Avoid possible infinite recursion when writing pyc files in assert rewrite

changelog/3506.bugfix.rst

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

src/_pytest/assertion/rewrite.py

@@ -64,11 +64,16 @@ 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):
         self.session = session
 
     def find_module(self, name, path=None):
+        if self._writing_pyc:
+            return None
         state = self.config._assertstate
         state.trace("find_module called for: %s" % name)
         names = name.rsplit(".", 1)
@@ -151,7 +156,11 @@ def find_module(self, name, path=None):
                 # Probably a SyntaxError in the test.
                 return None
             if write:
-                _write_pyc(state, co, source_stat, pyc)
+                self._writing_pyc = True
+                try:
+                    _write_pyc(state, co, source_stat, pyc)
+                finally:
+                    self._writing_pyc = False
         else:
             state.trace("found cached rewritten pyc for %r" % (fn,))
         self.modules[name] = co, pyc

testing/test_assertrewrite.py

@@ -1124,3 +1124,32 @@ 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_foo(): pass")
+    testdir.makepyfile(test_bar="def test_bar(): 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_bar") 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