fixes: #345 InvokeMethodWithReply segfault (#382)

* fixes: #345 InvokeMethodWithReply segfault

The callback function used to retrieve the value of the reply was
free-ed by the Golang GC.
This was quite hard to fix, had to uses runtime.KeepAlive on a struct
that contains the callback function.
runtime.KeepAlive, ensures that the object is not freed, and its
finalizer is not run, before the point in the program where KeepAlive is
called. Since the returned value is obtained with the callback function,
I used the `defer` keyword to make the function as not used for the gc
once we have the reply value.

* adress PR comment

Author: pchampio

embedder/embedder.go

@@ -346,17 +346,20 @@ func (flu *FlutterEngine) MarkExternalTextureFrameAvailable(textureID int64) Res
 
 // DataCallback is a function called when a PlatformMessage response send back
 // to the embedder.
-type DataCallback func(binaryReply []byte)
+type DataCallback struct {
+	// Handle func
+	Handle func(binaryReply []byte)
+}
 
 // CreatePlatformMessageResponseHandle creates a platform message response
 // handle that allows the embedder to set a native callback for a response to a
 // message.
 // Must be collected via `ReleasePlatformMessageResponseHandle` after the call
 // to `SendPlatformMessage`.
-func (flu *FlutterEngine) CreatePlatformMessageResponseHandle(callback DataCallback) (PlatformMessageResponseHandle, error) {
+func (flu *FlutterEngine) CreatePlatformMessageResponseHandle(callback *DataCallback) (PlatformMessageResponseHandle, error) {
 	var responseHandle *C.FlutterPlatformMessageResponseHandle
 
-	callbackPointer := uintptr(unsafe.Pointer(&callback))
+	callbackPointer := uintptr(unsafe.Pointer(callback))
 	defer func() {
 		runtime.KeepAlive(callbackPointer)
 	}()

embedder/embedder_proxy.go

@@ -97,6 +97,6 @@ func proxy_post_task_callback(task C.FlutterTask, targetTimeNanos C.uint64_t, us
 //export proxy_desktop_binary_reply
 func proxy_desktop_binary_reply(data *C.uint8_t, dataSize C.size_t, userData unsafe.Pointer) {
 	callbackPointer := *(*uintptr)(userData)
-	handler := *(*DataCallback)(unsafe.Pointer(callbackPointer))
-	handler(C.GoBytes(unsafe.Pointer(data), C.int(dataSize)))
+	callback := (*DataCallback)(unsafe.Pointer(callbackPointer))
+	callback.Handle(C.GoBytes(unsafe.Pointer(data), C.int(dataSize)))
 }

messenger.go

@@ -3,6 +3,7 @@ package flutter
 import (
 	"errors"
 	"fmt"
+	"runtime"
 	"sync"
 
 	"github.com/go-flutter-desktop/go-flutter/embedder"
@@ -37,9 +38,13 @@ func newMessenger(engine *embedder.FlutterEngine) *messenger {
 func (m *messenger) SendWithReply(channel string, binaryMessage []byte) (binaryReply []byte, err error) {
 	reply := make(chan []byte)
 	defer close(reply)
-	responseHandle, err := m.engine.CreatePlatformMessageResponseHandle(func(binaryMessage []byte) {
-		reply <- binaryMessage
-	})
+	callbackHandle := &embedder.DataCallback{
+		Handle: func(binaryMessage []byte) {
+			reply <- binaryMessage
+		},
+	}
+	defer runtime.KeepAlive(callbackHandle)
+	responseHandle, err := m.engine.CreatePlatformMessageResponseHandle(callbackHandle)
 	if err != nil {
 		return nil, err
 	}