329 lines
10 KiB
Go
329 lines
10 KiB
Go
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// Package handler provides implementations of the jrpc2.Assigner interface,
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// and support for adapting functions to the jrpc2.Handler interface.
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package handler
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import (
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"context"
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"encoding/json"
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"errors"
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"fmt"
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"reflect"
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"strings"
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"bitbucket.org/creachadair/stringset"
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"github.com/creachadair/jrpc2"
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"github.com/creachadair/jrpc2/code"
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)
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// A Func adapts a function having the correct signature to a jrpc2.Handler.
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type Func func(context.Context, *jrpc2.Request) (interface{}, error)
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// Handle implements the jrpc2.Handler interface by calling m.
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func (m Func) Handle(ctx context.Context, req *jrpc2.Request) (interface{}, error) {
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return m(ctx, req)
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}
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// A Map is a trivial implementation of the jrpc2.Assigner interface that looks
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// up method names in a map of static jrpc2.Handler values.
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type Map map[string]jrpc2.Handler
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// Assign implements part of the jrpc2.Assigner interface.
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func (m Map) Assign(_ context.Context, method string) jrpc2.Handler { return m[method] }
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// Names implements part of the jrpc2.Assigner interface.
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func (m Map) Names() []string { return stringset.FromKeys(m).Elements() }
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// A ServiceMap combines multiple assigners into one, permitting a server to
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// export multiple services under different names.
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//
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// Example:
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// m := handler.ServiceMap{
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// "Foo": handler.NewService(fooService), // methods Foo.A, Foo.B, etc.
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// "Bar": handler.NewService(barService), // methods Bar.A, Bar.B, etc.
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// }
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//
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type ServiceMap map[string]jrpc2.Assigner
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// Assign splits the inbound method name as Service.Method, and passes the
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// Method portion to the corresponding Service assigner. If method does not
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// have the form Service.Method, or if Service is not set in m, the lookup
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// fails and returns nil.
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func (m ServiceMap) Assign(ctx context.Context, method string) jrpc2.Handler {
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parts := strings.SplitN(method, ".", 2)
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if len(parts) == 1 {
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return nil
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} else if ass, ok := m[parts[0]]; ok {
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return ass.Assign(ctx, parts[1])
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}
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return nil
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}
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// Names reports the composed names of all the methods in the service, each
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// having the form Service.Method.
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func (m ServiceMap) Names() []string {
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var all stringset.Set
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for svc, assigner := range m {
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for _, name := range assigner.Names() {
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all.Add(svc + "." + name)
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}
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}
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return all.Elements()
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}
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// New adapts a function to a jrpc2.Handler. The concrete value of fn must be a
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// function with one of the following type signatures:
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//
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// func(context.Context) error
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// func(context.Context) Y
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// func(context.Context) (Y, error)
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// func(context.Context, X) error
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// func(context.Context, X) Y
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// func(context.Context, X) (Y, error)
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// func(context.Context, ...X) (Y, error)
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// func(context.Context, *jrpc2.Request) (Y, error)
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// func(context.Context, *jrpc2.Request) (interface{}, error)
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//
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// for JSON-marshalable types X and Y. New will panic if the type of fn does
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// not have one of these forms. The resulting method will handle encoding and
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// decoding of JSON and report appropriate errors.
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//
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// Functions adapted by in this way can obtain the *jrpc2.Request value using
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// the jrpc2.InboundRequest helper on the context value supplied by the server.
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func New(fn interface{}) Func {
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m, err := newHandler(fn)
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if err != nil {
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panic(err)
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}
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return m
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}
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// NewService adapts the methods of a value to a map from method names to
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// Handler implementations as constructed by New. It will panic if obj has no
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// exported methods with a suitable signature.
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func NewService(obj interface{}) Map {
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out := make(Map)
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val := reflect.ValueOf(obj)
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typ := val.Type()
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// This considers only exported methods, as desired.
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for i, n := 0, val.NumMethod(); i < n; i++ {
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mi := val.Method(i)
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if v, err := newHandler(mi.Interface()); err == nil {
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out[typ.Method(i).Name] = v
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}
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}
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if len(out) == 0 {
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panic("no matching exported methods")
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}
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return out
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}
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var (
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ctxType = reflect.TypeOf((*context.Context)(nil)).Elem() // type context.Context
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errType = reflect.TypeOf((*error)(nil)).Elem() // type error
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reqType = reflect.TypeOf((*jrpc2.Request)(nil)) // type *jrpc2.Request
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)
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func newHandler(fn interface{}) (Func, error) {
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if fn == nil {
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return nil, errors.New("nil method")
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}
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// Special case: If fn has the exact signature of the Handle method, don't do
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// any (additional) reflection at all.
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if f, ok := fn.(func(context.Context, *jrpc2.Request) (interface{}, error)); ok {
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return Func(f), nil
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}
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// Check that fn is a function of one of the correct forms.
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typ, err := checkFunctionType(fn)
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if err != nil {
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return nil, err
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}
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// Construct a function to unpack the parameters from the request message,
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// based on the signature of the user's callback.
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var newinput func(req *jrpc2.Request) ([]reflect.Value, error)
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if typ.NumIn() == 1 {
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// Case 1: The function does not want any request parameters.
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// Nothing needs to be decoded, but verify no parameters were passed.
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newinput = func(req *jrpc2.Request) ([]reflect.Value, error) {
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if req.HasParams() {
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return nil, jrpc2.Errorf(code.InvalidParams, "no parameters accepted")
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}
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return nil, nil
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}
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} else if a := typ.In(1); a == reqType {
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// Case 2: The function wants the underlying *jrpc2.Request value.
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newinput = func(req *jrpc2.Request) ([]reflect.Value, error) {
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return []reflect.Value{reflect.ValueOf(req)}, nil
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}
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} else {
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// Check whether the function wants a pointer to its argument. We need
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// to create one either way to support unmarshaling, but we need to
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// indirect it back off if the callee didn't want it.
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// Case 3a: The function wants a bare value, not a pointer.
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argType := typ.In(1)
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undo := reflect.Value.Elem
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if argType.Kind() == reflect.Ptr {
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// Case 3b: The function wants a pointer.
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undo = func(v reflect.Value) reflect.Value { return v }
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argType = argType.Elem()
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}
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newinput = func(req *jrpc2.Request) ([]reflect.Value, error) {
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in := reflect.New(argType).Interface()
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if err := req.UnmarshalParams(in); err != nil {
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return nil, jrpc2.Errorf(code.InvalidParams, "invalid parameters: %v", err)
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}
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arg := reflect.ValueOf(in)
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return []reflect.Value{undo(arg)}, nil
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}
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}
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// Construct a function to decode the result values.
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var decodeOut func([]reflect.Value) (interface{}, error)
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switch typ.NumOut() {
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case 1:
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if typ.Out(0) == errType {
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// A function that returns only error: Result is always nil.
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decodeOut = func(vals []reflect.Value) (interface{}, error) {
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oerr := vals[0].Interface()
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if oerr != nil {
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return nil, oerr.(error)
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}
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return nil, nil
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}
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} else {
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// A function that returns a single non-error: err is always nil.
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decodeOut = func(vals []reflect.Value) (interface{}, error) {
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return vals[0].Interface(), nil
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}
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}
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default:
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// A function that returns a value and an error.
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decodeOut = func(vals []reflect.Value) (interface{}, error) {
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out, oerr := vals[0].Interface(), vals[1].Interface()
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if oerr != nil {
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return nil, oerr.(error)
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}
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return out, nil
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}
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}
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f := reflect.ValueOf(fn)
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call := f.Call
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if typ.IsVariadic() {
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call = f.CallSlice
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}
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return Func(func(ctx context.Context, req *jrpc2.Request) (interface{}, error) {
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rest, ierr := newinput(req)
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if ierr != nil {
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return nil, ierr
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}
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args := append([]reflect.Value{reflect.ValueOf(ctx)}, rest...)
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return decodeOut(call(args))
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}), nil
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}
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func checkFunctionType(fn interface{}) (reflect.Type, error) {
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typ := reflect.TypeOf(fn)
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if typ.Kind() != reflect.Func {
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return nil, errors.New("not a function")
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} else if np := typ.NumIn(); np == 0 || np > 2 {
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return nil, errors.New("wrong number of parameters")
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} else if no := typ.NumOut(); no < 1 || no > 2 {
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return nil, errors.New("wrong number of results")
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} else if typ.In(0) != ctxType {
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return nil, errors.New("first parameter is not context.Context")
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} else if no == 2 && typ.Out(1) != errType {
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return nil, errors.New("result is not of type error")
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}
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return typ, nil
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}
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// Args is a wrapper that decodes an array of positional parameters into
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// concrete locations.
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//
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// Unmarshaling a JSON value into an Args value v succeeds if the JSON encodes
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// an array with length len(v), and unmarshaling each subvalue i into the
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// corresponding v[i] succeeds. As a special case, if v[i] == nil the
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// corresponding value is discarded.
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//
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// Marshaling an Args value v into JSON succeeds if each element of the slice
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// is JSON marshalable, and yields a JSON array of length len(v) containing the
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// JSON values corresponding to the elements of v.
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//
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// Usage example:
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//
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// func Handler(ctx context.Context, req *jrpc2.Request) (interface{}, error) {
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// var x, y int
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// var s string
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//
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// if err := req.UnmarshalParams(&handler.Args{&x, &y, &s}); err != nil {
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// return nil, err
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// }
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// // do useful work with x, y, and s
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// }
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//
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type Args []interface{}
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// UnmarshalJSON supports JSON unmarshaling for a.
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func (a Args) UnmarshalJSON(data []byte) error {
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var elts []json.RawMessage
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if err := json.Unmarshal(data, &elts); err != nil {
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return fmt.Errorf("decoding args: %w", err)
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} else if len(elts) != len(a) {
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return fmt.Errorf("wrong number of args (got %d, want %d)", len(elts), len(a))
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}
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for i, elt := range elts {
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if a[i] == nil {
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continue
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} else if err := json.Unmarshal(elt, a[i]); err != nil {
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return fmt.Errorf("decoding argument %d: %w", i+1, err)
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}
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}
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return nil
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}
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// MarshalJSON supports JSON marshaling for a.
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func (a Args) MarshalJSON() ([]byte, error) {
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if len(a) == 0 {
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return []byte(`[]`), nil
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}
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return json.Marshal([]interface{}(a))
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}
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// Obj is a wrapper that maps object fields into concrete locations.
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//
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// Unmarshaling a JSON text into an Obj value v succeeds if the JSON encodes an
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// object, and unmarshaling the value for each key k of the object into v[k]
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// succeeds. If k does not exist in v, it is ignored.
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//
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// Marshaling an Obj into JSON works as for an ordinary map.
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type Obj map[string]interface{}
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// UnmarshalJSON supports JSON unmarshaling into o.
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func (o Obj) UnmarshalJSON(data []byte) error {
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var base map[string]json.RawMessage
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if err := json.Unmarshal(data, &base); err != nil {
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return fmt.Errorf("decoding object: %v", err)
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}
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for key, val := range base {
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arg, ok := o[key]
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if !ok {
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continue
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} else if err := json.Unmarshal(val, arg); err != nil {
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return fmt.Errorf("decoding %q: %v", key, err)
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}
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}
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return nil
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}
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