[Python-Dev] Mysterious Python pyc file corruption problems

Guido van Rossum guido at python.org
Thu May 16 23:19:14 CEST 2013

This reminds me of the following bug, which can happen when two
processes are both writing the .pyc file and a third is reading it.
First some background.

When writing a .pyc file, we use the following strategy:
- open the file for writing
- write a dummy header (four null bytes)
- write the .py file's mtime
- write the marshalled code object
- replace the dummy heaer with the correct magic word

Even py_compile.py (used by compileall.py) uses this strategy.

When reading a .pyc file, we ignore it when the magic word isn't there
(or when the mtime doesn't match that of the .py file exactly), and
then we will write it back like described above.

Now consider the following scenario. It involves *three* processes.

- Two unrelated processes both start and want to import the same module.
- They both see the .pyc file is missing/corrupt and decide to write it.
- The first process finishing writing the file, writing the correct header.
- Now a third process wants to import the module, sees the valid
header, and starts reading the file.
- However, while this is going on, the second process gets ready to
write the file.
- The second process truncates the file, writes the dummy header, and
then stalls.
- At this point the third process (which thought it was reading a
valid file) sees an unexpected EOF because the file has been

Now, this would explain the EOFError, but not necessarily the
ValueError with "unknown type code". However, it looks like marshal
doesn't always check for EOF immediately (sometimes it calls getc()
without checking the result, and sometimes it doesn't check the error
state after calling r_string()), so I think all the errors are
actually explainable from this scenario.

--Guido van Rossum (python.org/~guido)

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