Avoid possible infinite recursion when writing pyc files in assert rewrite

What happens is that atomic_write on Python 2.7 on Windows will try
to convert the paths to unicode, but this triggers the import of
the encoding module for the file system codec, which in turn triggers
the rewrite, which in turn again tries to import the module, and so on.

This short-circuits the cases where we try to import another file when
writing a pyc file; I don't expect this to affect anything because
the only modules that could be affected are those imported by
atomic_writes.

Fix pytest-dev#3506

Author: nicoddemus

changelog/3506.bugfix.rst

@@ -0,0 +1 @@
+Fix possible infinite recursion when writing ``.pyc`` files.

src/_pytest/assertion/rewrite.py

@@ -64,6 +64,8 @@ def __init__(self, config):
         self._rewritten_names = set()
         self._register_with_pkg_resources()
         self._must_rewrite = set()
+        # flag to guard against trying to rewrite a pyc file while we are already writing another pyc file,
+        # which might result in infinite recursion (#3506)
         self._writing_pyc = False
 
     def set_session(self, session):

testing/test_assertrewrite.py

@@ -1124,3 +1124,31 @@ def test_simple_failure():
 
         result = testdir.runpytest()
         result.stdout.fnmatch_lines("*E*assert (1 + 1) == 3")
+
+
+def test_rewrite_infinite_recursion(testdir, pytestconfig, monkeypatch):
+    """Fix infinite recursion when writing pyc files: if an import happens to be triggered when writing the pyc
+    file, this would cause another call to the hook, which would trigger another pyc writing, which could
+    trigger another import, and so on. (#3506)"""
+    from _pytest.assertion import rewrite
+
+    testdir.syspathinsert()
+    testdir.makepyfile(test_foo="def test(): pass")
+
+    original_write_pyc = rewrite._write_pyc
+
+    write_pyc_called = []
+
+    def spy_write_pyc(*args, **kwargs):
+        # make a note that we have called _write_pyc
+        write_pyc_called.append(True)
+        # try to import a module at this point: we should not try to rewrite this module
+        assert hook.find_module("test_foo") is None
+        return original_write_pyc(*args, **kwargs)
+
+    monkeypatch.setattr(rewrite, "_write_pyc", spy_write_pyc)
+    monkeypatch.setattr(sys, "dont_write_bytecode", False)
+
+    hook = AssertionRewritingHook(pytestconfig)
+    assert hook.find_module("test_foo") is not None
+    assert len(write_pyc_called) == 1