Add options for Open and implement Range for all remotes

crypt/crypt.go

@@ -4,6 +4,7 @@ package crypt
 import (
 	"fmt"
 	"io"
+	"io/ioutil"
 	"path"
 	"sync"
 
@@ -297,12 +298,59 @@ func (o *Object) Hash(hash fs.HashType) (string, error) {
 }
 
 // Open opens the file for read.  Call Close() on the returned io.ReadCloser
-func (o *Object) Open() (io.ReadCloser, error) {
+func (o *Object) Open(options ...fs.OpenOption) (io.ReadCloser, error) {
+	var offset int64
+	for _, option := range options {
+		switch x := option.(type) {
+		case *fs.SeekOption:
+			offset = x.Offset
+		default:
+			if option.Mandatory() {
+				fs.Log(o, "Unsupported mandatory option: %v", option)
+			}
+		}
+	}
 	in, err := o.Object.Open()
 	if err != nil {
-		return in, err
+		return nil, err
 	}
-	return o.f.cipher.DecryptData(in)
+
+	// This reads the header and checks it is OK
+	rc, err := o.f.cipher.DecryptData(in)
+	if err != nil {
+		return nil, err
+	}
+
+	// If seeking required, then...
+	if offset != 0 {
+		// FIXME could cache the unseeked decrypter as we re-read the header on every seek
+		decrypter := rc.(*decrypter)
+
+		// Seek the decrypter and work out where to seek the
+		// underlying file and how many bytes to discard
+		underlyingOffset, discard := decrypter.seek(offset)
+
+		// Re-open stream with a seek of underlyingOffset
+		err = in.Close()
+		if err != nil {
+			return nil, err
+		}
+		in, err := o.Object.Open(&fs.SeekOption{Offset: underlyingOffset})
+		if err != nil {
+			return nil, err
+		}
+
+		// Update the stream
+		decrypter.rc = in
+
+		// Discard the bytes
+		_, err = io.CopyN(ioutil.Discard, decrypter, discard)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return rc, err
 }
 
 // Update in to the object with the modTime given of the given size