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errno is usually implemented as thread local variable, more precisely as a macro that calls a function which returns the memory address of a thread local variable. Each C runtime library has its own function and TLS which introduces an issue when a Python extension uses a different CRT than the core. AFAIK that an issue only an issue on Windows with other versions of Visual Studio.
This means that mixed CRTs break the three PyErr_SetFromErrno* functions as they always access the errno of the core's CRT. The patch adds a Py_errno macro that must be used in extensions before a PyErr_SetFromErrno() function is used.
Example:
fd = open(filename, O_RDONLY);
if (fd == -1) {
 Py_errno = errno;
 return PyErr_SetFromErrnoWithFilename(PyExc_OSError, filename);
}
The issue was discovered by me a about two weeks ago and further analyzed by Martin. http://mail.python.org/pipermail/python-dev/2012-August/121460.html