chunker: partially implement no-rename transactions (#4675)

Some storage providers e.g. S3 don't have an efficient rename operation.
Before this change, when chunker finished an upload, the server-side copy
and delete operations that renamed temporary chunks to their final names
could take a significant amount of time.
This PR records transaction identifier (versioning) in the metadata of
chunker composite objects striving to remove the need for rename
operations on such backends.
This approach will be triggered be the new "transactions" configuration
option, which can be "rename" (the default) or "norename".
We implement the new approach for uploads (Put operations).
The chunker Move operation still uses the rename operation of
underlying backend. Filling this gap is left for a later PR.

Co-authored-by: Ivan Andreev <ivandeex@gmail.com>

backend/chunker/chunker.go

@@ -47,7 +47,8 @@ import (
 // The following types of chunks are supported:
 // data and control, active and temporary.
 // Chunk type is identified by matching chunk file name
-// based on the chunk name format configured by user.
+// based on the chunk name format configured by user and transaction
+// style being used.
 //
 // Both data and control chunks can be either temporary (aka hidden)
 // or active (non-temporary aka normal aka permanent).
@@ -63,6 +64,12 @@ import (
 // which is transparently converted to the new format. In its maximum
 // length of 13 decimals it makes a 7-digit base-36 number.
 //
+// When transactions is set to the norename style, data chunks will
+// keep their temporary chunk names (with the transacion identifier
+// suffix). To distinguish them from temporary chunks, the txn field
+// of the metadata file is set to match the transaction identifier of
+// the data chunks.
+//
 // Chunker can tell data chunks from control chunks by the characters
 // located in the "hash placeholder" position of configured format.
 // Data chunks have decimal digits there.
@@ -101,7 +108,7 @@ const maxMetadataSize = 1023
 const maxMetadataSizeWritten = 255
 
 // Current/highest supported metadata format.
-const metadataVersion = 1
+const metadataVersion = 2
 
 // optimizeFirstChunk enables the following optimization in the Put:
 // If a single chunk is expected, put the first chunk using the
@@ -224,6 +231,31 @@ It has the following fields: ver, size, nchunks, md5, sha1.`,
 					Help:  "Warn user, skip incomplete file and proceed.",
 				},
 			},
+		}, {
+			Name:     "transactions",
+			Advanced: true,
+			Default:  "rename",
+			Help:     `Choose how chunker should handle temporary files during transactions.`,
+			Hide:     fs.OptionHideCommandLine,
+			Examples: []fs.OptionExample{
+				{
+					Value: "rename",
+					Help:  "Rename temporary files after a successful transaction.",
+				}, {
+					Value: "norename",
+					Help: `Leave temporary file names and write transaction ID to metadata file.
+Metadata is required for no rename transactions (meta format cannot be "none").
+If you are using norename transactions you should be careful not to downgrade Rclone
+as older versions of Rclone don't support this transaction style and will misinterpret
+files manipulated by norename transactions.
+This method is EXPERIMENTAL, don't use on production systems.`,
+				}, {
+					Value: "auto",
+					Help: `Rename or norename will be used depending on capabilities of the backend.
+If meta format is set to "none", rename transactions will always be used.
+This method is EXPERIMENTAL, don't use on production systems.`,
+				},
+			},
 		}},
 	})
 }
@@ -271,7 +303,7 @@ func NewFs(ctx context.Context, name, rpath string, m configmap.Mapper) (fs.Fs,
 	cache.PinUntilFinalized(f.base, f)
 	f.dirSort = true // processEntries requires that meta Objects prerun data chunks atm.
 
-	if err := f.configure(opt.NameFormat, opt.MetaFormat, opt.HashType); err != nil {
+	if err := f.configure(opt.NameFormat, opt.MetaFormat, opt.HashType, opt.Transactions); err != nil {
 		return nil, err
 	}
 
@@ -316,6 +348,7 @@ type Options struct {
 	MetaFormat   string        `config:"meta_format"`
 	HashType     string        `config:"hash_type"`
 	FailHard     bool          `config:"fail_hard"`
+	Transactions string        `config:"transactions"`
 }
 
 // Fs represents a wrapped fs.Fs
@@ -337,12 +370,13 @@ type Fs struct {
 	opt          Options        // copy of Options
 	features     *fs.Features   // optional features
 	dirSort      bool           // reserved for future, ignored
+	useNoRename  bool           // can be set with the transactions option
 }
 
 // configure sets up chunker for given name format, meta format and hash type.
 // It also seeds the source of random transaction identifiers.
 // configure must be called only from NewFs or by unit tests.
-func (f *Fs) configure(nameFormat, metaFormat, hashType string) error {
+func (f *Fs) configure(nameFormat, metaFormat, hashType, transactionMode string) error {
 	if err := f.setChunkNameFormat(nameFormat); err != nil {
 		return errors.Wrapf(err, "invalid name format '%s'", nameFormat)
 	}
@@ -352,6 +386,9 @@ func (f *Fs) configure(nameFormat, metaFormat, hashType string) error {
 	if err := f.setHashType(hashType); err != nil {
 		return err
 	}
+	if err := f.setTransactionMode(transactionMode); err != nil {
+		return err
+	}
 
 	randomSeed := time.Now().UnixNano()
 	f.xactIDRand = rand.New(rand.NewSource(randomSeed))
@@ -411,6 +448,27 @@ func (f *Fs) setHashType(hashType string) error {
 	return nil
 }
 
+func (f *Fs) setTransactionMode(transactionMode string) error {
+	switch transactionMode {
+	case "rename":
+		f.useNoRename = false
+	case "norename":
+		if !f.useMeta {
+			return errors.New("incompatible transaction options")
+		}
+		f.useNoRename = true
+	case "auto":
+		f.useNoRename = !f.CanQuickRename()
+		if f.useNoRename && !f.useMeta {
+			f.useNoRename = false
+			return errors.New("using norename transactions requires metadata")
+		}
+	default:
+		return fmt.Errorf("unsupported transaction mode '%s'", transactionMode)
+	}
+	return nil
+}
+
 // setChunkNameFormat converts pattern based chunk name format
 // into Printf format and Regular expressions for data and
 // control chunks.
@@ -693,6 +751,7 @@ func (f *Fs) processEntries(ctx context.Context, origEntries fs.DirEntries, dirP
 	byRemote := make(map[string]*Object)
 	badEntry := make(map[string]bool)
 	isSubdir := make(map[string]bool)
+	txnByRemote := map[string]string{}
 
 	var tempEntries fs.DirEntries
 	for _, dirOrObject := range sortedEntries {
@@ -705,12 +764,18 @@ func (f *Fs) processEntries(ctx context.Context, origEntries fs.DirEntries, dirP
 				object := f.newObject("", entry, nil)
 				byRemote[remote] = object
 				tempEntries = append(tempEntries, object)
+				if f.useNoRename {
+					txnByRemote[remote], err = object.readXactID(ctx)
+					if err != nil {
+						return nil, err
+					}
+				}
 				break
 			}
 			// this is some kind of chunk
 			// metobject should have been created above if present
-			isSpecial := xactID != "" || ctrlType != ""
 			mainObject := byRemote[mainRemote]
+			isSpecial := xactID != txnByRemote[mainRemote] || ctrlType != ""
 			if mainObject == nil && f.useMeta && !isSpecial {
 				fs.Debugf(f, "skip orphan data chunk %q", remote)
 				break
@@ -811,6 +876,7 @@ func (f *Fs) scanObject(ctx context.Context, remote string, quickScan bool) (fs.
 	var (
 		o             *Object
 		baseObj       fs.Object
+		currentXactID string
 		err           error
 		sameMain      bool
 	)
@@ -856,7 +922,14 @@ func (f *Fs) scanObject(ctx context.Context, remote string, quickScan bool) (fs.
 		return nil, errors.Wrap(err, "can't detect composite file")
 	}
 
+	if f.useNoRename {
+		currentXactID, err = o.readXactID(ctx)
+		if err != nil {
+			return nil, err
+		}
+	}
 	caseInsensitive := f.features.CaseInsensitive
+
 	for _, dirOrObject := range entries {
 		entry, ok := dirOrObject.(fs.Object)
 		if !ok {
@@ -878,7 +951,7 @@ func (f *Fs) scanObject(ctx context.Context, remote string, quickScan bool) (fs.
 		if !sameMain {
 			continue // skip alien chunks
 		}
-		if ctrlType != "" || xactID != "" {
+		if ctrlType != "" || xactID != currentXactID {
 			if f.useMeta {
 				// temporary/control chunk calls for lazy metadata read
 				o.unsure = true
@@ -993,12 +1066,57 @@ func (o *Object) readMetadata(ctx context.Context) error {
 		}
 		o.md5 = metaInfo.md5
 		o.sha1 = metaInfo.sha1
+		o.xactID = metaInfo.xactID
 	}
 
 	o.isFull = true // cache results
+	o.xIDCached = true
 	return nil
 }
 
+// readXactID returns the transaction ID stored in the passed metadata object
+func (o *Object) readXactID(ctx context.Context) (xactID string, err error) {
+	// if xactID has already been read and cahced return it now
+	if o.xIDCached {
+		return o.xactID, nil
+	}
+	// Avoid reading metadata for backends that don't use xactID to identify permanent chunks
+	if !o.f.useNoRename {
+		return "", errors.New("readXactID requires norename transactions")
+	}
+	if o.main == nil {
+		return "", errors.New("readXactID requires valid metaobject")
+	}
+	if o.main.Size() > maxMetadataSize {
+		return "", nil // this was likely not a metadata object, return empty xactID but don't throw error
+	}
+	reader, err := o.main.Open(ctx)
+	if err != nil {
+		return "", err
+	}
+	data, err := ioutil.ReadAll(reader)
+	_ = reader.Close() // ensure file handle is freed on windows
+	if err != nil {
+		return "", err
+	}
+
+	switch o.f.opt.MetaFormat {
+	case "simplejson":
+		if data != nil && len(data) > maxMetadataSizeWritten {
+			return "", nil // this was likely not a metadata object, return empty xactID but don't throw error
+		}
+		var metadata metaSimpleJSON
+		err = json.Unmarshal(data, &metadata)
+		if err != nil {
+			return "", nil // this was likely not a metadata object, return empty xactID but don't throw error
+		}
+		xactID = metadata.XactID
+	}
+	o.xactID = xactID
+	o.xIDCached = true
+	return xactID, nil
+}
+
 // put implements Put, PutStream, PutUnchecked, Update
 func (f *Fs) put(
 	ctx context.Context, in io.Reader, src fs.ObjectInfo, remote string, options []fs.OpenOption,
@@ -1151,7 +1269,8 @@ func (f *Fs) put(
 	// If previous object was chunked, remove its chunks
 	f.removeOldChunks(ctx, baseRemote)
 
-	// Rename data chunks from temporary to final names
+	if !f.useNoRename {
+		// The transaction suffix will be removed for backends with quick rename operations
 		for chunkNo, chunk := range c.chunks {
 			chunkRemote := f.makeChunkName(baseRemote, chunkNo, "", "")
 			chunkMoved, errMove := f.baseMove(ctx, chunk, chunkRemote, delFailed)
@@ -1160,6 +1279,8 @@ func (f *Fs) put(
 			}
 			c.chunks[chunkNo] = chunkMoved
 		}
+		xactID = ""
+	}
 
 	if !f.useMeta {
 		// Remove stale metadata, if any
@@ -1178,7 +1299,7 @@ func (f *Fs) put(
 	switch f.opt.MetaFormat {
 	case "simplejson":
 		c.updateHashes()
-		metadata, err = marshalSimpleJSON(ctx, sizeTotal, len(c.chunks), c.md5, c.sha1)
+		metadata, err = marshalSimpleJSON(ctx, sizeTotal, len(c.chunks), c.md5, c.sha1, xactID)
 	}
 	if err == nil {
 		metaInfo := f.wrapInfo(src, baseRemote, int64(len(metadata)))
@@ -1190,6 +1311,7 @@ func (f *Fs) put(
 
 	o := f.newObject("", metaObject, c.chunks)
 	o.size = sizeTotal
+	o.xactID = xactID
 	return o, nil
 }
 
@@ -1593,7 +1715,7 @@ func (f *Fs) copyOrMove(ctx context.Context, o *Object, remote string, do copyMo
 	var metadata []byte
 	switch f.opt.MetaFormat {
 	case "simplejson":
-		metadata, err = marshalSimpleJSON(ctx, newObj.size, len(newChunks), md5, sha1)
+		metadata, err = marshalSimpleJSON(ctx, newObj.size, len(newChunks), md5, sha1, o.xactID)
 		if err == nil {
 			metaInfo := f.wrapInfo(metaObject, "", int64(len(metadata)))
 			err = newObj.main.Update(ctx, bytes.NewReader(metadata), metaInfo)
@@ -1809,7 +1931,13 @@ func (f *Fs) ChangeNotify(ctx context.Context, notifyFunc func(string, fs.EntryT
 		//fs.Debugf(f, "ChangeNotify: path %q entryType %d", path, entryType)
 		if entryType == fs.EntryObject {
 			mainPath, _, _, xactID := f.parseChunkName(path)
-			if mainPath != "" && xactID == "" {
+			metaXactID := ""
+			if f.useNoRename {
+				metaObject, _ := f.base.NewObject(ctx, mainPath)
+				dummyObject := f.newObject("", metaObject, nil)
+				metaXactID, _ = dummyObject.readXactID(ctx)
+			}
+			if mainPath != "" && xactID == metaXactID {
 				path = mainPath
 			}
 		}
@@ -1835,7 +1963,9 @@ type Object struct {
 	chunks    []fs.Object // active data chunks if file is composite, or wrapped file as a single chunk if meta format is 'none'
 	size      int64       // cached total size of chunks in a composite file or -1 for non-chunked files
 	isFull    bool        // true if metadata has been read
+	xIDCached bool        // true if xactID has been read
 	unsure    bool        // true if need to read metadata to detect object type
+	xactID    string      // transaction ID for "norename" or empty string for "renamed" chunks
 	md5       string
 	sha1      string
 	f         *Fs
@@ -2166,6 +2296,7 @@ type ObjectInfo struct {
 	src     fs.ObjectInfo
 	fs      *Fs
 	nChunks int    // number of data chunks
+	xactID  string // transaction ID for "norename" or empty string for "renamed" chunks
 	size    int64  // overrides source size by the total size of data chunks
 	remote  string // overrides remote name
 	md5     string // overrides MD5 checksum
@@ -2266,6 +2397,7 @@ type metaSimpleJSON struct {
 	// optional extra fields
 	MD5    string `json:"md5,omitempty"`
 	SHA1   string `json:"sha1,omitempty"`
+	XactID string `json:"txn,omitempty"` // transaction ID for norename transactions
 }
 
 // marshalSimpleJSON
@@ -2275,8 +2407,11 @@ type metaSimpleJSON struct {
 // - if file contents can be mistaken as meta object
 // - if consistent hashing is On but wrapped remote can't provide given hash
 //
-func marshalSimpleJSON(ctx context.Context, size int64, nChunks int, md5, sha1 string) ([]byte, error) {
+func marshalSimpleJSON(ctx context.Context, size int64, nChunks int, md5, sha1, xactID string) ([]byte, error) {
 	version := metadataVersion
+	if xactID == "" && version == 2 {
+		version = 1
+	}
 	metadata := metaSimpleJSON{
 		// required core fields
 		Version:  &version,
@@ -2285,6 +2420,7 @@ func marshalSimpleJSON(ctx context.Context, size int64, nChunks int, md5, sha1 s
 		// optional extra fields
 		MD5:    md5,
 		SHA1:   sha1,
+		XactID: xactID,
 	}
 	data, err := json.Marshal(&metadata)
 	if err == nil && data != nil && len(data) >= maxMetadataSizeWritten {
@@ -2362,6 +2498,7 @@ func unmarshalSimpleJSON(ctx context.Context, metaObject fs.Object, data []byte)
 	info.nChunks = *metadata.ChunkNum
 	info.md5 = metadata.MD5
 	info.sha1 = metadata.SHA1
+	info.xactID = metadata.XactID
 	return info, true, nil
 }
 
@@ -2394,6 +2531,11 @@ func (f *Fs) Precision() time.Duration {
 	return f.base.Precision()
 }
 
+// CanQuickRename returns true if the Fs supports a quick rename operation
+func (f *Fs) CanQuickRename() bool {
+	return f.base.Features().Move != nil
+}
+
 // Check the interfaces are satisfied
 var (
 	_ fs.Fs              = (*Fs)(nil)

backend/chunker/chunker_internal_test.go

@@ -468,9 +468,15 @@ func testPreventCorruption(t *testing.T, f *Fs) {
 		return obj
 	}
 	billyObj := newFile("billy")
+	billyTxn := billyObj.(*Object).xactID
+	if f.useNoRename {
+		require.True(t, billyTxn != "")
+	} else {
+		require.True(t, billyTxn == "")
+	}
 
 	billyChunkName := func(chunkNo int) string {
-		return f.makeChunkName(billyObj.Remote(), chunkNo, "", "")
+		return f.makeChunkName(billyObj.Remote(), chunkNo, "", billyTxn)
 	}
 
 	err := f.Mkdir(ctx, billyChunkName(1))
@@ -487,6 +493,8 @@ func testPreventCorruption(t *testing.T, f *Fs) {
 	// accessing chunks in strict mode is prohibited
 	f.opt.FailHard = true
 	billyChunk4Name := billyChunkName(4)
+	_, err = f.base.NewObject(ctx, billyChunk4Name)
+	require.NoError(t, err)
 	billyChunk4, err := f.NewObject(ctx, billyChunk4Name)
 	assertOverlapError(err)
 
@@ -520,7 +528,8 @@ func testPreventCorruption(t *testing.T, f *Fs) {
 
 	// recreate billy in case it was anyhow corrupted
 	willyObj := newFile("willy")
-	willyChunkName := f.makeChunkName(willyObj.Remote(), 1, "", "")
+	willyTxn := willyObj.(*Object).xactID
+	willyChunkName := f.makeChunkName(willyObj.Remote(), 1, "", willyTxn)
 	f.opt.FailHard = false
 	willyChunk, err := f.NewObject(ctx, willyChunkName)
 	f.opt.FailHard = true
@@ -561,17 +570,20 @@ func testChunkNumberOverflow(t *testing.T, f *Fs) {
 	modTime := fstest.Time("2001-02-03T04:05:06.499999999Z")
 	contents := random.String(100)
 
-	newFile := func(f fs.Fs, name string) (fs.Object, string) {
+	newFile := func(f fs.Fs, name string) (obj fs.Object, filename string, txnID string) {
-		filename := path.Join(dir, name)
+		filename = path.Join(dir, name)
 		item := fstest.Item{Path: filename, ModTime: modTime}
-		_, obj := fstests.PutTestContents(ctx, t, f, &item, contents, true)
+		_, obj = fstests.PutTestContents(ctx, t, f, &item, contents, true)
 		require.NotNil(t, obj)
-		return obj, filename
+		if chunkObj, isChunkObj := obj.(*Object); isChunkObj {
+			txnID = chunkObj.xactID
+		}
+		return
 	}
 
 	f.opt.FailHard = false
-	file, fileName := newFile(f, "wreaker")
+	file, fileName, fileTxn := newFile(f, "wreaker")
-	wreak, _ := newFile(f.base, f.makeChunkName("wreaker", wreakNumber, "", ""))
+	wreak, _, _ := newFile(f.base, f.makeChunkName("wreaker", wreakNumber, "", fileTxn))
 
 	f.opt.FailHard = false
 	fstest.CheckListingWithRoot(t, f, dir, nil, nil, f.Precision())
@@ -650,7 +662,7 @@ func testMetadataInput(t *testing.T, f *Fs) {
 		}
 	}
 
-	metaData, err := marshalSimpleJSON(ctx, 3, 1, "", "")
+	metaData, err := marshalSimpleJSON(ctx, 3, 1, "", "", "")
 	require.NoError(t, err)
 	todaysMeta := string(metaData)
 	runSubtest(todaysMeta, "today")
@@ -664,7 +676,7 @@ func testMetadataInput(t *testing.T, f *Fs) {
 	runSubtest(futureMeta, "future")
 }
 
-// test that chunker refuses to change on objects with future/unknowm metadata
+// Test that chunker refuses to change on objects with future/unknown metadata
 func testFutureProof(t *testing.T, f *Fs) {
 	if f.opt.MetaFormat == "none" {
 		t.Skip("this test requires metadata support")
@@ -738,6 +750,100 @@ func testFutureProof(t *testing.T, f *Fs) {
 	}
 }
 
+// The newer method of doing transactions without renaming should still be able to correctly process chunks that were created with renaming
+// If you attempt to do the inverse, however, the data chunks will be ignored causing commands to perform incorrectly
+func testBackwardsCompatibility(t *testing.T, f *Fs) {
+	if !f.useMeta {
+		t.Skip("Can't do norename transactions without metadata")
+	}
+	const dir = "backcomp"
+	ctx := context.Background()
+	saveOpt := f.opt
+	saveUseNoRename := f.useNoRename
+	defer func() {
+		f.opt.FailHard = false
+		_ = operations.Purge(ctx, f.base, dir)
+		f.opt = saveOpt
+		f.useNoRename = saveUseNoRename
+	}()
+	f.opt.ChunkSize = fs.SizeSuffix(10)
+
+	modTime := fstest.Time("2001-02-03T04:05:06.499999999Z")
+	contents := random.String(250)
+	newFile := func(f fs.Fs, name string) (fs.Object, string) {
+		filename := path.Join(dir, name)
+		item := fstest.Item{Path: filename, ModTime: modTime}
+		_, obj := fstests.PutTestContents(ctx, t, f, &item, contents, true)
+		require.NotNil(t, obj)
+		return obj, filename
+	}
+
+	f.opt.FailHard = false
+	f.useNoRename = false
+	file, fileName := newFile(f, "renamefile")
+
+	f.opt.FailHard = false
+	item := fstest.NewItem(fileName, contents, modTime)
+
+	var items []fstest.Item
+	items = append(items, item)
+
+	f.useNoRename = true
+	fstest.CheckListingWithRoot(t, f, dir, items, nil, f.Precision())
+	_, err := f.NewObject(ctx, fileName)
+	assert.NoError(t, err)
+
+	f.opt.FailHard = true
+	_, err = f.List(ctx, dir)
+	assert.NoError(t, err)
+
+	f.opt.FailHard = false
+	_ = file.Remove(ctx)
+}
+
+func testChunkerServerSideMove(t *testing.T, f *Fs) {
+	if !f.useMeta {
+		t.Skip("Can't test norename transactions without metadata")
+	}
+
+	ctx := context.Background()
+	const dir = "servermovetest"
+	subRemote := fmt.Sprintf("%s:%s/%s", f.Name(), f.Root(), dir)
+
+	subFs1, err := fs.NewFs(ctx, subRemote+"/subdir1")
+	assert.NoError(t, err)
+	fs1, isChunkerFs := subFs1.(*Fs)
+	assert.True(t, isChunkerFs)
+	fs1.useNoRename = false
+	fs1.opt.ChunkSize = fs.SizeSuffix(3)
+
+	subFs2, err := fs.NewFs(ctx, subRemote+"/subdir2")
+	assert.NoError(t, err)
+	fs2, isChunkerFs := subFs2.(*Fs)
+	assert.True(t, isChunkerFs)
+	fs2.useNoRename = true
+	fs2.opt.ChunkSize = fs.SizeSuffix(3)
+
+	modTime := fstest.Time("2001-02-03T04:05:06.499999999Z")
+	item := fstest.Item{Path: "movefile", ModTime: modTime}
+	contents := "abcdef"
+	_, file := fstests.PutTestContents(ctx, t, fs1, &item, contents, true)
+
+	dstOverwritten, _ := fs2.NewObject(ctx, "movefile")
+	dstFile, err := operations.Move(ctx, fs2, dstOverwritten, "movefile", file)
+	assert.NoError(t, err)
+	assert.Equal(t, int64(len(contents)), dstFile.Size())
+
+	r, err := dstFile.Open(ctx)
+	assert.NoError(t, err)
+	assert.NotNil(t, r)
+	data, err := ioutil.ReadAll(r)
+	assert.NoError(t, err)
+	assert.Equal(t, contents, string(data))
+	_ = r.Close()
+	_ = operations.Purge(ctx, f.base, dir)
+}
+
 // InternalTest dispatches all internal tests
 func (f *Fs) InternalTest(t *testing.T) {
 	t.Run("PutLarge", func(t *testing.T) {
@@ -764,6 +870,12 @@ func (f *Fs) InternalTest(t *testing.T) {
 	t.Run("FutureProof", func(t *testing.T) {
 		testFutureProof(t, f)
 	})
+	t.Run("BackwardsCompatibility", func(t *testing.T) {
+		testBackwardsCompatibility(t, f)
+	})
+	t.Run("ChunkerServerSideMove", func(t *testing.T) {
+		testChunkerServerSideMove(t, f)
+	})
 }
 
 var _ fstests.InternalTester = (*Fs)(nil)

docs/content/chunker.md

@@ -151,6 +151,9 @@ Note that `list` assembles composite directory entries only when chunk names
 match the configured format and treats non-conforming file names as normal
 non-chunked files.
 
+When using `norename` transactions, chunk names will additionally have a unique
+file version suffix. For example, `BIG_FILE_NAME.rclone_chunk.001_bp562k`.
+
 
 ### Metadata
 
@@ -170,6 +173,7 @@ for composite files. Meta objects carry the following fields:
 - `nchunks` - number of data chunks in file
 - `md5`     - MD5 hashsum of composite file (if present)
 - `sha1`    - SHA1 hashsum (if present)
+- `txn`     - identifies current version of the file
 
 There is no field for composite file name as it's simply equal to the name
 of meta object on the wrapped remote. Please refer to respective sections
@@ -242,8 +246,8 @@ use modification time of the first data chunk.
 
 ### Migrations
 
-The idiomatic way to migrate to a different chunk size, hash type or
+The idiomatic way to migrate to a different chunk size, hash type, transaction
-chunk naming scheme is to:
+style or chunk naming scheme is to:
 
 - Collect all your chunked files under a directory and have your
   chunker remote point to it.
@@ -303,6 +307,8 @@ Chunker included in rclone releases up to `v1.54` can sometimes fail to
 detect metadata produced by recent versions of rclone. We recommend users
 to keep rclone up-to-date to avoid data corruption.
 
+Changing `transactions` is dangerous and requires explicit migration.
+
 {{< rem autogenerated options start" - DO NOT EDIT - instead edit fs.RegInfo in backend/chunker/chunker.go then run make backenddocs" >}}
 ### Standard Options
 

fstest/test_all/config.yaml

@@ -42,6 +42,10 @@ backends:
    remote:   "TestChunkerChunk3bNometaLocal:"
    fastlist: true
    maxfile:  6k
+ - backend:  "chunker"
+   remote:   "TestChunkerChunk3bNoRenameLocal:"
+   fastlist: true
+   maxfile:  6k
  - backend:  "chunker"
    remote:   "TestChunkerMailru:"
    fastlist: true