PyOS_StdioReadline() contains code such as:

   p = (char *)PyMem_REALLOC(p, n + incr);

The code looses its pointer to p when realloc fails and has no chance to free the memory in p.

Also the code sets PyExc_OverflowError when incr > INT_MAX but it doesn't return NULL to signal the error.

A similar issue: #14909.

Is it sufficient to check incr > INT_MAX to guard against overflow?

The patch can be a little simplified (the "else" keyword is redundant), but in general it LGTM. Let's push.

New changeset 5859a3ec5b7e by Christian Heimes in branch '3.3':
Issue #18368: PyOS_StdioReadline() no longer leaks memory when realloc() fails.
http://hg.python.org/cpython/rev/5859a3ec5b7e

New changeset 6dbc4d6ff31e by Christian Heimes in branch 'default':
Issue #18368: PyOS_StdioReadline() no longer leaks memory when realloc() fails.
http://hg.python.org/cpython/rev/6dbc4d6ff31e

Serhiy:
Thanks for the review

Kristján:
Yes, it's enough to check for incr > INT_MAX. The buffer size is incremented to a value of <= (2*n)+2 in each round. The start value is 100. The loop is terminated once the buffer size reaches INT_MAX-2.