simplify context retrieval

context.go

@@ -8,10 +8,19 @@ type key int
 
 const v2rayKey key = 1
 
-// FromContext returns a Instance from the given context, or nil if the context doesn't contain one.
+// FromContext returns an Instance from the given context, or nil if the context doesn't contain one.
 func FromContext(ctx context.Context) *Instance {
 	if s, ok := ctx.Value(v2rayKey).(*Instance); ok {
 		return s
 	}
 	return nil
 }
+
+// MustFromContext returns an Instance from the given context, or panics if not present.
+func MustFromContext(ctx context.Context) *Instance {
+	v := FromContext(ctx)
+	if v == nil {
+		panic("V is not in context.")
+	}
+	return v
+}