sftpd: add support for upload resume

we support resume only if the client sets the correct offset while resuming
the upload.
Based on the specs the offset is optional for resume, but all the tested
clients sets a right offset.
If an invalid offset is given we interrupt the transfer with the error
"Invalid write offset ..."

See https://github.com/pkg/sftp/issues/295

This commit add a new upload mode: "atomic with resume support", this acts
as atomic but if there is an upload error the temporary file is renamed
to the requested path and not deleted, this way a client can reconnect
and resume the upload

sftpd/internal_test.go

@@ -98,21 +98,44 @@ func TestRemoveNonexistentQuotaScan(t *testing.T) {
 func TestGetOSOpenFlags(t *testing.T) {
 	var flags sftp.FileOpenFlags
 	flags.Write = true
-	flags.Append = true
 	flags.Excl = true
 	osFlags := getOSOpenFlags(flags)
-	if osFlags&os.O_WRONLY == 0 || osFlags&os.O_APPEND == 0 || osFlags&os.O_EXCL == 0 {
+	if osFlags&os.O_WRONLY == 0 || osFlags&os.O_EXCL == 0 {
+		t.Errorf("error getting os flags from sftp file open flags")
+	}
+	flags.Append = true
+	// append flag should be ignored to allow resume
+	if osFlags&os.O_WRONLY == 0 || osFlags&os.O_EXCL == 0 {
 		t.Errorf("error getting os flags from sftp file open flags")
 	}
 }
 
-func TestUploadResume(t *testing.T) {
-	c := Connection{}
-	var flags sftp.FileOpenFlags
-	_, err := c.handleSFTPUploadToExistingFile(flags, "", "", 0)
-	if err != sftp.ErrSshFxOpUnsupported {
-		t.Errorf("file resume is not supported")
+func TestUploadResumeInvalidOffset(t *testing.T) {
+	testfile := "testfile"
+	file, _ := os.Create(testfile)
+	transfer := Transfer{
+		file:          file,
+		path:          file.Name(),
+		start:         time.Now(),
+		bytesSent:     0,
+		bytesReceived: 0,
+		user: dataprovider.User{
+			Username: "testuser",
+		},
+		connectionID:   "",
+		transferType:   transferUpload,
+		lastActivity:   time.Now(),
+		isNewFile:      false,
+		protocol:       protocolSFTP,
+		transferError:  nil,
+		isFinished:     false,
+		minWriteOffset: 10,
 	}
+	_, err := transfer.WriteAt([]byte("test"), 0)
+	if err == nil {
+		t.Errorf("upload with invalid offset must fail")
+	}
+	os.Remove(testfile)
 }
 
 func TestUploadFiles(t *testing.T) {
@@ -617,19 +640,20 @@ func TestSCPUploadFiledata(t *testing.T) {
 	}
 	file, _ := os.Create(testfile)
 	transfer := Transfer{
-		file:          file,
-		path:          file.Name(),
-		start:         time.Now(),
-		bytesSent:     0,
-		bytesReceived: 0,
-		user:          scpCommand.connection.User,
-		connectionID:  "",
-		transferType:  transferDownload,
-		lastActivity:  time.Now(),
-		isNewFile:     true,
-		protocol:      connection.protocol,
-		transferError: nil,
-		isFinished:    false,
+		file:           file,
+		path:           file.Name(),
+		start:          time.Now(),
+		bytesSent:      0,
+		bytesReceived:  0,
+		user:           scpCommand.connection.User,
+		connectionID:   "",
+		transferType:   transferDownload,
+		lastActivity:   time.Now(),
+		isNewFile:      true,
+		protocol:       connection.protocol,
+		transferError:  nil,
+		isFinished:     false,
+		minWriteOffset: 0,
 	}
 	addTransfer(&transfer)
 	err := scpCommand.getUploadFileData(2, &transfer)
@@ -684,6 +708,8 @@ func TestSCPUploadFiledata(t *testing.T) {
 }
 
 func TestUploadError(t *testing.T) {
+	oldUploadMode := uploadMode
+	uploadMode = uploadModeAtomic
 	connection := Connection{
 		User: dataprovider.User{
 			Username: "testuser",
@@ -694,19 +720,20 @@ func TestUploadError(t *testing.T) {
 	fileTempName := "temptestfile"
 	file, _ := os.Create(fileTempName)
 	transfer := Transfer{
-		file:          file,
-		path:          testfile,
-		start:         time.Now(),
-		bytesSent:     0,
-		bytesReceived: 100,
-		user:          connection.User,
-		connectionID:  "",
-		transferType:  transferUpload,
-		lastActivity:  time.Now(),
-		isNewFile:     true,
-		protocol:      connection.protocol,
-		transferError: nil,
-		isFinished:    false,
+		file:           file,
+		path:           testfile,
+		start:          time.Now(),
+		bytesSent:      0,
+		bytesReceived:  100,
+		user:           connection.User,
+		connectionID:   "",
+		transferType:   transferUpload,
+		lastActivity:   time.Now(),
+		isNewFile:      true,
+		protocol:       connection.protocol,
+		transferError:  nil,
+		isFinished:     false,
+		minWriteOffset: 0,
 	}
 	addTransfer(&transfer)
 	transfer.TransferError(fmt.Errorf("fake error"))
@@ -722,6 +749,7 @@ func TestUploadError(t *testing.T) {
 	if !os.IsNotExist(err) {
 		t.Errorf("file uploaded must be deleted after an error: %v", err)
 	}
+	uploadMode = oldUploadMode
 }
 
 func TestConnectionStatusStruct(t *testing.T) {