindTorBinary indicates that we could not find the tor binary.
+	cannotFindTorBinary bool
 }
 
 // Measurer performs the measurement.
@@ -147,6 +150,9 @@ func (m *Measurer) Run(ctx context.Context, args *model.ExperimentArgs) error {
 		case tk := <-tkch:
 			measurement.TestKeys = tk
 			callbacks.OnProgress(1.0, "torsf experiment is finished")
+			if tk.cannotFindTorBinary {
+				return tunnel.ErrCannotFindTorBinary
+			}
 			return nil
 		case <-ticker.C:
 			if !m.config.DisableProgress {
@@ -228,7 +234,9 @@ func (m *Measurer) bootstrap(ctx context.Context, timeout time.Duration, sess mo
 			"ClientTransportPlugin", ptl.AsClientTransportPluginArgument(),
 			"Bridge", sfdialer.AsBridgeArgument(),
 		},
+		TorBinary: sess.TorBinary(),
 	})
+	tk.cannotFindTorBinary = errors.Is(err, tunnel.ErrCannotFindTorBinary)
 	tk.TorVersion = debugInfo.Version
 	m.readTorLogs(sess.Logger(), tk, debugInfo.LogFilePath)
 	if err != nil {

internal/experiment/vanillator/vanillator.go

@@ -23,7 +23,7 @@ import (
 // We may want to have a single implementation for both nettests in the future.
 
 // testVersion is the experiment version.
-const testVersion = "0.2.0"
+const testVersion = "0.3.0"
 
 // Config contains the experiment config.
 type Config struct {
@@ -68,6 +68,9 @@ type TestKeys struct {
 
 	// TransportName is always set to "vanilla" for this experiment.
 	TransportName string `json:"transport_name"`
+
+	// cannotFindTorBinary indicates that we could not find the tor binary.
+	cannotFindTorBinary bool
 }
 
 // Measurer performs the measurement.
@@ -123,6 +126,9 @@ func (m *Measurer) Run(ctx context.Context, args *model.ExperimentArgs) error {
 		case tk := <-tkch:
 			measurement.TestKeys = tk
 			callbacks.OnProgress(1.0, "vanilla_tor experiment is finished")
+			if tk.cannotFindTorBinary {
+				return tunnel.ErrCannotFindTorBinary
+			}
 			return nil
 		case <-ticker.C:
 			if !m.config.DisableProgress {
@@ -168,7 +174,9 @@ func (m *Measurer) bootstrap(ctx context.Context, timeout time.Duration,
 		Session:   sess,
 		TunnelDir: path.Join(m.baseTunnelDir(sess), "vanillator"),
 		Logger:    sess.Logger(),
+		TorBinary: sess.TorBinary(),
 	})
+	tk.cannotFindTorBinary = errors.Is(err, tunnel.ErrCannotFindTorBinary)
 	tk.TorVersion = debugInfo.Version
 	m.readTorLogs(sess.Logger(), tk, debugInfo.LogFilePath)
 	if err != nil {

internal/tunnel/tor.go

@@ -12,6 +12,11 @@ import (
 	"time"
 )
 
+// ErrCannotFindTorBinary is an error emitted when we cannot find the
+// tor binary. We use this error in vanillator and torsf to detect
+// cases where this happens and avoid submitting measurements.
+var ErrCannotFindTorBinary = errors.New("tunnel: cannot find tor binary")
+
 // torProcess is a running tor process.
 type torProcess interface {
 	io.Closer

internal/tunnel/tordesktop.go

@@ -10,6 +10,7 @@ package tunnel
 //
 
 import (
+	"fmt"
 	"strings"
 
 	"github.com/cretz/bine/tor"
@@ -21,7 +22,7 @@ func getTorStartConf(config *Config, dataDir string, extraArgs []string) (*tor.S
 	exePath, err := config.torBinary()
 	if err != nil {
 		config.logger().Warnf("cannot find tor binary: %s", err.Error())
-		return nil, err
+		return nil, fmt.Errorf("%w: %s", ErrCannotFindTorBinary, err.Error())
 	}
 	config.logger().Infof("tunnel: tor: exec: %s %s %s", exePath,
 		dataDir, strings.Join(extraArgs, " "))