为什么我的主题行会出现分段错误

Question

我正在为 RocksDB 创建一个本地节点扩展，我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;

但是当我引入一个减少一个额外的优化时 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)

有没有想过为什么会发生这个问题？

提前致谢！

编辑

为了确定，我还检查了以下版本（同样失败）：

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

根据评论中的要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，完整清单如下：

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在 return 中得到一个字符串。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关于为什么这样做不好的解释？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？

Answer 1

But when I introduce an optimization that reduces one extra memcpy

不清楚您认为优化了哪些额外 memcpy。

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

昂贵得多

如果字符串很长，您实际上根本没有优化任何东西，而是无缘无故地使代码变慢。

I get segfaults:

添加 v = new std::string; ... ; delete v; 引入 SIGSEGV 的事实可能表明您有一些 other 堆损坏正在进行，直到您稍微改变一下。 Valgrind 是你的朋友。

为什么我的主题行会出现分段错误

Why do I get segmentation faults for the subject line

c++

node.js

segmentation-fault

rocksdb

node-native-addon