python - Check if a file is not open nor being used by another process

Question

I my application, i have below requests: 1. There has one thread will regularly record some logs in file. The log file will be rollovered in certain interval. for keeping the log files small. 2. There has another thread also will regularly to process these log files. ex: Move the log files to other place, parse the log's content to generate some log reports.

But, there has a condition is the second thread can not process the log file that's using to record the log. in code side, the pseudocode similars like below:

#code in second thread to process the log files
for logFile in os.listdir(logFolder):
     if not file_is_open(logFile) or file_is_use(logFile):
          ProcessLogFile(logFile) # move log file to other place, and generate log report....

So, how do i check is a file is already open or is used by other process? I did some research in internet. And have some results:

try:
   myfile = open(filename, "r+") # or "a+", whatever you need
except IOError:
    print "Could not open file! Please close Excel!"

I tried this code, but it doesn't work, no matter i use "r+" or "a+" flag

try:
   os.remove(filename) # try to remove it directly
except OSError as e:
    if e.errno == errno.ENOENT: # file doesn't exist
        break

This code can work, but it can not reach my request, since i don't want to delete the file to check if it is open.

Answer 1

An issue with trying to find out if a file is being used by another process is the possibility of a race condition. You could check a file, decide that it is not in use, then just before you open it another process (or thread) leaps in and grabs it (or even deletes it).

Ok, let's say you decide to live with that possibility and hope it does not occur. To check files in use by other processes is operating system dependant.

On Linux it is fairly easy, just iterate through the PIDs in /proc. Here is a generator that iterates over files in use for a specific PID:

def iterate_fds(pid):
    dir = '/proc/'+str(pid)+'/fd'
    if not os.access(dir,os.R_OK|os.X_OK): return

    for fds in os.listdir(dir):
        for fd in fds:
            full_name = os.path.join(dir, fd)
            try:
                file = os.readlink(full_name)
                if file == '/dev/null' or 
                  re.match(r'pipe:[d+]',file) or 
                  re.match(r'socket:[d+]',file):
                    file = None
            except OSError as err:
                if err.errno == 2:     
                    file = None
                else:
                    raise(err)

            yield (fd,file)

On Windows it is not quite so straightforward, the APIs are not published. There is a sysinternals tool (handle.exe) that can be used, but I recommend the PyPi module psutil, which is portable (i.e., it runs on Linux as well, and probably on other OS):

import psutil

for proc in psutil.process_iter():
    try:
        # this returns the list of opened files by the current process
        flist = proc.open_files()
        if flist:
            print(proc.pid,proc.name)
            for nt in flist:
                print("",nt.path)

    # This catches a race condition where a process ends
    # before we can examine its files    
    except psutil.NoSuchProcess as err:
        print("****",err)