add rsync support ...

and better document quota management issues for system commands.

rsync and git are not enabled in the default config so don't install
them in sample Dockerfiles, simply add a comment to facilitate their
installation if needed

Fixes #44

sftpd/internal_test.go

@@ -431,6 +431,46 @@ func TestSSHCommandQuotaScan(t *testing.T) {
 	}
 }
 
+func TestRsyncOptions(t *testing.T) {
+	conn := Connection{
+		User: dataprovider.User{
+			Permissions: []string{dataprovider.PermAny},
+			HomeDir:     os.TempDir(),
+		},
+	}
+	sshCmd := sshCommand{
+		command:    "rsync",
+		connection: conn,
+		args:       []string{"--server", "-vlogDtprze.iLsfxC", ".", "/"},
+	}
+	cmd, err := sshCmd.getSystemCommand()
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if !utils.IsStringInSlice("--safe-links", cmd.cmd.Args) {
+		t.Errorf("--safe-links must be added if the user has the create symlinks permission")
+	}
+	conn = Connection{
+		User: dataprovider.User{
+			Permissions: []string{dataprovider.PermDownload, dataprovider.PermUpload, dataprovider.PermCreateDirs,
+				dataprovider.PermListItems, dataprovider.PermOverwrite, dataprovider.PermDelete, dataprovider.PermRename},
+			HomeDir: os.TempDir(),
+		},
+	}
+	sshCmd = sshCommand{
+		command:    "rsync",
+		connection: conn,
+		args:       []string{"--server", "-vlogDtprze.iLsfxC", ".", "/"},
+	}
+	cmd, err = sshCmd.getSystemCommand()
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if !utils.IsStringInSlice("--munge-links", cmd.cmd.Args) {
+		t.Errorf("--munge-links must be added if the user has the create symlinks permission")
+	}
+}
+
 func TestSystemCommandErrors(t *testing.T) {
 	buf := make([]byte, 65535)
 	stdErrBuf := make([]byte, 65535)