使用文件锁实现进程间同步

简介

在实际应用中，会出现这种应用场景：希望shell下执行的脚本对某些竞争资源提供保护，避免出现冲突。本文将通过fcntl模块的文件整体上锁机制来实现这种进程间同步功能。

fcntl系统函数介绍

Linux系统提供了文件整体上锁(flock)和更细粒度的记录上锁(fcntl)功能，底层功能均可由fcntl函数实现。

首先来了解记录上锁。记录上锁是读写锁的一种扩展类型，它可用于有亲缘关系或无亲缘关系的进程间共享某个文件的读与写。被锁住的文件通过其描述字访问，执行上锁操作的函数是fcntl。这种类型的锁在内核中维护，其宿主标识为fcntl调用进程的进程ID。这意味着这些锁用于不同进程间的上锁，而不是同一进程内不同线程间的上锁。
fcntl记录上锁即可用于读也可用于写，对于文件的任意字节，最多只能存在一种类型的锁（读锁或写锁）。而且，一个给定字节可以有多个读写锁，但只能有一个写入锁。
对于一个打开着某个文件的给定进程来说，当它关闭该文件的任何一个描述字或者终止时，与该文件关联的所有锁都被删除。锁不能通过fork由子进程继承。

NAME       fcntl - manipulate file descriptorSYNOPSIS       #include <unistd.h>       #include <fcntl.h>       int fcntl(int fd, int cmd, ... /* arg */ );DESCRIPTION       fcntl() performs one of the operations described below on the open file descriptor fd.  The operation is determined by cmd.       fcntl() can take an optional third argument.  Whether or not this argument is required is determined by cmd.  The required argument type       is indicated in parentheses after each cmd name (in most cases, the required type is int, and we identify the argument  using  the  name       arg), or void is specified if the argument is not required.   Advisory record locking       Linux implements traditional ("process-associated") UNIX record locks, as standardized by POSIX.  For a Linux-specific alternative  with       better semantics, see the discussion of open file description locks below.       F_SETLK, F_SETLKW, and F_GETLK are used to acquire, release, and test for the existence of record locks (also known as byte-range, file-       segment, or file-region locks).  The third argument, lock, is a pointer to a structure that  has  at  least  the  following  fields  (in       unspecified order).           struct flock {               ...               short l_type;    /* Type of lock: F_RDLCK,                                   F_WRLCK, F_UNLCK */               short l_whence;  /* How to interpret l_start:                                   SEEK_SET, SEEK_CUR, SEEK_END */               off_t l_start;   /* Starting offset for lock */               off_t l_len;     /* Number of bytes to lock */               pid_t l_pid;     /* PID of process blocking our lock                                   (set by F_GETLK and F_OFD_GETLK) */               ...           };

其次，文件上锁源自Berkeley的Unix实现支持给整个文件上锁或解锁的文件上锁(file locking)，但没有给文件内的字节范围上锁或解锁的能力。

fcntl模块及基于文件锁的同步功能。

Python fcntl模块提供了基于文件描述符的文件和I/O控制功能。它是Unix系统调用fcntl()和ioctl()的接口。因此，我们可以基于文件锁来提供进程同步的功能。

import fcntlclass Lock(object):    def __init__(self, file_name):        self.file_name = file_name        self.handle = open(file_name, 'w')    def lock(self):        fcntl.flock(self.handle, fcntl.LOCK_EX)    def unlock(self):        fcntl.flock(self.handle, fcntl.LOCK_UN)    def __del__(self):        try:            self.handle.close()        except:            pass   

应用

我们做一个简单的场景应用：需要从指定的服务器上下载软件版本到/exports/images目录下，因为这个脚本可以在多用户环境执行。我们不希望下载出现冲突，并仅在该目录下保留一份指定的软件版本。下面是基于文件锁的参考实现：

if __name__ == "__main__":    parser = OptionParser()    group = OptionGroup(parser, "FTP download tool", "Download build from ftp server")    group.add_option("--server", type="string", help="FTP server's IP address")    group.add_option("--username", type="string", help="User name")    group.add_option("--password", type="string", help="User's password")    group.add_option("--buildpath", type="string", help="Build path in the ftp server")    group.add_option("--buildname", type="string", help="Build name to be downloaded")    parser.add_option_group(group)    (options, args) = parser.parse_args()    local_dir = "/exports/images"    lock_file = "/var/tmp/flock.txt"    flock = Lock(lock_file)    flock.lock()        if os.path.isfile(os.path.join(local_dir, options.buildname)):        log.info("build exists, nothing needs to be done")        log.info("Download completed")        flock.unlock()        exit(0)        log.info("start to download build " + options.buildname)    t = paramiko.Transport((options.server, 22))    t.connect(username=options.username, password=options.password)    sftp = paramiko.SFTPClient.from_transport(t)    sftp.get(os.path.join(options.buildpath, options.buildname),              os.path.join(local_dir, options.buildname))    sftp.close()    t.close()    log.info("Download completed")    flock.unlock()