[Python-3000] uuid creation not thread-safe? (original) (raw)

lcaamano lcaamano at gmail.com
Sun Jul 29 03:04:29 CEST 2007

On Jul 20, 1:52 pm, "Guido van Rossum" <gu... at python.org> wrote:

I discovered what appears to be a thread-unsafety inuuid.py. This is in the trunk as well as in 3.x; I'm using the trunk here for easy reference. There's some code around like 395:

import ctypes, ctypes.util buffer = ctypes.createstringbuffer(16) This creates a global buffer which is used as the output parameter to later calls to uuidgeneraterandom() and uuidgeneratetime(). For example, around line 481, in uuid1(): uuidgeneratetime(buffer) returnUUID(bytes=buffer.raw) Clearly if two threads do this simultaneously they are overwriting buffer in unpredictable order. There are a few other occurrences of this too. I find it somewhat disturbing that what seems a fairly innocent function that doesn't appear to have global state is nevertheless not thread-safe. Would it be wise to fix this, e.g. by allocating a fresh output buffer inside uuid1() and other callers?

I didn't find any reply to this, which is odd, so forgive me if it's old news.

I agree with you that it's not thread safe and that a local buffer in the stack should fix it.

Just for reference, the thread-safe uuid extension we've been using since python 2.1, which I don't recall where we borrow it from, uses a local buffer in the stack. It looks like this:

-----begin uuid.c--------------

static char uuid__doc__ [] = "DCE compatible Universally Unique Identifier module";

#include "Python.h" #include <uuid/uuid.h>

static char uuidgen__doc__ [] = "Create a new DCE compatible UUID value";

static PyObject * uuidgen(void) { uuid_t out; char buf[48];

uuid_generate(out);
uuid_unparse(out, buf);
return PyString_FromString(buf);

}

static PyMethodDef uuid_methods[] = { {"uuidgen", uuidgen, 0, uuidgen__doc__}, {NULL, NULL} /* Sentinel */ };

DL_EXPORT(void) inituuid(void) { Py_InitModule4("uuid", uuid_methods, uuid__doc__, (PyObject *)NULL, PYTHON_API_VERSION); }

-----end uuid.c--------------

It also seems that using uuid_generate()/uuid_unparse() should be faster than using uuid_generate_random() and then creating a python object to call its str method. If so, it would be nice if the uuid.py module also provided equivalent fast versions that returned strings instead of objects.

-- Luis P Caamano Atlanta, GA, USA

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