[#283] pkg/session: Implement work with token contexts

Implement `Context` / `SetContext` methods on `Token` which reads / sets
token context. Support container context (`ContainerContext`).

Add helper function `GetContainerContext` for easy reading of the container
context.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/session/session.go

@@ -135,6 +135,50 @@ func (t *Token) Signature() *pkg.Signature {
 	)
 }
 
+// SetContext sets context of the Token.
+//
+// Supported contexts:
+//  - *ContainerContext.
+//
+// Resets context if it is not supported.
+func (t *Token) SetContext(v interface{}) {
+	var cV2 session.SessionTokenContext
+
+	switch c := v.(type) {
+	case *ContainerContext:
+		cV2 = c.ToV2()
+	}
+
+	t.setBodyField(func(body *session.SessionTokenBody) {
+		body.SetContext(cV2)
+	})
+}
+
+// Context returns context of the Token.
+//
+// Supports same contexts as SetContext.
+//
+// Returns nil if context is not supported.
+func (t *Token) Context() interface{} {
+	switch v := (*session.SessionToken)(t).
+		GetBody().
+		GetContext(); c := v.(type) {
+	default:
+		return nil
+	case *session.ContainerSessionContext:
+		return ContainerContextFromV2(c)
+	}
+}
+
+// GetContainerContext is a helper function that casts
+// Token context to ContainerContext.
+//
+// Returns nil if context is not a ContainerContext.
+func GetContainerContext(t *Token) *ContainerContext {
+	c, _ := t.Context().(*ContainerContext)
+	return c
+}
+
 // Marshal marshals Token into a protobuf binary form.
 //
 // Buffer is allocated when the argument is empty.

pkg/session/session_test.go

@@ -86,3 +86,55 @@ func TestToken_VerifySignature(t *testing.T) {
 		require.True(t, tok.VerifySignature())
 	})
 }
+
+var unsupportedContexts = []interface{}{
+	123,
+	true,
+	session.NewToken(),
+}
+
+var nonContainerContexts = unsupportedContexts
+
+func TestToken_Context(t *testing.T) {
+	tok := session.NewToken()
+
+	for _, item := range []struct {
+		ctx      interface{}
+		v2assert func(interface{})
+	}{
+		{
+			ctx: sessiontest.ContainerContext(),
+			v2assert: func(c interface{}) {
+				require.Equal(t, c.(*session.ContainerContext).ToV2(), tok.ToV2().GetBody().GetContext())
+			},
+		},
+	} {
+		tok.SetContext(item.ctx)
+
+		require.Equal(t, item.ctx, tok.Context())
+
+		item.v2assert(item.ctx)
+	}
+
+	for _, c := range unsupportedContexts {
+		tok.SetContext(c)
+
+		require.Nil(t, tok.Context())
+	}
+}
+
+func TestGetContainerContext(t *testing.T) {
+	tok := session.NewToken()
+
+	c := sessiontest.ContainerContext()
+
+	tok.SetContext(c)
+
+	require.Equal(t, c, session.GetContainerContext(tok))
+
+	for _, c := range nonContainerContexts {
+		tok.SetContext(c)
+
+		require.Nil(t, session.GetContainerContext(tok))
+	}
+}