maximum recursion depth exceeded in 1050 string join

Original report by jayvdb (@jayvdb?) on Launchpad:

Running current pyflakes on astroid results in maximum recursion depth exceeded on a test module.

https://bitbucket.org/logilab/astroid/src/6d4e198bdc7091f36c2c24d911c5ee92b64847c2/astroid/tests/testdata/python2/data/joined_strings.py

$ pyflakes  astroid/tests/testdata/python2/data/joined_strings.py
Traceback (most recent call last):
  File "/usr/bin/pyflakes", line 9, in <module>
    load_entry_point('pyflakes==1.0.0', 'console_scripts', 'pyflakes')()
  File "/usr/lib/python2.7/site-packages/pyflakes-1.0.0-py2.7.egg/pyflakes/api.py", line 172, in main
    warnings = checkRecursive(args, reporter)
  File "/usr/lib/python2.7/site-packages/pyflakes-1.0.0-py2.7.egg/pyflakes/api.py", line 129, in checkRecursive
    warnings += checkPath(sourcePath, reporter)
  File "/usr/lib/python2.7/site-packages/pyflakes-1.0.0-py2.7.egg/pyflakes/api.py", line 96, in checkPath
    return check(codestr, filename, reporter)
  File "/usr/lib/python2.7/site-packages/pyflakes-1.0.0-py2.7.egg/pyflakes/api.py", line 57, in check
    w = checker.Checker(tree, filename)
  File "/usr/lib/python2.7/site-packages/pyflakes-1.0.0-py2.7.egg/pyflakes/checker.py", line 294, in __init__
    self.handleChildren(tree)
  File "/usr/lib/python2.7/site-packages/pyflakes-1.0.0-py2.7.egg/pyflakes/checker.py", line 567, in handleChildren
    self.handleNode(node, tree)
  File "/usr/lib/python2.7/site-packages/pyflakes-1.0.0-py2.7.egg/pyflakes/checker.py", line 609, in handleNode
    handler(node)
...
  File "/usr/lib/python2.7/site-packages/pyflakes-1.0.0-py2.7.egg/pyflakes/checker.py", line 609, in handleNode
    handler(node)
  File "/usr/lib/python2.7/site-packages/pyflakes-1.0.0-py2.7.egg/pyflakes/checker.py", line 566, in handleChildren
    for node in iter_child_nodes(tree, omit=omit):
  File "/usr/lib/python2.7/site-packages/pyflakes-1.0.0-py2.7.egg/pyflakes/checker.py", line 80, in iter_child_nodes
    if isinstance(field, ast.AST):
RuntimeError: maximum recursion depth exceeded while calling a Python object

Setting sys.setrecursionlimit(2500) fixes the problem

Reproduce the failure by running pyflakes on astroid/tests/testdata/python2/data/joined_strings.py. Start in pyflakes/checker.py, especially iter_child_nodes and handleChildren, which appear in the traceback. Done means the file no longer raises a recursion-depth error without relying on a higher recursion limit, with the relevant tests passing.