Service capabilities / error behaviors

spec/Section 4 -- Introspection.md

@@ -85,13 +85,14 @@ operation.
 
 ## Schema Introspection
 
-The schema introspection system is accessible from the meta-fields `__schema`
-and `__type` which are accessible from the type of the root of a query
-operation.
+The schema introspection system is accessible from the meta-fields `__schema`,
+`__type` and `__service` which are accessible from the type of the root of a
+query operation.
 
 ```graphql
 __schema: __Schema!
 __type(name: String!): __Type
+__service: __Service!
 ```
 
 Like all meta-fields, these are implicit and do not appear in the fields list in
@@ -218,6 +219,15 @@ enum __DirectiveLocation {
   INPUT_OBJECT
   INPUT_FIELD_DEFINITION
 }
+
+type __Service {
+  capabilities: [__Capability!]!
+}
+
+type __Capability {
+  identifier: String!
+  value: String
+}
 ```
 
 ### The \_\_Schema Type
@@ -500,3 +510,74 @@ Fields\:
     {true}, deprecated arguments are also returned.
 - `isRepeatable` must return a Boolean that indicates if the directive may be
   used repeatedly at a single location.
+
+### The \_\_Service Type
+
+The `__Service` type is returned from the `__service` meta-field and provides
+information about the GraphQL service, most notably about its capabilities. This
+type was added after the original release of GraphQL, so older schemas may not
+support it.
+
+Fields\:
+
+- `capabilities` must return a list of `__Capability` indicating the
+  capabilities supported by the service.
+
+### The \_\_Capability Type
+
+:: A _capability_ describes a feature supported by the GraphQL service but not
+directly expressible in the type system. This may include experimental GraphQL
+features (such as new syntax or behavior), protocol support (such as GraphQL
+over WebSockets), or additional operational metadata (such as release
+identifiers). Capabilities may be supplied by the GraphQL implementation, the
+service, or both.
+
+The `__Service.capabilities` field exposes a _capability_ list. A _capability_
+consists of a capability identifier and optionally a string value.
+
+**Capability identifier**
+
+A capability identifier is a string value composed of two or more segments
+separated by a period (`.`). Each segment must begin with an ASCII letter, and
+must contain only ASCII letters, digits and hyphens. These constraints are
+inspired by reverse domain name notation to encourage global uniqueness and
+collision-resistance. Unlike the domain name system, capability identifiers are
+case sensitive. Identifiers beginning with the prefix {"org.graphql."} are
+reserved and must not be used outside of official GraphQL Foundation
+specifications. Identifiers beginning with the prefix {"org.graphql.rfc."} are
+reserved for RFC proposals.
+
+Identifiers defined by specific projects, vendors, or implementations should
+begin with a prefix derived from a DNS name they control (e.g.,
+{"com.example."}).
+
+Clients should compare identifiers using exact string equality and should ignore
+unknown identifiers.
+
+Implementers should not change the meaning of capability identifiers; instead, a
+new capability identifier should be used when the meaning changes. Implementers
+should ensure that capability identifiers remain stable and version-agnostic
+where possible; capability versioning, if needed, can be indicated using dot
+suffixes (e.g.{ "org.example.capability.v2"}).
+
+This system enables incremental feature adoption and richer tooling
+interoperability, while avoiding tight coupling to specific implementations.
+
+**Capability value**
+
+The value assigned to a given capability may either be {null} to simply indicate
+the capability is supported, or a string to provide additional information. For
+example {"org.graphql.onError"} does not require additional information and thus
+uses the value {null}, whereas {"org.graphql.defaultErrorBehavior"} needs the
+{value} to indicate which _error behavior_ is the default.
+
+**Specified capabilities**
+
+This version of the specification defines the following capabilities:
+
+- {"org.graphql.defaultErrorBehavior"} - indicates the _default error behavior_
+  of the service via the {value}. If not present, must be assumed to be
+  {"PROPAGATE"}.
+- {"org.graphql.onError"} - indicates that the service allows the client to
+  specify {onError} in a request to indicate the _error behavior_ the service
+  should use for the request. {value} is {null}.

spec/Section 6 -- Execution.md

@@ -4,6 +4,8 @@ A GraphQL service generates a response from a request via execution.
 
 :: A _request_ for execution consists of a few pieces of information:
 
+<!-- https://github.com/prettier/prettier/issues/17286 -->
+<!-- prettier-ignore -->
 - {schema}: The schema to use, typically solely provided by the GraphQL service.
 - {document}: A {Document} which must contain GraphQL {OperationDefinition} and
   may contain {FragmentDefinition}.
@@ -15,13 +17,24 @@ A GraphQL service generates a response from a request via execution.
   being executed. Conceptually, an initial value represents the "universe" of
   data available via a GraphQL Service. It is common for a GraphQL Service to
   always use the same initial value for every request.
+- {onError} (optional): The _error behavior_ to apply to the request; see
+  [Handling Execution Errors](#sec-Handling-Execution-Errors). If {onError} is
+  provided and its value is not one of {"PROPAGATE"}, {"NO_PROPAGATE"}, or
+  {"HALT"}, then a _request error_ must be raised.
 - {extensions} (optional): A map reserved for implementation-specific additional
   information.
 
 Given this information, the result of {ExecuteRequest(schema, document,
 operationName, variableValues, initialValue)} produces the response, to be
 formatted according to the Response section below.
 
+Note: Previous versions of this specification did not define the {onError}
+request attribute. Clients can detect support for {onError} by checking for the
+{"org.graphql.onError"} capability. If this capability is not present, or if
+capabilities themselves are not supported by introspection, then clients should
+not include {onError} in the request and must assume the _error behavior_ is
+{"PROPAGATE"}.
+
 Implementations should not add additional properties to a _request_, which may
 conflict with future editions of the GraphQL specification. Instead,
 {extensions} provides a reserved location for implementation-specific additional
@@ -392,13 +405,24 @@ is explained in greater detail in the Field Collection section below.
 </a>
 
 If during {ExecuteSelectionSet()} a _response position_ with a non-null type
-raises an _execution error_ then that error must propagate to the parent
-response position (the entire selection set in the case of a field, or the
-entire list in the case of a list position), either resolving to {null} if
-allowed or being further propagated to a parent response position.
-
-If this occurs, any sibling response positions which have not yet executed or
-have not yet yielded a value may be cancelled to avoid unnecessary work.
+raises an _execution error_, the error must be added to the {"errors"} list in
+the _response_ and then handled according to the _error behavior_ of the
+request:
+
+<!-- https://github.com/prettier/prettier/issues/17286 -->
+<!-- prettier-ignore -->
+- {"NO\_PROPAGATE"}: The _response position_ must be set to {null}. (The client
+  is responsible for interpreting this {null} in conjunction with the {"errors"}
+  list to distinguish error results from intentional {null} values.)
+- {"PROPAGATE"}: The _execution error_ must propagate to the parent _response
+  position_ (the entire selection set in the case of a field, or the entire list
+  in the case of a list position). The parent position resolves to {null} if
+  allowed, or else the error is further propagated to a parent response
+  position. Any sibling response positions that have not yet executed or have
+  not yet yielded a value may be cancelled to avoid unnecessary work.
+- {"HALT"}: The entire _request_ must be cancelled. The {"data"} entry in the
+  _response_ must be {null}. Any _response position_ that has not yet executed
+  or has not yet yielded a value may be cancelled to avoid unnecessary work.
 
 Note: See [Handling Execution Errors](#sec-Handling-Execution-Errors) for more
 about this behavior.
@@ -823,28 +847,64 @@ MergeSelectionSets(fields):
 </a>
 
 An _execution error_ is an error raised during field execution, value resolution
-or coercion, at a specific _response position_. While these errors must be
-reported in the response, they are "handled" by producing partial {"data"} in
-the _response_.
+or coercion, at a specific _response position_. These errors must be added to
+the {"errors"} list in the _response_, and are "handled" according to the _error
+behavior_ of the request.
+
+Note: An _execution error_ is distinct from a _request error_ which results in a
+response with no {"data"}.
+
+If a _response position_ resolves to {null} because of an execution error which
+has already been added to the {"errors"} list in the response, the {"errors"}
+list must not be further affected. That is, only one error should be added to
+the errors list per _response position_.
+
+:: The _error behavior_ of a request indicates how an _execution error_ is
+handled. It may be specified using the optional {onError} attribute of the
+_request_. If omitted, the _default error behavior_ of the service applies.
+Valid values for _error behavior_ are {"PROPAGATE"}, {"NO_PROPAGATE"} and
+{"HALT"}.
+
+<!-- https://github.com/prettier/prettier/issues/17286 -->
+<!-- prettier-ignore -->
+:: The _default error behavior_ of a service is implementation-defined. For
+compatibility with existing clients, services should default to {"PROPAGATE"}
+which reflects prior behavior. <!-- For new services, {"NO_PROPAGATE"} is
+recommended. --> The default error behavior is indicated via the
+{"org.graphql.defaultErrorBehavior"} _capability_.
+
+Note: {"HALT"} is not recommended as the _default error behavior_ because it
+prevents generating partial responses which may still contain useful data.
+
+Regardless of error behavior, if a _response position_ with a non-null type
+results in {null} due to the result of {ResolveFieldValue()} then an execution
+error must be raised at that position as specified in {CompleteValue()}.
 
-Note: This is distinct from a _request error_ which results in a response with
-no data.
+The _error behavior_ of a request applies to every _execution error_ raised
+during execution. The following sections describe the behavior of each valid
+value:
 
-If an execution error is raised while resolving a field (either directly or
-nested inside any lists), it is handled as though the _response position_ at
-which the error occurred resolved to {null}, and the error must be added to the
-{"errors"} list in the response.
+**{"NO_PROPAGATE"}**
+
+<!-- https://github.com/prettier/prettier/issues/17286 -->
+<!-- prettier-ignore -->
+With {"NO\_PROPAGATE"}, a `Non-Null` _response position_ will have the value
+{null} if and only if an error occurred at that position.
+
+Note: Clients must inspect the {"errors"} list and use the {"path"} of each
+error result to distinguish between intentional {null} values and those
+resulting from an _execution error_.
+
+**{"PROPAGATE"}**
+
+With {"PROPAGATE"}, a `Non-Null` _response position_ must not contain {null} in
+the _response_.
 
 If the result of resolving a _response position_ is {null} (either due to the
 result of {ResolveFieldValue()} or because an execution error was raised), and
 that position is of a `Non-Null` type, then an execution error is raised at that
 position. The error must be added to the {"errors"} list in the response.
 
-If a _response position_ resolves to {null} because of an execution error which
-has already been added to the {"errors"} list in the response, the {"errors"}
-list must not be further affected. That is, only one error should be added to
-the errors list per _response position_.
-
 Since `Non-Null` response positions cannot be {null}, execution errors are
 propagated to be handled by the parent _response position_. If the parent
 response position may be {null} then it resolves to {null}, otherwise if it is a
@@ -859,3 +919,9 @@ position_ must resolve to {null}. If the `List` type is also wrapped in a
 If every _response position_ from the root of the request to the source of the
 execution error has a `Non-Null` type, then the {"data"} entry in the response
 should be {null}.
+
+**{"HALT"}**
+
+With {"HALT"}, execution must cease immediately when the first _execution error_
+is raised. That error must be added to the {"errors"} list, and {"data"} must be
+{null}.