feat(ftp): support seek/range request (#3811)

drivers/ftp/util.go

@@ -1,6 +1,13 @@
 package ftp
 
-import "github.com/jlaffaye/ftp"
+import (
+	"io"
+	"os"
+	"sync"
+	"time"
+
+	"github.com/jlaffaye/ftp"
+)
 
 // do others that not defined in Driver interface
 
@@ -11,7 +18,7 @@ func (d *FTP) login() error {
 			return nil
 		}
 	}
-	conn, err := ftp.Dial(d.Address)
+	conn, err := ftp.Dial(d.Address, ftp.DialWithShutTimeout(10*time.Second))
 	if err != nil {
 		return err
 	}
@@ -22,3 +29,81 @@ func (d *FTP) login() error {
 	d.conn = conn
 	return nil
 }
+
+// An FTP file reader that implements io.ReadSeekCloser for seeking.
+type FTPFileReader struct {
+	conn   *ftp.ServerConn
+	resp   *ftp.Response
+	offset int64
+	mu     sync.Mutex
+	path   string
+}
+
+func NewFTPFileReader(conn *ftp.ServerConn, path string) *FTPFileReader {
+	return &FTPFileReader{
+		conn: conn,
+		path: path,
+	}
+}
+
+func (r *FTPFileReader) Read(buf []byte) (n int, err error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	if r.resp == nil {
+		r.resp, err = r.conn.RetrFrom(r.path, uint64(r.offset))
+		if err != nil {
+			return 0, err
+		}
+	}
+
+	n, err = r.resp.Read(buf)
+	r.offset += int64(n)
+	return
+}
+
+func (r *FTPFileReader) Seek(offset int64, whence int) (int64, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	oldOffset := r.offset
+	var newOffset int64
+	switch whence {
+	case io.SeekStart:
+		newOffset = offset
+	case io.SeekCurrent:
+		newOffset = oldOffset + offset
+	case io.SeekEnd:
+		size, err := r.conn.FileSize(r.path)
+		if err != nil {
+			return oldOffset, err
+		}
+		newOffset = offset + int64(size)
+	default:
+		return -1, os.ErrInvalid
+	}
+
+	if newOffset < 0 {
+		// offset out of range
+		return oldOffset, os.ErrInvalid
+	}
+	if newOffset == oldOffset {
+		// offset not changed, so return directly
+		return oldOffset, nil
+	}
+	r.offset = newOffset
+
+	if r.resp != nil {
+		// close the existing ftp data connection, otherwise the next read will be blocked
+		_ = r.resp.Close() // we do not care about whether it returns an error
+		r.resp = nil
+	}
+	return newOffset, nil
+}
+
+func (r *FTPFileReader) Close() error {
+	if r.resp != nil {
+		return r.resp.Close()
+	}
+	return nil
+}