Replace custom atomicWrite by more robust library call.

[benjaminfuchs]

Issue

We are sometimes seeing following exception when we run our build with many cores:

Failed to execute script clcache_main
         Traceback (most recent call last):
           File "clcache_main.py", line 2, in <module>
           File "clcache\__main__.py", line 1596, in main
           File "clcache\__main__.py", line 1623, in processCompileRequest
           File "clcache\__main__.py", line 1675, in scheduleJobs
           File "concurrent\futures\_base.py", line 398, in result
           File "concurrent\futures\_base.py", line 357, in __get_result
           File "concurrent\futures\thread.py", line 55, in run
           File "clcache\__main__.py", line 1696, in processSingleSource
           File "clcache\__main__.py", line 1760, in processDirect
           File "clcache\__main__.py", line 1785, in ensureArtifactsExist
           File "clcache\__main__.py", line 756, in __exit__
           File "clcache\__main__.py", line 666, in save
           File "contextlib.py", line 66, in __exit__
           File "clcache\__main__.py", line 125, in atomicWrite
         PermissionError: [WinError 5] Access is denied: 'XXXX\\stats.txt.new' -> 'XXXX\\stats.txt'

Searching a bit makes it look like there is a race condition:

Anyway, the os.replace docs could state in general that: "[o]n Windows, a PermissionError will be raised if dst is a read-only file, or if either src or dst is currently open, or if src is a directory with an open file". https://bugs.python.org/issue27886

Looking with that in mind at the atomicWrite function:

@contextlib.contextmanager
def atomicWrite(fileName):
    tempFileName = fileName + '.new'
    with open(tempFileName, 'w') as f:
# One thread can have a open file handler on the tempFile thats always "stats.txt" ...
        yield f
    os.replace(tempFileName, fileName)
# ... while another thread is calling os.replace() causing PermssionError.

Solution

Replacing the atomicWrite function with atomic_write from the atomicwrites package seems to solve the issue (https://pypi.org/project/atomicwrites/). The package takes care of such race conditions so we no longer have to think about it.

[frerich]

The atomicwrites package is a great find, I didn't even know that exists!

The patch seems plausible to me, merging this. Thanks!

[izmmisha]

Hi, just checked this patch with this simple test:

import concurrent.futures
import tempfile
import os

#os.environ['TRACKER_ENABLED']="1"

from clcache.__main__ import (
    Statistics,
)

STATS_FILE = 'C:\\Users\\im\\clcache\\stats.txt' #problem
#STATS_FILE = tempfile.NamedTemporaryFile(dir='C:\\Users\\im\\clcache', suffix='.txt').name #problem
#STATS_FILE = tempfile.NamedTemporaryFile(suffix='.txt').name #no problem
#STATS_FILE = tempfile.NamedTemporaryFile(dir='C:\\Users\\im\\clcache').name #problem without txt extension too
#STATS_FILE = 'C:\\clcache\\stats.txt' #no problem

def change_stats():
    stats = Statistics(STATS_FILE)
    with stats.lock, stats as s:
        s.registerCallWithInvalidArgument()


if __name__ == '__main__':
    print(STATS_FILE)
    while True:
        with concurrent.futures.ThreadPoolExecutor(4) as executor:
            futures = [executor.submit(change_stats) for _ in range(1000)]
            for future in concurrent.futures.as_completed(futures):
                future.result()

Without this fix:

C:\Users\im\clcache\stats.txt
Traceback (most recent call last):
  File "..\awtest.py", line 29, in <module>
    future.result()
  File "C:\Python34\lib\concurrent\futures\_base.py", line 395, in result
    return self.__get_result()
  File "C:\Python34\lib\concurrent\futures\_base.py", line 354, in __get_result
    raise self._exception
  File "C:\Python34\lib\concurrent\futures\thread.py", line 54, in run
    result = self.fn(*self.args, **self.kwargs)
  File "..\awtest.py", line 20, in change_stats
    s.registerCallWithInvalidArgument()
  File "C:\Python34\lib\site-packages\clcache\__main__.py", line 795, in __exit__
    self._stats.save()
  File "C:\Python34\lib\site-packages\clcache\__main__.py", line 703, in save
    json.dump(self._dict, f, sort_keys=True, indent=4)
  File "C:\Python34\lib\contextlib.py", line 66, in __exit__
    next(self.gen)
  File "C:\Python34\lib\site-packages\clcache\__main__.py", line 162, in atomicWrite
    os.replace(tempFileName, fileName)
PermissionError: [WinError 5] Отказано в доступе: 'C:\\Users\\im\\clcache\\stats.txt.new' -> 'C:\\Users\\im\\clcache\\stats.txt'

With this fix:

C:\repository\git\clcache>\Python34\python.exe ..\awtest.py
C:\Users\im\clcache\stats.txt
Traceback (most recent call last):
  File "..\awtest.py", line 29, in <module>
    future.result()
  File "C:\Python34\lib\concurrent\futures\_base.py", line 395, in result
    return self.__get_result()
  File "C:\Python34\lib\concurrent\futures\_base.py", line 354, in __get_result
    raise self._exception
  File "C:\Python34\lib\concurrent\futures\thread.py", line 54, in run
    result = self.fn(*self.args, **self.kwargs)
  File "..\awtest.py", line 20, in change_stats
    s.registerCallWithInvalidArgument()
  File "C:\Python34\lib\site-packages\clcache\__main__.py", line 749, in __exit__
    self._stats.save()
  File "C:\Python34\lib\site-packages\clcache\__main__.py", line 659, in save
    json.dump(self._dict, f, sort_keys=True, indent=4)
  File "C:\Python34\lib\contextlib.py", line 66, in __exit__
    next(self.gen)
  File "C:\Python34\lib\site-packages\atomicwrites\__init__.py", line 152, in _open
    self.commit(f)
  File "C:\Python34\lib\site-packages\atomicwrites\__init__.py", line 177, in commit
    replace_atomic(f.name, self._path)
  File "C:\Python34\lib\site-packages\atomicwrites\__init__.py", line 89, in replace_atomic
    return _replace_atomic(src, dst)
  File "C:\Python34\lib\site-packages\atomicwrites\__init__.py", line 71, in _replace_atomic
    _windows_default_flags | _MOVEFILE_REPLACE_EXISTING
  File "C:\Python34\lib\site-packages\atomicwrites\__init__.py", line 66, in _handle_errors
    raise WinError()
PermissionError: [WinError 5] Отказано в доступе.

[benjaminfuchs]

@izmmisha Thanks for the test. I did some test runs with it. Both implementations with 200.000 calls to change_stats():

old implementation: 10 fails
new implementation: 2 fails

Did you also see such kind of frequency of fails?

I think the race condition that tempFileName was always stats.txt.new for every thread is solved.
But there is still a race condition if 2 threads try to write the new content to stats.txt.
I think this can only be solved by some kind of mutex guarding (what is aleardy done in some other parts of clcache).

[izmmisha]

It need more investigation, the problem exists only in user home directory (except temp directory), the simplest solution right now is move clcache folder out of user home directory.

[benjaminfuchs]

@izmmisha Looking at this post nodejs/node-gyp#1124 maybe the default CLCACHE_DIR location should be in %LOCALAPPDATA% on windows? I ran the test with this folder and didn't get any errors. No idea what magic happens in %USERPROFILE%.

[punitchandra]

When can this be released?