[New-bugs-announce] [issue20057] wrong behavior with fork and mmap

[Brett Tiplitz]

New submission from Brett Tiplitz:

When running the example mmap library (with a slight modification, plus I did not handle all the changes for the 3.3 string handling as the example posted does not work with 3.x) 

When looking at the subprocess, the spawned process will have all the mmap'd file descriptors open.  The spawned process has the responsibility of closing any FD's that are in use.  However, since the shared memory segment get's closed and the program has no knowledge of private FD's, the mmap's private FD becomes a leak in the FD table.  It seems python should set the close-on-exec attribute on the dup'd FD that it maintains.  Examples of fixing this issue are found on http://stackoverflow.com/questions/1643304/how-to-set-close-on-exec-by-default
import mmap,os

# write a simple example file
with open("hello.txt", "wb") as f:
    f.write(bytes("Hello Python!\n", 'UTF-8'))

with open("hello.txt", "r+b") as f:
    # memory-map the file, size 0 means whole file
    os.system("/bin/ls -l /proc/"+str(os.getpid())+"/fd")

    mm = mmap.mmap(f.fileno(), 0)
    os.system("/bin/ls -l /proc/"+str(os.getpid())+"/fd")
    os.system("/bin/ls -l /proc/self/fd")

    # read content via standard file methods
    t1 = mm.readline() # used to print out
  # prints "Hello Python!"
    # read content via slice notation
    t2=mm[:5]
#    print mm[:5]  # prints "Hello"
    # update content using slice notation;
    # note that new content must have same size
    mm[6:] = bytes(" world!\n", 'UTF-8')
    # ... and read again using standard file methods
    mm.seek(0)
    t3=mm.readline()
 #   print mm.readline()  # prints "Hello  world!"
    # close the map
    mm.close()
~

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components: Library (Lib)
messages: 206871
nosy: btiplitz
priority: normal
severity: normal
status: open
title: wrong behavior with fork and mmap
type: behavior
versions: Python 2.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue20057>
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