Berkeley DB: 在c api调用中异常退出导致卡在 futex_wait 调用

Question

C开发，使用berkeley db 4.3 (/usr/lib64/libdb-4.3.so) on RHEL5.6 with kernel 2.6.18-238_xen_AMD64.

在我的测试中 (写入 1,000,000 key/value pairs), 如果某个进程在对db进行操作的过程中异常退出(ctrl + c, kill, or assert fails) , 此后对db的打开都会被block. 使用strace可以看出，进程卡在了一个 futex(ptr_to_something, FUTEX_WAIT, 2, NULL) 调用。之前它刚打开 __db.00x(e.g __db.001, __db.002, __db.003) .

目前我所知的唯一方法就是删掉 __db.00x 文件，后续测试表明数据库并未损坏，仍可以正常读写。这个出方法基本可以满足我的需求，但是我想知道是否有更好、或者说更优雅的处理方案。

下面列出一些strace的输出，以及用户操作db的代码，也许会有帮助。

some of the strace stderr

...  
open("__db.001", O_RDWR)                = 3  
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0  
fstat(3, {st_mode=S_IFREG|0640, st_size=24576, ...}) = 0  
close(3)                                = 0  
open("__db.001", O_RDWR)                = 3  
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0  
mmap(NULL, 24576, PROT_READ|PROT_WRITE, MAP_SHARED, 3, 0) = 0x2afcc4149000  
close(3)                                = 0
futex(0x2afcc4149000, FUTEX_WAIT, 2, NULL **[[stuck here]]**

code to operate the database

typedef DB* db_handle;
db_handle bdb_open(const char *filename, u_int32_t cache_size_mb)
{
    int ret;
    DB_ENV *env;
    db_handle dbp;
    u_int32_t flags = DB_CREATE | DB_THREAD | DB_INIT_LOCK | DB_INIT_MPOOL | DB_INIT_LOCK ;
    u_int32_t gb = cache_size_mb / 1024, mb = cache_size_mb % 1024;

    if (ret = db_env_create(&env, 0)) {
        fprintf(stderr, "db_env_create:%d, %s\n", ret, db_strerror(ret));
        exit(EXIT_FAILURE);
    }

    if (ret = env->set_timeout(env, 3 * 1000000, DB_SET_LOCK_TIMEOUT)) {
        fprintf(stderr, "env->set_timeout:%d, %s\n", ret, db_strerror(ret));
        exit(EXIT_FAILURE);
    }

    if (ret = env->set_lk_detect(env, DB_LOCK_DEFAULT)) { /* this seems to be of no use in my case */
        fprintf(stderr, "env->set_lk_detect:%d, %s\n", ret, db_strerror(ret));
        exit(EXIT_FAILURE);
    }

    if (ret = env->set_cachesize(env, gb, mb * 1024 * 1024, 0)) {
        fprintf(stderr, "env->set_cachesize:%d, %s\n", ret, db_strerror(ret));
        exit(EXIT_FAILURE);
    }

    if ((ret = env->open(env, NULL, flags, 0)) != 0) {
        fprintf(stderr, "db_env_open:%d, %s\n", ret, db_strerror(ret));
        exit(EXIT_FAILURE);
    }

    if (ret = db_create(&dbp, env, 0)) {
        fprintf(stderr, "db_create:%d, %s\n", ret, db_strerror(ret));
        exit(EXIT_FAILURE);
    }

    if (ret = dbp->open(dbp, NULL, filename, NULL, DB_BTREE, flags, 0664)) {
        fprintf(stderr, "dbp->open:%d, %s\n", ret, db_strerror(ret));
        exit(EXIT_FAILURE);
    }

    return dbp;
}

int bdb_put(db_handle db, void* key, u_int32_t keylen, void* val, u_int32_t vallen)
{
    DBT dkey, dval;
    bzero(&dkey, sizeof(dkey));
    bzero(&dval, sizeof(dval));
    dkey.data = key, dkey.size = keylen;
    dval.data = val, dval.size = vallen;
    return db->put(db, NULL, &dkey, &dval, 0);
}

int bdb_get(db_handle db, void* key, const u_int32_t keylen,
                          void* buf, u_int32_t buflen, u_int32_t* nwrite)
{
    DBT dkey, dval;
    bzero(&dkey, sizeof(dkey));
    bzero(&dval, sizeof(dval));
    dkey.data = key, dkey.size = keylen;
    dval.data = buf, dval.ulen = buflen, dval.flags = DB_DBT_USERMEM;
    int ret = db->get(db, NULL, &dkey, &dval, 0);
    if (ret == 0 && nwrite != NULL)
        *nwrite = dval.size;
    return ret;
}

Answer 1

晾了两个月，自己了结一下吧。貌似无解。

根据官方文档FAQ 第一个问题里给出的第一个链接：

if the system fails, the database environment and the databases may be left in a corrupted state. In this case, recovery must be performed on the database environment before any further action is taken, in order to:
- recover the database environment resources,
- release any locks or mutexes that may have been held to avoid starvation as the remaining threads of control convoy behind the held locks, and
- resolve any partially completed operations that may have left a database in an inconsistent or corrupted state.

也就是说，正在操作它的某个进程挂掉以后，数据库可能是处于不可用状态，必须启动一个修复进程。

For details on performing recovery, see the Recovery procedures

多线程的操作也可能导致类似的问题，但是不一定需要重启整个进程，上面的几个链接中也有具体的说明。

因此综合来看，BerkeleyDB并不适合多进程读写，最好是只有一个进程进行读写，然后对外提供接口，例如memcachedb。

感谢 xinzsky 对早期某个版本FAQ的 翻译。

Answer 2

因为BDB是嵌入式的数据库操作，多个进程使用就相当于每个进程的代码都要存在一整套BDB的操作，错误处理代码，这个对事务性操作尤其重要。