Improve accuracy of documentation on any.proto.

PiperOrigin-RevId: 867605540

Author: varomodt

src/google/protobuf/any.proto

@@ -42,121 +42,65 @@ option csharp_namespace = "Google.Protobuf.WellKnownTypes";
 // `Any` contains an arbitrary serialized protocol buffer message along with a
 // URL that describes the type of the serialized message.
 //
-// Protobuf library provides support to pack/unpack Any values in the form
-// of utility functions or additional generated methods of the Any type.
-//
-// Example 1: Pack and unpack a message in C++.
-//
-//     Foo foo = ...;
-//     Any any;
-//     any.PackFrom(foo);
-//     ...
-//     if (any.UnpackTo(&foo)) {
-//       ...
-//     }
-//
-// Example 2: Pack and unpack a message in Java.
-//
-//     Foo foo = ...;
-//     Any any = Any.pack(foo);
-//     ...
-//     if (any.is(Foo.class)) {
-//       foo = any.unpack(Foo.class);
-//     }
-//     // or ...
-//     if (any.isSameTypeAs(Foo.getDefaultInstance())) {
-//       foo = any.unpack(Foo.getDefaultInstance());
-//     }
-//
-//  Example 3: Pack and unpack a message in Python.
-//
-//     foo = Foo(...)
-//     any = Any()
-//     any.Pack(foo)
-//     ...
-//     if any.Is(Foo.DESCRIPTOR):
-//       any.Unpack(foo)
-//       ...
-//
-//  Example 4: Pack and unpack a message in Go
-//
-//      foo := &pb.Foo{...}
-//      any, err := anypb.New(foo)
-//      if err != nil {
-//        ...
-//      }
-//      ...
-//      foo := &pb.Foo{}
-//      if err := any.UnmarshalTo(foo); err != nil {
-//        ...
-//      }
-//
-// The pack methods provided by protobuf library will by default use
-// 'type.googleapis.com/full.type.name' as the type URL and the unpack
-// methods only use the fully qualified type name after the last '/'
-// in the type URL, for example "foo.bar.com/x/y.z" will yield type
-// name "y.z".
-//
-// JSON
-// ====
-// The JSON representation of an `Any` value uses the regular
-// representation of the deserialized, embedded message, with an
-// additional field `@type` which contains the type URL. Example:
-//
-//     package google.profile;
-//     message Person {
-//       string first_name = 1;
-//       string last_name = 2;
-//     }
-//
-//     {
-//       "@type": "type.googleapis.com/google.profile.Person",
-//       "firstName": <string>,
-//       "lastName": <string>
-//     }
-//
-// If the embedded message type is well-known and has a custom JSON
-// representation, that representation will be embedded adding a field
-// `value` which holds the custom JSON in addition to the `@type`
-// field. Example (for message [google.protobuf.Duration][]):
-//
-//     {
-//       "@type": "type.googleapis.com/google.protobuf.Duration",
-//       "value": "1.212s"
-//     }
-//
+// In its binary encoding, an `Any` is an ordinary message; but in other wire
+// forms like JSON, it has a special encoding. The format of the type URL is
+// described on the `type_url` field.
+//
+// Protobuf APIs provide utilities to interact with `Any` values:
+//
+// - A 'pack' operation accepts a message and constructs a generic `Any` wrapper
+//   around it.
+// - An 'unpack' operation reads the content of an `Any` message, either into an
+//   existing message or a new one. Unpack operations must check the type of the
+//   value they unpack against the declared `type_url`.
+// - An 'is' operation decides whether an `Any` contains a message of the given
+//   type, i.e. whether it can 'unpack' that type.
+//
+// The JSON format representation of an `Any` follows one of these cases:
+//
+// - For types without special-cased JSON encodings, the JSON format
+//   representation of the `Any` is the same as that of the message, with an
+//   additional `@type` field which contains the type URL.
+// - For types with special-cased JSON encodings (typically called 'well-known'
+//   types, listed in https://protobuf.dev/programming-guides/json/#any), the
+//   JSON format representation has a key `@type` which contains the type URL
+//   and a key `value` which contains the JSON-serialized value.
+//
+// The text format representation of an `Any` is like a message with one field
+// whose name is the type URL in brackets. For example, an `Any` containing a
+// `foo.Bar` message may be written `[type.googleapis.com/foo.Bar] { a: 2 }`.
 message Any {
-  // A URL/resource name that uniquely identifies the type of the serialized
-  // protocol buffer message. This string must contain at least
-  // one "/" character. The last segment of the URL's path must represent
-  // the fully qualified name of the type (as in
-  // `path/google.protobuf.Duration`). The name should be in a canonical form
-  // (e.g., leading "." is not accepted).
+  // Identifies the type of the serialized Protobuf message with a URI reference
+  // consisting of a prefix ending in a slash and the fully-qualified type name.
   //
-  // In practice, teams usually precompile into the binary all types that they
-  // expect it to use in the context of Any. However, for URLs which use the
-  // scheme `http`, `https`, or no scheme, one can optionally set up a type
-  // server that maps type URLs to message definitions as follows:
+  // Example: type.googleapis.com/google.protobuf.StringValue
   //
-  // * If no scheme is provided, `https` is assumed.
-  // * An HTTP GET on the URL must yield a [google.protobuf.Type][]
-  //   value in binary format, or produce an error.
-  // * Applications are allowed to cache lookup results based on the
-  //   URL, or have them precompiled into a binary to avoid any
-  //   lookup. Therefore, binary compatibility needs to be preserved
-  //   on changes to types. (Use versioned type names to manage
-  //   breaking changes.)
+  // This string must contain at least one `/` character, and the content after
+  // the last `/` must be the fully-qualified name of the type in canonical
+  // form, without a leading dot. Do not write a scheme on these URI references
+  // so that clients do not attempt to contact them.
   //
-  // Note: this functionality is not currently available in the official
-  // protobuf release, and it is not used for type URLs beginning with
-  // type.googleapis.com. As of May 2023, there are no widely used type server
-  // implementations and no plans to implement one.
+  // The prefix is arbitrary and Protobuf implementations are expected to
+  // simply strip off everything up to and including the last `/` to identify
+  // the type. `type.googleapis.com/` is a common default prefix that some
+  // legacy implementations require. This prefix does not indicate the origin of
+  // the type, and URIs containing it are not expected to respond to any
+  // requests.
   //
-  // Schemes other than `http`, `https` (or the empty scheme) might be
-  // used with implementation specific semantics.
+  // All type URL strings must be legal URI references with the additional
+  // restriction (for the text format) that the content of the reference
+  // must consist only of alphanumeric characters, percent-encoded escapes, and
+  // characters in the following set (not including the outer backticks):
+  // `/-.~_!$&()*+,;=`. Despite our allowing percent encodings, implementations
+  // should not unescape them to prevent confusion with existing parsers. For
+  // example, `type.googleapis.com%2FFoo` should be rejected.
   //
+  // In the original design of `Any` the possibility of launching a type
+  // resolution service at these type URLs was considered but Protobuf never
+  // implemented one and considers contacting these URLs to be problematic and
+  // a potential security issue. Do not attempt to contact type URLs.
   string type_url = 1;
 
-  // Must be a valid serialized protocol buffer of the above specified type.
+  // Holds a Protobuf serialization of the type described by type_url.
   bytes value = 2;
 }