rpc: check RPC service functions more carefully (#7587)

Require that RPC functions take a context as their first argument, and return
an error as either their only result, or the second of two results.

This does not change how functions are dispatched, but will make it a little
easier to make more invasive changes in the near future.

rpc/jsonrpc/server/parse_test.go

@@ -134,7 +134,7 @@ func TestParseJSONArray(t *testing.T) {
 }
 
 func TestParseJSONRPC(t *testing.T) {
-	demo := func(ctx context.Context, height int, name string) {}
+	demo := func(ctx context.Context, height int, name string) error { return nil }
 	call := NewRPCFunc(demo, "height", "name")
 
 	cases := []struct {
@@ -171,7 +171,7 @@ func TestParseJSONRPC(t *testing.T) {
 }
 
 func TestParseURI(t *testing.T) {
-	demo := func(ctx context.Context, height int, name string) {}
+	demo := func(ctx context.Context, height int, name string) error { return nil }
 	call := NewRPCFunc(demo, "height", "name")
 
 	cases := []struct {

rpc/jsonrpc/server/rpc_func.go

@@ -1,6 +1,9 @@
 package server
 
 import (
+	"context"
+	"errors"
+	"fmt"
 	"net/http"
 	"reflect"
 
@@ -23,7 +26,7 @@ func RegisterRPCFuncs(mux *http.ServeMux, funcMap map[string]*RPCFunc, logger lo
 
 // Function introspection
 
-// RPCFunc contains the introspected type information for a function
+// RPCFunc contains the introspected type information for a function.
 type RPCFunc struct {
 	f        reflect.Value  // underlying rpc function
 	args     []reflect.Type // type of each function arg
@@ -32,47 +35,80 @@ type RPCFunc struct {
 	ws       bool           // websocket only
 }
 
-// NewRPCFunc wraps a function for introspection.
-// f is the function, args are comma separated argument names
+// NewRPCFunc constructs an RPCFunc for f, which must be a function whose type
+// signature matches one of these schemes:
+//
+//     func(context.Context, T1, T2, ...) error
+//     func(context.Context, T1, T2, ...) (R, error)
+//
+// for arbitrary types T_i and R. The number of argNames must exactly match the
+// number of non-context arguments to f. Otherwise, NewRPCFunc panics.
+//
+// The parameter names given are used to map JSON object keys to the
+// corresonding parameter of the function. The names do not need to match the
+// declared names, but must match what the client sends in a request.
 func NewRPCFunc(f interface{}, argNames ...string) *RPCFunc {
-	return newRPCFunc(f, argNames, false)
+	rf, err := newRPCFunc(f, argNames)
+	if err != nil {
+		panic("invalid RPC function: " + err.Error())
+	}
+	return rf
 }
 
-// NewWSRPCFunc wraps a function for introspection and use in the websockets.
+// NewWSRPCFunc behaves as NewRPCFunc, but marks the resulting function for use
+// via websocket.
 func NewWSRPCFunc(f interface{}, argNames ...string) *RPCFunc {
-	return newRPCFunc(f, argNames, true)
+	rf := NewRPCFunc(f, argNames...)
+	rf.ws = true
+	return rf
 }
 
-func newRPCFunc(f interface{}, argNames []string, wsOnly bool) *RPCFunc {
+var (
+	ctxType = reflect.TypeOf((*context.Context)(nil)).Elem()
+	errType = reflect.TypeOf((*error)(nil)).Elem()
+)
+
+// newRPCFunc constructs an RPCFunc for f. See the comment at NewRPCFunc.
+func newRPCFunc(f interface{}, argNames []string) (*RPCFunc, error) {
+	if f == nil {
+		return nil, errors.New("nil function")
+	}
+
+	// Check the type and signature of f.
+	fv := reflect.ValueOf(f)
+	if fv.Kind() != reflect.Func {
+		return nil, errors.New("not a function")
+	}
+
+	ft := fv.Type()
+	if np := ft.NumIn(); np == 0 {
+		return nil, errors.New("wrong number of parameters")
+	} else if ft.In(0) != ctxType {
+		return nil, errors.New("first parameter is not context.Context")
+	} else if np-1 != len(argNames) {
+		return nil, fmt.Errorf("have %d names for %d parameters", len(argNames), np-1)
+	}
+
+	if no := ft.NumOut(); no < 1 || no > 2 {
+		return nil, errors.New("wrong number of results")
+	} else if ft.Out(no-1) != errType {
+		return nil, errors.New("last result is not error")
+	}
+
+	args := make([]reflect.Type, ft.NumIn())
+	for i := 0; i < ft.NumIn(); i++ {
+		args[i] = ft.In(i)
+	}
+	outs := make([]reflect.Type, ft.NumOut())
+	for i := 0; i < ft.NumOut(); i++ {
+		outs[i] = ft.Out(i)
+	}
 	return &RPCFunc{
-		f:        reflect.ValueOf(f),
-		args:     funcArgTypes(f),
-		returns:  funcReturnTypes(f),
+		f:        fv,
+		args:     args,
+		returns:  outs,
 		argNames: argNames,
-		ws:       wsOnly,
-	}
-}
-
-// return a function's argument types
-func funcArgTypes(f interface{}) []reflect.Type {
-	t := reflect.TypeOf(f)
-	n := t.NumIn()
-	typez := make([]reflect.Type, n)
-	for i := 0; i < n; i++ {
-		typez[i] = t.In(i)
-	}
-	return typez
-}
-
-// return a function's return types
-func funcReturnTypes(f interface{}) []reflect.Type {
-	t := reflect.TypeOf(f)
-	n := t.NumOut()
-	typez := make([]reflect.Type, n)
-	for i := 0; i < n; i++ {
-		typez[i] = t.Out(i)
-	}
-	return typez
+	}, nil
 }
 
 //-------------------------------------------------------------