Resources must not form a resource reference cycle

This rule enforces that resources do not create reference cycles of mutable references as mandated in AIP-121.

Details

This rule scans the fields of every resource and ensures that any references to other resources do not create a mutable cycle between them.

Examples

Incorrect code for this rule:

message Book {
  option (google.api.resource) = {
    type: "library.googleapis.com/Book"
    pattern: "books/{book}"
  };

  string name = 1;

  // Incorrect. Creates potential reference cycle.
  string author = 2 [
    (google.api.resource_reference).type = "library.googleapis.com/Author"
  ];
}

message Author {
  option (google.api.resource) = {
    type: "library.googleapis.com/Author"
    pattern: "authors/{author}"
  };

  string name = 1;

  // Incorrect. Creates potential reference cycle.
  string book = 2 [
    (google.api.resource_reference).type = "library.googleapis.com/Book"
  ];
}

Correct code for this rule:

message Book {
  option (google.api.resource) = {
    type: "library.googleapis.com/Book"
    pattern: "books/{book}"
  };

  string name = 1;

  // Correct because the other reference is OUTPUT_ONLY.
  string author = 2 [
    (google.api.resource_reference).type = "library.googleapis.com/Author"
  ];
}

message Author {
  option (google.api.resource) = {
    type: "library.googleapis.com/Author"
    pattern: "authors/{author}"
  };

  string name = 1;

  // Correct because an OUTPUT_ONLY reference breaks the mutation cycle.
  string book = 2 [
    (google.api.resource_reference).type = "library.googleapis.com/Book",
    (google.api.field_behavior) = OUTPUT_ONLY
  ];
}

Disabling

If you need to violate this rule, use a leading comment above the service. Remember to also include an aip.dev/not-precedent comment explaining why.

message Book {
  option (google.api.resource) = {
    type: "library.googleapis.com/Book"
    pattern: "books/{book}"
  };

  string name = 1;

  // (-- api-linter: core::0121::no-mutable-cycles=disabled
  //     aip.dev/not-precedent: We need to do this because reasons. --)
  string author = 2 [
    (google.api.resource_reference).type = "library.googleapis.com/Author"
  ];
}

message Author {
  option (google.api.resource) = {
    type: "library.googleapis.com/Author"
    pattern: "authors/{author}"
  };

  string name = 1;

  // (-- api-linter: core::0121::no-mutable-cycles=disabled
  //     aip.dev/not-precedent: We need to do this because reasons. --)
  string book = 2 [
    (google.api.resource_reference).type = "library.googleapis.com/Book"
  ];
}

If you need to violate this rule for an entire file, place the comment at the top of the file.