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/transfer.go

@@ -1,6 +1,7 @@
 package sftpd
 
 import (
+	"fmt"
 	"os"
 	"time"
 
@@ -14,27 +15,23 @@ const (
 	transferDownload
 )
 
-const (
-	uploadModeStandard = iota
-	uploadModeAtomic
-)
-
 // Transfer contains the transfer details for an upload or a download.
 // It implements the io Reader and Writer interface to handle files downloads and uploads
 type Transfer struct {
-	file          *os.File
-	path          string
-	start         time.Time
-	bytesSent     int64
-	bytesReceived int64
-	user          dataprovider.User
-	connectionID  string
-	transferType  int
-	lastActivity  time.Time
-	isNewFile     bool
-	protocol      string
-	transferError error
-	isFinished    bool
+	file           *os.File
+	path           string
+	start          time.Time
+	bytesSent      int64
+	bytesReceived  int64
+	user           dataprovider.User
+	connectionID   string
+	transferType   int
+	lastActivity   time.Time
+	isNewFile      bool
+	protocol       string
+	transferError  error
+	isFinished     bool
+	minWriteOffset int64
 }
 
 // TransferError is called if there is an unexpected error.
@@ -60,6 +57,10 @@ func (t *Transfer) ReadAt(p []byte, off int64) (n int, err error) {
 // It handles upload bandwidth throttling too
 func (t *Transfer) WriteAt(p []byte, off int64) (n int, err error) {
 	t.lastActivity = time.Now()
+	if off < t.minWriteOffset {
+		logger.Warn(logSender, t.connectionID, "Invalid write offset %v minimum valid value %v", off, t.minWriteOffset)
+		return 0, fmt.Errorf("Invalid write offset %v", off)
+	}
 	written, e := t.file.WriteAt(p, off)
 	t.bytesReceived += int64(written)
 	t.handleThrottle()
@@ -82,7 +83,7 @@ func (t *Transfer) Close() error {
 		numFiles = 1
 	}
 	if t.transferType == transferUpload && t.file.Name() != t.path {
-		if t.transferError == nil {
+		if t.transferError == nil || uploadMode == uploadModeAtomicWithResume {
 			err = os.Rename(t.file.Name(), t.path)
 			logger.Debug(logSender, t.connectionID, "atomic upload completed, rename: %#v -> %#v, error: %v",
 				t.file.Name(), t.path, err)