带有 Dart ffi 的 MySqlXC 连接器带来错误“NoSuchMethodError: The method 'FfiTrampoline' was called on null.”

问题描述

我正在尝试使用 Dart 的 ffi 包和 MysqL 的本机 C-Connector 直接连接到带有 Dart 的 MysqL。

我的 Dart 代码是：

import 'dart:ffi';
import 'package:ffi/ffi.dart';

typedef CFun = Pointer Function(Pointer<Utf8>,Int32,Pointer<Utf8>,Pointer<Pointer>?);
typedef DartFun = Pointer Function(Pointer<Utf8>,int,Pointer<Pointer>?);

void main() {
  final lib = DynamicLibrary.open('/usr/lib64/libMysqLcppconn8.so.2');
  final fc = lib.lookup<NativeFunction<CFun>>('MysqLx_get_session');
  print('We have C: $fc');
  final DartFun fd = fc.asFunction();
  print('We have Dart: $fd');
  Pointer<Pointer>? error;
  Pointer? session;
  session = fd('localhost'.toNativeUtf8(),33060,'*user*'.toNativeUtf8(),'*password*'.toNativeUtf8(),''.toNativeUtf8(),error);
}

运行时我得到这个输出：

We have Pointer<NativeFunction<(Pointer<Utf8>,Pointer<Pointer<NativeType>>?) => Pointer<NativeType>>>: address=0x7f5b9c78f770
We have Closure: (Pointer<Utf8>,Pointer<Pointer<NativeType>>?) => Pointer<NativeType>
Unhandled exception:
NoSuchMethodError: The method 'FfiTrampoline' was called on null.
Receiver: null
Tried calling: FfiTrampoline()
#0      FfiTrampoline (dart:ffi)
#1      main (package:greudb/src/connection.dart:37:14)
#2      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:283:19)
#3      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)

在 C 中尝试相同的方法，效果很好：

#include <MysqL-cppconn/MysqLx/xapi.h>
#include <stdio.h>

int main()
{
    MysqLx_session_t *session = 0;
    MysqLx_error_t *error = 0;
    session = MysqLx_get_session("localhost","*user*","*password*","",&error);
    if(error != 0) {
        printf("%s/n",MysqLx_error_message(error));
    }
    return 0;
}

我猜我的 Dart 函数签名不正确，但无论我尝试什么都无济于事。 MysqL 的 C 文档在这里：https://dev.mysql.com/doc/dev/connector-cpp/8.0/group__xapi__sess.html

我使用的是 Linux Fedora 33、MysqL 社区服务器 8、MysqL Connector C/C++ 8、Dart 202.8488。

解决方法

稍微环顾四周后，it appears 您可以使用以下方法为 error 创建适当的类型：

final error = calloc<Pointer>();

在幕后使用适当的 calloc.allocate<Pointer>(...) 调用 byteCount。由于这是非托管内存，您需要记住通过调用来释放它：

calloc.free(error);

一旦你完成它。

幸运的是，mysqlx API 似乎将所有结构视为不透明，因此您可以通过执行以下操作来确定实际错误是什么：

final message = errorMessage(error.value);
print(message.toDartString());

关于释放内存的话题，调用Pointer<Utf8>创建的toNativeUtf8()值也需要释放。

将所有这些放在一起：

import 'dart:ffi';
import 'package:ffi/ffi.dart';

typedef _GetSession = Pointer Function(Pointer<Utf8>,Int32,Pointer<Utf8>,Pointer<Pointer>);
typedef GetSession = Pointer Function(Pointer<Utf8>,int,Pointer<Pointer>);

typedef _CloseSession = Void Function(Pointer);
typedef CloseSession = void Function(Pointer);

typedef _ErrorMessage = Pointer<Utf8> Function(Pointer);
typedef ErrorMessage = Pointer<Utf8> Function(Pointer);

void main() {
  final lib = DynamicLibrary.open('/usr/lib64/libmysqlcppconn8.so.2');
  final GetSession getSession = lib.lookup<NativeFunction<_GetSession>>('mysqlx_get_session').asFunction();
  final CloseSession closeSession = lib.lookup<NativeFunction<_CloseSession>>('mysqlx_session_close').asFunction();
  final ErrorMessage errorMessage = lib.lookup<NativeFunction<_ErrorMessage>>('mysqlx_error_message').asFunction();

  final host = 'localhost'.toNativeUtf8();
  final user = 'user'.toNativeUtf8();
  final password = 'password'.toNativeUtf8();
  final database = ''.toNativeUtf8();
  final error = calloc<Pointer>();

  final session = getSession(host,33060,user,password,database,error);

  calloc.free(host);
  calloc.free(user);
  calloc.free(password);
  calloc.free(database);

  if (error.value != nullptr) {
    final message = errorMessage(error.value);
    print(message.toDartString());

    calloc.free(error);
    return;
  }

  // Do something with session

  closeSession(session);
}