Kernel based rename undo

A more robust version below:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <signal.h>
#include <aio.h>
#include <errno.h>

/* XXX this is just a demo, no error checking */

static int sd=-1;
static int count=0;
static char filler[2]={0,0};

/* locks */
static struct flock 
  fwl={.l_type=F_WRLCK,.l_whence=SEEK_SET,.l_start=0,.l_len=1},
  ful={.l_type=F_UNLCK,.l_whence=SEEK_SET,.l_start=0,.l_len=1},
  bwl={.l_type=F_WRLCK,.l_whence=SEEK_SET,.l_start=1,.l_len=1},
  brl={.l_type=F_RDLCK,.l_whence=SEEK_SET,.l_start=1,.l_len=1},
  bul={.l_type=F_UNLCK,.l_whence=SEEK_SET,.l_start=1,.l_len=1};

static void aiodone(int signum,siginfo_t *info,void *context){
  /* signal counter down */
  count--;
}

#define BUF_SIZE 50

static void config(struct aiocb *cb){
  int fd;
  ssize_t len;
  char buf[BUF_SIZE];

  /* critical section */
  while(fcntl(sd,F_SETLKW,&fwl));

  /* don't care if it fails, any version is OK */
  link("foo","foo~");

  /* read existing file */
  fd=open("foo",O_RDONLY);
  len=read(fd,buf,BUF_SIZE);
  close(fd);

  /* write to new file */
  fd=open("foo.new",O_WRONLY|O_CREAT|O_TRUNC,S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH);
  write(fd,buf,len);

  /* AIO control block setup */
  memset(cb,0,sizeof(*cb));
  cb->aio_sigevent.sigev_notify=SIGEV_SIGNAL;
  cb->aio_sigevent.sigev_signo=SIGRTMIN;
  cb->aio_fildes=fd;

  /* signal counter up */
  count++;

  /* initiate sync and close */
  aio_fsync(O_SYNC,cb);
  close(fd);

  /* put the new file in place */
  rename("foo.new","foo");

  /* end critical section */
  while(fcntl(sd,F_SETLKW,&ful));
}

#define LOOPS 10
#define TRIES 20

int main(int argc,char **argv){
  int i;
  struct aiocb cb[LOOPS];
  struct sigaction act;

  /* setup shared file, restore */
  if((sd=shm_open("foo",O_RDWR|O_CREAT|O_EXCL,S_IRUSR|S_IWUSR))==-1){
    int tries=TRIES;
    struct stat f;

    /* not the first to arrive, open and wait for restore */
    sd=shm_open("foo",O_RDWR,S_IRUSR|S_IWUSR);

    fstat(sd,&f);
    while(tries-- && f.st_size<sizeof(filler)){
      sleep(1);
      fstat(sd,&f);
    }

    /* something's really screwed */
    if(!tries)
      return 1;
  } else{ /* first to arrive, restore */
    /* don't care if we fail */
    if(!rename("foo~","foo"))
      fprintf(stderr,"Restored.\n");

    /* setup lock file */
    write(sd,&filler,sizeof(filler));
  }

  /* signal handler setup */
  memset(&act,0,sizeof(act));
  act.sa_flags=SA_SIGINFO;
  act.sa_sigaction=aiodone;
  sigaction(SIGRTMIN,&act,NULL);

  /* we need the backup file to be there */
  while(fcntl(sd,F_SETLKW,&brl));

  /* program may run config many times */
  for(i=0;i<LOOPS;i++){
    config(&cb[i]);

    /* do something really useful here */
  }

  /* wait for AIO completion */
  while(count)
    sleep(1);

  /* unlock the backup file */
  while(fcntl(sd,F_SETLKW,&bul));

  /* try to remove backup file */
  if(!fcntl(sd,F_SETLK,&fwl)){
    if(!fcntl(sd,F_SETLK,&bwl)){
      unlink("foo~");
      while(fcntl(sd,F_SETLKW,&bul));
    }
    while(fcntl(sd,F_SETLKW,&ful));
  }

  /* clean up shared memory */
  close(sd);

  return 0;
}