feat(AIP-193): require ErrorInfo (#1094)

feat(AIP-193): require ErrorInfo

This change makes `ErrorInfo` a required component of each error's
details. It adds rationale for the change as well.

Fixes: #1088

Author: shwoodard

aip/general/0193.md

@@ -44,15 +44,23 @@ strings **may** change over time; however, if an error message does not have a
 machine-readable identifier _in addition to_ the `code` field, changing the
 error message **must** be considered a backwards-incompatible change.
 
-**Note:** The [`ErrorInfo`][ErrorInfo] message is the recommended way to send a
-machine-readable identifier.
+#### ErrorInfo
+
+The [`ErrorInfo`][ErrorInfo] message is the required way to send a
+machine-readable identifier. All error responses **must** include an
+`ErrorInfo` payload in the `details` field. Variable information
+**should** be included in the `metadata` field on `ErrorInfo` and
+**must** be included if it appears within an error message.
 
 #### Uniqueness
 
 Each type of detail payload **must** be included at most once. For
-example, there **must not** be more than one `ErrorInfo` message in the
-`details` field, but there **may** be an `ErrorInfo` and a
-[`PreconditionFailure`][PreconditionFailure].
+example, there **must not** be more than one [`BadRequest`][BadRequest]
+message in the `details` field, but there **may** be a `BadRequest` and
+a [`PreconditionFailure`][PreconditionFailure].
+
+**Note:** `ErrorInfo` represents a special case. There **must** be exactly one
+`ErrorInfo`. It is required.
 
 ### Localization
 
@@ -86,13 +94,29 @@ if the resource or parent exists.
 If the user does have proper permission, but the requested resource or parent
 does not exist, the service **must** error with `NOT_FOUND` (HTTP 404).
 
+## Rationale
+
+`ErrorInfo` is required because it further identifies an error. The
+`Status` field, with under 20 [allowed values][Code], is rarely enough
+to disambiguate one error from another accross an entire API Service.
+Error messages often contain dynamic segments that express values, such
+as project numbers or locations, and these values are often read from
+the string message using brittle extraction methods, like RegExp. There
+needs to be a machine readable component of *every* error response that
+enables clients to pull such information programatically--without
+parsing the string error message. For these reasons, `ErrorInfo` is
+required for *every* error response.
+
 ## Further reading
 
 - For which error codes to retry, see [AIP-194](https://aip.dev/194).
-- For how to retry errors in client libraries, see [AIP-4221](https://aip.dev/client-libraries/4221).
+- For how to retry errors in client libraries, see
+  [AIP-4221](https://aip.dev/client-libraries/4221).
 
 ## Changelog
 
+- **2023-05-10**: Require [`ErrorInfo`][ErrorInfo] for all error
+  responses.
 - **2023-05-04**: Require uniqueness by message type for error details.
 - **2022-11-04**: Added guidance around PERMISSION_DENIED errors previously
   found in other AIPs.
@@ -108,6 +132,7 @@ does not exist, the service **must** error with `NOT_FOUND` (HTTP 404).
 [details]: https://github.com/googleapis/googleapis/blob/master/google/rpc/error_details.proto
 [ErrorInfo]: https://github.com/googleapis/googleapis/blob/6f3fcc058ff29989f6d3a71557a44b5e81b897bd/google/rpc/error_details.proto#L27-L76
 [PreconditionFailure]: https://github.com/googleapis/googleapis/blob/6f3fcc058ff29989f6d3a71557a44b5e81b897bd/google/rpc/error_details.proto#L139-L166
+[BadRequest]: https://github.com/googleapis/googleapis/blob/477a59d764428136ba1d857a9633c0d231de6efa/google/rpc/error_details.proto#L168-L218
 [grpc status code documentation]: https://github.com/grpc/grpc/blob/master/doc/statuscodes.md
 [Code]: https://github.com/googleapis/googleapis/blob/master/google/rpc/code.proto
 [Status]: https://github.com/googleapis/googleapis/blob/master/google/rpc/status.proto