service.go

// Copyright 2019 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package jrpc

import (
	"context"
	"errors"
	"fmt"
	"reflect"
	"runtime"
	"unicode"

	"git.tuxpa.in/a/zlog/log"
)

var (
	contextType = reflect.TypeOf((*context.Context)(nil)).Elem()
	errorType   = reflect.TypeOf((*error)(nil)).Elem()
	stringType  = reflect.TypeOf("")
)

// A helper function that mimics the behavior of the handlers in the go-ethereum rpc package
// if you don't know how to use this, just use the chi-like interface instead.
func RegisterStruct(r Router, name string, rcvr any) error {
	rcvrVal := reflect.ValueOf(rcvr)
	if name == "" {
		return fmt.Errorf("no service name for type %s", rcvrVal.Type().String())
	}
	callbacks := suitableCallbacks(rcvrVal)
	if len(callbacks) == 0 {
		return fmt.Errorf("service %T doesn't have any suitable methods/subscriptions to expose", rcvr)
	}
	r.Route(name, func(r Router) {
		for nm, cb := range callbacks {
			r.Handle(nm, cb)
		}
	})
	return nil
}

// suitableCallbacks iterates over the methods of the given type. It determines if a method
// satisfies the criteria for a RPC callback or a subscription callback and adds it to the
// collection of callbacks. See server documentation for a summary of these criteria.
func suitableCallbacks(receiver reflect.Value) map[string]Handler {
	typ := receiver.Type()
	callbacks := make(map[string]Handler)
	for m := 0; m < typ.NumMethod(); m++ {
		method := typ.Method(m)
		if method.PkgPath != "" {
			continue // method not exported
		}
		cb := newCallback(receiver, method.Func)
		if cb == nil {
			continue // function invalid
		}
		name := formatName(method.Name)
		callbacks[name] = cb
	}
	return callbacks
}

// newCallback turns fn (a function) into a callback object. It returns nil if the function
// is unsuitable as an RPC callback.
func newCallback(receiver, fn reflect.Value) Handler {
	fntype := fn.Type()
	c := &callback{fn: fn, rcvr: receiver, errPos: -1}
	// Determine parameter types. They must all be exported or builtin types.
	c.makeArgTypes()

	// Verify return types. The function must return at most one error
	// and/or one other non-error value.
	outs := make([]reflect.Type, fntype.NumOut())
	for i := 0; i < fntype.NumOut(); i++ {
		outs[i] = fntype.Out(i)
	}
	if len(outs) > 2 {
		return nil
	}
	// If an error is returned, it must be the last returned value.
	switch {
	case len(outs) == 1 && isErrorType(outs[0]):
		c.errPos = 0
	case len(outs) == 2:
		if isErrorType(outs[0]) || !isErrorType(outs[1]) {
			return nil
		}
		c.errPos = 1
	}
	return c
}

// makeArgTypes composes the argTypes list.
func (c *callback) makeArgTypes() {
	fntype := c.fn.Type()
	// Skip receiver and context.Context parameter (if present).
	firstArg := 0
	if c.rcvr.IsValid() {
		firstArg++
	}
	if fntype.NumIn() > firstArg && fntype.In(firstArg) == contextType {
		c.hasCtx = true
		firstArg++
	}
	// Add all remaining parameters.
	c.argTypes = make([]reflect.Type, fntype.NumIn()-firstArg)
	for i := firstArg; i < fntype.NumIn(); i++ {
		c.argTypes[i-firstArg] = fntype.In(i)
	}
}

// call invokes the callback.
func (c *callback) call(ctx context.Context, method string, args []reflect.Value) (res any, errRes error) {
	// Create the argument slice.
	fullargs := make([]reflect.Value, 0, 2+len(args))
	if c.rcvr.IsValid() {
		fullargs = append(fullargs, c.rcvr)
	}
	if c.hasCtx {
		fullargs = append(fullargs, reflect.ValueOf(ctx))
	}
	fullargs = append(fullargs, args...)

	// Catch panic while running the callback.
	defer func() {
		if err := recover(); err != nil {
			const size = 64 << 10
			buf := make([]byte, size)
			buf = buf[:runtime.Stack(buf, false)]
			log.Error().Str("method", method).Interface("err", err).Hex("buf", buf).Msg("crashed")
			errRes = errors.New("method handler crashed")
		}
	}()
	// Run the callback.
	// fn is a pointer to a function. I'm not sure if I like this...
	results := c.fn.Call(fullargs)
	if len(results) == 0 {
		return nil, nil
	}
	if c.errPos >= 0 && !results[c.errPos].IsNil() {
		// Method has returned non-nil error value.
		err := results[c.errPos].Interface().(error)
		return reflect.Value{}, err
	}
	return results[0].Interface(), nil
}

// Does t satisfy the error interface?
func isErrorType(t reflect.Type) bool {
	for t.Kind() == reflect.Ptr {
		t = t.Elem()
	}
	return t.Implements(errorType)
}

// formatName converts to first character of name to lowercase.
func formatName(name string) string {
	ret := []rune(name)
	if len(ret) > 0 {
		ret[0] = unicode.ToLower(ret[0])
	}
	return string(ret)
}