refactor(tunnel): simplify psiphon implementation (#289)

Simplify interaction within the package by avoiding to have
a psiphon specific config. Use a Config instead.

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

internal/engine/tunnel/config.go

@@ -0,0 +1,58 @@
+package tunnel
+
+import (
+	"context"
+	"os"
+
+	"github.com/ooni/psiphon/oopsi/github.com/Psiphon-Labs/psiphon-tunnel-core/ClientLibrary/clientlib"
+)
+
+// Config contains the configuration for creating a Tunnel instance.
+type Config struct {
+	// Name is the mandatory name of the tunnel. We support
+	// "tor" and "psiphon" tunnels.
+	Name string
+
+	// Session is the current measurement session.
+	Session Session
+
+	// WorkDir is the directory in which the tunnel SHOULD
+	// store its state, if any.
+	WorkDir string
+
+	// testMkdirAll allows us to mock os.MkdirAll in testing code.
+	testMkdirAll func(path string, perm os.FileMode) error
+
+	// testRemoveAll allows us to mock os.RemoveAll in testing code.
+	testRemoveAll func(path string) error
+
+	// testStartPsiphon allows us to mock psiphon's clientlib.StartTunnel.
+	testStartPsiphon func(ctx context.Context, config []byte,
+		workdir string) (*clientlib.PsiphonTunnel, error)
+}
+
+// mkdirAll calls either testMkdirAll or os.MkdirAll.
+func (c *Config) mkdirAll(path string, perm os.FileMode) error {
+	if c.testMkdirAll != nil {
+		return c.testMkdirAll(path, perm)
+	}
+	return os.MkdirAll(path, perm)
+}
+
+// removeAll calls either testRemoveAll or os.RemoveAll.
+func (c *Config) removeAll(path string) error {
+	if c.testRemoveAll != nil {
+		return c.testRemoveAll(path)
+	}
+	return os.RemoveAll(path)
+}
+
+// startPsiphon calls either testStartPsiphon or psiphon's clientlib.StartTunnel.
+func (c *Config) startPsiphon(ctx context.Context, config []byte,
+	workdir string) (*clientlib.PsiphonTunnel, error) {
+	if c.testStartPsiphon != nil {
+		return c.testStartPsiphon(ctx, config, workdir)
+	}
+	return clientlib.StartTunnel(ctx, config, "", clientlib.Parameters{
+		DataRootDirectory: &workdir}, nil, nil)
+}