protobuf/tests/test.proto

// A series of messages with various kinds of cycles in them.
//      +-+---+    +---+
//      V |   |    |   |
// A -> B-+-> C -> D<--+
// ^          |    |
// +----------+----+
//
// This tests the following cases:
//  - B and C are together in multiple cycles
//  - B and D are cycles to themselves.

message A {
  optional B b = 1;
}

message B {
  optional B b = 1;
  optional C c = 2;
}

message C {
  optional A a = 1;
  optional B b = 2;
  optional D d = 3;
}

message D {
  optional A a = 1;
  optional D d = 2;
}