cleanup(miniooni): rename kvstore2 to engine (#1436)

Noticed when preparing the SplinterCon demo. We should have the same
name for the engine state directory for miniooni and ooniprobe.

Otherwise, we need to tell people about this difference.

Extracted from: #1423

Closes: ooni/probe#2646

internal/cmd/miniooni/session.go

@@ -9,6 +9,7 @@ import (
 	"github.com/apex/log"
 	"github.com/ooni/probe-cli/v3/internal/engine"
 	"github.com/ooni/probe-cli/v3/internal/kvstore"
+	"github.com/ooni/probe-cli/v3/internal/legacy/kvstore2dir"
 	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/runtimex"
 	"github.com/ooni/probe-cli/v3/internal/version"
@@ -27,9 +28,12 @@ func newSessionOrPanic(ctx context.Context, currentOptions *Options,
 		proxyURL = mustParseURL(currentOptions.Proxy)
 	}
 
-	kvstore2dir := filepath.Join(miniooniDir, "kvstore2")
-	kvstore, err := kvstore.NewFS(kvstore2dir)
-	runtimex.PanicOnError(err, "cannot create kvstore2 directory")
+	// We renamed kvstore2 to engine in the 3.20 development cycle
+	_ = kvstore2dir.Move(miniooniDir)
+
+	enginedir := filepath.Join(miniooniDir, "engine")
+	kvstore, err := kvstore.NewFS(enginedir)
+	runtimex.PanicOnError(err, "cannot create engine directory")
 
 	tunnelDir := filepath.Join(miniooniDir, "tunnel")
 	err = os.MkdirAll(tunnelDir, 0700)

internal/legacy/kvstore2dir/kvstore2dir.go

@@ -0,0 +1,34 @@
+// Package kvstore2dir migrates $OONI_HOME/kvstore2 to $OONI_HOME/engine. This ensures
+// that miniooni and ooniprobe use the same directory name for the engine state.
+package kvstore2dir
+
+import (
+	"os"
+	"path/filepath"
+)
+
+type statBuf interface {
+	IsDir() bool
+}
+
+func simplifiedStat(path string) (statBuf, error) {
+	return os.Stat(path)
+}
+
+var (
+	osStat   = simplifiedStat
+	osRename = os.Rename
+)
+
+// Move moves $OONI_HOME/kvstore2 to $OONI_HOME/engine, if possible.
+func Move(dir string) error {
+	kvstore2dir := filepath.Join(dir, "kvstore2")
+	if stat, err := osStat(kvstore2dir); err != nil || !stat.IsDir() {
+		return nil
+	}
+	enginedir := filepath.Join(dir, "engine")
+	if _, err := osStat(enginedir); err == nil {
+		return nil
+	}
+	return osRename(kvstore2dir, enginedir)
+}

internal/legacy/kvstore2dir/kvstore2dir_test.go

@@ -0,0 +1,102 @@
+package kvstore2dir
+
+import (
+	"errors"
+	"io"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+type booleanStatBuf bool
+
+var _ statBuf = booleanStatBuf(true)
+
+// IsDir implements statBuf.
+func (v booleanStatBuf) IsDir() bool {
+	return bool(v)
+}
+
+func TestMove(t *testing.T) {
+	// testcase is a test case implemented by this function
+	type testcase struct {
+		name     string
+		osStat   func(name string) (statBuf, error)
+		osRename func(oldpath string, newpath string) error
+		expect   error
+	}
+
+	cases := []testcase{{
+		name: "when we cannot stat kvstore2",
+		osStat: func(name string) (statBuf, error) {
+			return nil, io.EOF
+		},
+		osRename: func(oldpath string, newpath string) error {
+			panic("should not be called")
+		},
+		expect: nil,
+	}, {
+		name: "when kvstore2 is not a directory",
+		osStat: func(name string) (statBuf, error) {
+			return booleanStatBuf(false), nil
+		},
+		osRename: func(oldpath string, newpath string) error {
+			panic("should not be called")
+		},
+		expect: nil,
+	}, {
+		name: "when we can find kvstore2 as a dir and engine",
+		osStat: func(name string) (statBuf, error) {
+			if name == filepath.Join("xo", "kvstore2") {
+				return booleanStatBuf(true), nil
+			}
+			return booleanStatBuf(true), nil
+		},
+		osRename: func(oldpath string, newpath string) error {
+			panic("should not be called")
+		},
+		expect: nil,
+	}, {
+		name: "when we can find kvstore2 as a dir without engine",
+		osStat: func(name string) (statBuf, error) {
+			if name == filepath.Join("xo", "kvstore2") {
+				return booleanStatBuf(true), nil
+			}
+			return nil, io.EOF
+		},
+		osRename: func(oldpath string, newpath string) error {
+			return nil
+		},
+		expect: nil,
+	}}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			// override and restore functions
+			osStat = tc.osStat
+			osRename = tc.osRename
+			defer func() {
+				osStat = simplifiedStat
+				osRename = os.Rename
+			}()
+
+			// invoke Move
+			err := Move("xo")
+
+			// check the result
+			if !errors.Is(err, tc.expect) {
+				t.Fatal("expected", tc.expect, "got", err)
+			}
+		})
+	}
+}
+
+func TestSimplifiedStat(t *testing.T) {
+	buf, err := simplifiedStat("kvstore2dir.go")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if buf.IsDir() {
+		t.Fatal("expected not dir")
+	}
+}