-2

我正在为 RocksDB 创建一个本机节点扩展，我已经确定了一个我无法解释的问题。所以我有以下完美运行的代码：

std::string v;

ROCKSDB_STATUS_THROWS(db->Get(*options, k, &v));

napi_value result;
NAPI_STATUS_THROWS(napi_create_buffer_copy(env, v.size(), v.c_str(), nullptr, &result));
return result;

但是当我引入一种减少额外的优化时，memcpy我得到了段错误：

std::string *v = new std::string();

ROCKSDB_STATUS_THROWS(db->Get(*options, k, v)); // <============= I get segfaults here

napi_value result;
NAPI_STATUS_THROWS(napi_create_external_buffer(env, v->size(), (void *)v->c_str(), rocksdb_get_finalize, v, &result));
return result;

这是Get方法签名：

rocksdb::Status rocksdb::DB::Get(const rocksdb::ReadOptions &options, const rocksdb::Slice &key, std::string *value)

任何想法为什么会发生这个问题？

先感谢您！

编辑

可以肯定的是，我还检查了以下版本（它也失败了）：

std::string *v = new std::string();

ROCKSDB_STATUS_THROWS(db->Get(*options, k, v));

napi_value result;
NAPI_STATUS_THROWS(napi_create_buffer_copy(env, v->size(), v->c_str(), nullptr, &result));

delete v;

编辑

根据评论中的请求，提供更完整的示例：

#include <napi-macros.h>
#include <node_api.h>

#include <rocksdb/db.h>
#include <rocksdb/convenience.h>
#include <rocksdb/write_batch.h>
#include <rocksdb/cache.h>
#include <rocksdb/filter_policy.h>
#include <rocksdb/cache.h>
#include <rocksdb/comparator.h>
#include <rocksdb/env.h>
#include <rocksdb/options.h>
#include <rocksdb/table.h>

#include "easylogging++.h"

INITIALIZE_EASYLOGGINGPP

...

/**
 * Runs when a rocksdb_get return value instance is garbage collected.
 */
static void rocksdb_get_finalize(napi_env env, void *data, void *hint)
{
    LOG_IF(logging_enabled, INFO) << LOCATION << " rocksdb_get_finalize (started)";
    if (hint)
    {
        LOG_IF(logging_enabled, INFO) << LOCATION << " rocksdb_get_finalize (finished)";
        delete (std::string *)hint;
    }
    LOG_IF(logging_enabled, INFO) << LOCATION << " rocksdb_get_finalize (finished)";
}

/**
 * Gets key / value pair from a database.
 */
NAPI_METHOD(rocksdb_get)
{
    LOG_IF(logging_enabled, INFO) << LOCATION << " rocksdb_get (started)";

    NAPI_ARGV(3);

    LOG_IF(logging_enabled, INFO) << LOCATION << " rocksdb_get (getting db argument)";
    rocksdb::DB *DECLARE_FROM_EXTERNAL_ARGUMENT(0, db);
    LOG_IF(logging_enabled, INFO) << LOCATION << " rocksdb_get (getting k argument)";
    DECLARE_SLICE_FROM_BUFFER_ARGUMENT(1, k);
    LOG_IF(logging_enabled, INFO) << LOCATION << " rocksdb_get (getting options argument)";
    rocksdb::ReadOptions *DECLARE_FROM_EXTERNAL_ARGUMENT(2, options);

    LOG_IF(logging_enabled, INFO) << LOCATION << " rocksdb_get (declaring v variable)";
    std::string *v = new std::string();

    LOG_IF(logging_enabled, INFO) << LOCATION << " rocksdb_get (getting value from database)";
    ROCKSDB_STATUS_THROWS(db->Get(*options, k, v));

    LOG_IF(logging_enabled, INFO) << LOCATION << " rocksdb_get (wrapping value with js wrapper)";
    napi_value result;
    NAPI_STATUS_THROWS(napi_create_external_buffer(env, v->size(), (void *)v->c_str(), rocksdb_get_finalize, v, &result));
    LOG_IF(logging_enabled, INFO) << LOCATION << " rocksdb_get (finished)";
    return result;
}

启动上述方法的代码在 TypeScript 中实现并在 NodeJS 中运行，这里是完整的清单：

import path from 'path';
import { bindings as rocks, Unique, BatchContext } from 'rocksdb';

import { MapOf } from '../types';
import { Command, CommandOptions, CommandOptionDeclaration, Persist, CommandEnvironment } from '../command';

// tslint:disable-next-line: no-empty-interface
export interface PullCommandOptions {
}

@Command
export class ExampleCommandNameCommand implements Command {
    public get description(): string {
        return "[An example command description]";
    }

    public get options(): CommandOptions<CommandOptionDeclaration> {
        const result: MapOf<PullCommandOptions, CommandOptionDeclaration> = new Map();

        return result;
    }

    public async run(environment: CommandEnvironment, opts: CommandOptions<unknown>): Promise<void> {
        // let options = opts as unknown as PullCommandOptions;

        let window = global as any;

        window.rocks = rocks;

        const configPath = path.resolve('log.conf');
        const configPathBuffer = Buffer.from(configPath);

        rocks.logger_config(configPathBuffer);
        rocks.logger_start();

        let db = window.db = rocks.rocksdb_open(Buffer.from('test.db', 'utf-8'), rocks.rocksdb_options_init());
        let readOptions = window.readOptions = rocks.rocksdb_read_options_init();
        let writeOptions = window.writeOptions = rocks.rocksdb_write_options_init();

        // ===== The line below launches the C++ method
        rocks.rocksdb_put(db, Buffer.from('Zookie'), Buffer.from('Cookie'), writeOptions);
        // ===== The line above launches the C++ method

        console.log(rocks.rocksdb_get(db, Buffer.from('Zookie'), readOptions).toString());

        let batch: Unique<BatchContext> | null = rocks.rocksdb_batch_init();

        rocks.rocksdb_batch_put(batch, Buffer.from('Cookie'), Buffer.from('Zookie'));
        rocks.rocksdb_batch_put(batch, Buffer.from('Pookie'), Buffer.from('Zookie'));
        rocks.rocksdb_batch_put(batch, Buffer.from('Zookie'), Buffer.from('Zookie'));
        rocks.rocksdb_batch_put(batch, Buffer.from('Hookie'), Buffer.from('Zookie'));
        await rocks.rocksdb_batch_write_async(db, batch, writeOptions);

        batch = null;

        let proceed = true;

        while (proceed) {
            await new Promise(resolve => setTimeout(resolve, 1000));
        }
    }
}

基本上这段代码代表了 KeyValueDatabase->Get("Some key") 方法的实现，你将一个字符串传递给它，你会得到一个字符串作为回报。但很明显，问题是围绕new std::string()电话跳舞，我想我可能会得到一些解释，为什么这样不好？如何在没有副本的情况下将字符串值从一个字符串移动到另一个字符串？

4

1 回答 1

1

但是当我引入一种减少额外的 memcpy 的优化时

目前尚不清楚memcpy您认为您正在优化哪些额外内容。

如果字符串很短，并且您正在使用std::string短字符串优化，那么您确实会优化出一个 short memcpy。但是，动态分配然后删除std::string可能比memcpy.

如果字符串很长，您实际上根本不会优化任何东西，而是无缘无故地使代码变慢。

我得到段错误：

v = new std::string; ... ; delete v;添加引入 a的事实SIGSEGV可能表明您正在发生其他一些堆损坏，直到您稍微改变一下才会注意到。Valgrind是你的朋友。

于 2019-09-30T03:47:47.320 回答