New method Client.Extensions to list server extensions

client.go

@@ -155,6 +155,9 @@ func NewClientPipe(rd io.Reader, wr io.WriteCloser, opts ...ClientOption) (*Clie
 			inflight: make(map[uint32]chan<- result),
 			closed:   make(chan struct{}),
 		},
+
+		ext: make(map[string]string),
+
 		maxPacket:             1 << 15,
 		maxConcurrentRequests: 64,
 	}
@@ -183,6 +186,8 @@ func NewClientPipe(rd io.Reader, wr io.WriteCloser, opts ...ClientOption) (*Clie
 type Client struct {
 	clientConn
 
+	ext map[string]string // Extensions (name -> data).
+
 	maxPacket             int // max packet size read or written.
 	maxConcurrentRequests int
 	nextid                uint32
@@ -223,14 +228,32 @@ func (c *Client) recvVersion() error {
 		return &unexpectedPacketErr{sshFxpVersion, typ}
 	}
 
-	version, _ := unmarshalUint32(data)
+	version, data := unmarshalUint32(data)
 	if version != sftpProtocolVersion {
 		return &unexpectedVersionErr{sftpProtocolVersion, version}
 	}
 
+	for len(data) > 0 {
+		var ext extensionPair
+		ext, data, err = unmarshalExtensionPair(data)
+		if err != nil {
+			return err
+		}
+		c.ext[ext.Name] = ext.Data
+	}
+
 	return nil
 }
 
+// HasExtension checks whether the server supports a named extension.
+//
+// The first return value is the extension data reported by the server
+// (typically a version number).
+func (c *Client) HasExtension(name string) (string, bool) {
+	data, ok := c.ext[name]
+	return data, ok
+}
+
 // Walk returns a new Walker rooted at root.
 func (c *Client) Walk(root string) *fs.Walker {
 	return fs.WalkFS(root, c)

client_integration_linux_test.go

@@ -3,6 +3,8 @@ package sftp
 import (
 	"syscall"
 	"testing"
+
+	"github.com/stretchr/testify/require"
 )
 
 func TestClientStatVFS(t *testing.T) {
@@ -13,6 +15,9 @@ func TestClientStatVFS(t *testing.T) {
 	defer cmd.Wait()
 	defer sftp.Close()
 
+	_, ok := sftp.HasExtension("statvfs@openssh.com")
+	require.True(t, ok, "server doesn't list statvfs extension")
+
 	vfs, err := sftp.StatVFS("/")
 	if err != nil {
 		t.Fatal(err)