replace the log file on the fly

changelog.py

#-*-encoding=utf-8-*-
import gdb 
import os
import pdb
import os.path

S_IRUSR = 0400
S_IWUSR = 0200
S_IXUSR = 0100

S_IRGRP = S_IRUSR >> 3
S_IWGRP = S_IWUSR >> 3
S_IXGRP = S_IXUSR >> 3

S_IROTH = S_IRGRP >> 3
S_IWOTH = S_IWGRP >> 3
S_IXOTH = S_IXGRP >> 3

O_RDONLY = 00
O_WRONLY = 01
O_RDWR = 02

O_CREAT = 0100
O_EXCL = 0200 
O_NOCTTY = 0400
O_TRUNC = 01000
O_APPEND = 02000
O_NONBLOCK = 04000
O_SYNC = 04010000
O_ASYNC = 020000
O_LARGEFILE = 0100000
O_DIRECTORY = 0200000
O_NOFOLLOW = 0400000
O_CLOEXEC = 02000000
O_DIRECT = 040000
O_NOATIME = 01000000
O_DSYNC = 010000
O_TMPFILE = 020200000


def find_log_fd(pid, log):
    fd_dir = "/proc/%d/fd" % pid
    for i in os.listdir(fd_dir):
        if os.readlink(os.path.join(fd_dir, i)) == log:
            return int(i)
    raise IOError("unable to find the log fd") 


def replace_log(info):
    gdb.execute("attach %d" % info["pid"], to_string=True)
    fd = find_log_fd(info["pid"], info["log"])
    func_open = 'call open("%s", %d, %d)' % (info["newlog"], info["flags"], info["mode"])    
    fdstr = gdb.execute(func_open, to_string=True)
    newfd = int(fdstr.split("=")[1].strip())
    func_dup2 = "call dup2(%d, %d)" % (newfd, fd) 
    gdb.execute(func_dup2, to_string=True)
    gdb.execute("call close(%d)" % (newfd), to_string=True)
    gdb.execute("detach")

"""

def test(pid):
    replace_log({
        "pid": pid,
        "log": "/data/project/py/github/gist/log",
        "newlog": "/data/project/py/github/gist/newlog",
        "flags": O_CREAT | O_SYNC | O_RDWR |O_TRUNC,
        "mode": S_IWUSR|S_IRUSR 
        })

test(15508)
"""

test.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/types.h>


char *msg[] = {"test\n"};

int main(int argc, char **argv)
{
	int fd;
	fd = open("log", O_CREAT|O_TRUNC|O_SYNC|O_RDWR, S_IWUSR|S_IRUSR);
	if (fd < 0) {
		printf("open log failed\n");
		exit(0);
	}
	printf("pid: %d\n",  getpid());
	while(1) {
		sleep(2); 
		write(fd, *msg, strlen(*msg)); 
	} 
}