Fix partial circular imports (GH-4392)

Follows the implementation in CPython, which does a lookup in sys.modules if the attribute lookup on a package fails.

Closes https://github.com/cython/cython/issues/4390

Cython/Utility/ImportExport.c

@@ -238,6 +238,39 @@ static PyObject* __Pyx_ImportFrom(PyObject* module, PyObject* name); /*proto*/
 static PyObject* __Pyx_ImportFrom(PyObject* module, PyObject* name) {
     PyObject* value = __Pyx_PyObject_GetAttrStr(module, name);
     if (unlikely(!value) && PyErr_ExceptionMatches(PyExc_AttributeError)) {
+        // 'name' may refer to a (sub-)module which has not finished initialization
+        // yet, and may not be assigned as an attribute to its parent, so try
+        // finding it by full name.
+        const char* module_name_str = 0;
+        PyObject* module_name = 0;
+        PyObject* module_dot = 0;
+        PyObject* full_name = 0;
+        PyErr_Clear();
+        module_name_str = PyModule_GetName(module);
+        if (unlikely(!module_name_str)) { goto modbad; }
+        module_name = PyUnicode_FromString(module_name_str);
+        if (unlikely(!module_name)) { goto modbad; }
+        module_dot = PyUnicode_Concat(module_name, PYUNICODE("."));
+        if (unlikely(!module_dot)) { goto modbad; }
+        full_name = PyUnicode_Concat(module_dot, name);
+        if (unlikely(!full_name)) { goto modbad; }
+        #if PY_VERSION_HEX < 0x030700A1 || (CYTHON_COMPILING_IN_PYPY && PYPY_VERSION_NUM  < 0x07030400)
+        {
+            PyObject *modules = PyImport_GetModuleDict();
+            if (unlikely(!modules))
+                goto modbad;
+            value = PyObject_GetItem(modules, full_name);
+        }
+        #else
+        value = PyImport_GetModule(full_name);
+        #endif
+
+      modbad:
+        Py_XDECREF(full_name);
+        Py_XDECREF(module_dot);
+        Py_XDECREF(module_name);
+    }
+    if (unlikely(!value)) {
         PyErr_Format(PyExc_ImportError,
         #if PY_MAJOR_VERSION < 3
             "cannot import name %.230s", PyString_AS_STRING(name));

tests/pypy2_bugs.txt

@@ -10,6 +10,7 @@ run.cdef_multiple_inheritance_nodict
 run.extstarargs
 run.cpython_capi
 run.isnot
+run.partial_circular_import
 
 # pypy 2 seems to be preferring .py files to .so files
 # https://foss.heptapod.net/pypy/pypy/issues/3185

tests/run/partial_circular_import.srctree

+PYTHON -c 'if __import__("sys").version_info >= (3,7): import pkg.A'
+PYTHON setup.py build_ext --inplace
+PYTHON -c "import pkg.A"
+
+######## setup.py ########
+
+from Cython.Build.Dependencies import cythonize
+from distutils.core import setup
+
+setup(
+    ext_modules = cythonize("*/*.py"),
+    )
+
+######## pkg/__init__.py ########
+
+######## pkg/A.py ########
+from . import B
+
+def verify(rel_B):
+    import pkg.B as abs_B
+    assert abs_B == rel_B
+verify(B)
+
+######## pkg/B.py ########
+from . import C
+
+def verify(rel_C):
+    import pkg.C as abs_C
+    assert abs_C == rel_C
+verify(C)
+
+######## pkg/C.py ########
+from . import B
+
+def verify(rel_B):
+    import pkg.B as abs_B
+    assert abs_B == rel_B
+
+verify(B)
+