Support the 'assert' statement in nogil sections, as long as it has a plain C...

Support the 'assert' statement in nogil sections, as long as it has a plain C condition and constant message (string) value.

Cython/Compiler/Nodes.py

@@ -6469,6 +6469,7 @@ class ReraiseStatNode(StatNode):
                 UtilityCode.load_cached("ReRaiseException", "Exceptions.c"))
             code.putln("__Pyx_ReraiseException(); %s" % code.error_goto(self.pos))
 
+
 class AssertStatNode(StatNode):
     #  assert statement
     #
@@ -6490,9 +6491,6 @@ class AssertStatNode(StatNode):
                 self.value = value.coerce_to_pyobject(env)
         return self
 
-    nogil_check = Node.gil_error
-    gil_message = "Raising exception"
-
     def generate_execution_code(self, code):
         code.putln("#ifndef CYTHON_WITHOUT_ASSERTIONS")
         code.putln("if (unlikely(!Py_OptimizeFlag)) {")
@@ -6500,6 +6498,11 @@ class AssertStatNode(StatNode):
         self.cond.generate_evaluation_code(code)
         code.putln(
             "if (unlikely(!%s)) {" % self.cond.result())
+        in_nogil = not code.funcstate.gil_owned
+        if in_nogil:
+            # Apparently, evaluating condition and value does not require the GIL,
+            # but raising the exception now does.
+            code.put_ensure_gil()
         if self.value:
             self.value.generate_evaluation_code(code)
             code.putln(
@@ -6509,6 +6512,8 @@ class AssertStatNode(StatNode):
         else:
             code.putln(
                 "PyErr_SetNone(PyExc_AssertionError);")
+        if in_nogil:
+            code.put_release_ensured_gil()
         code.putln(
             code.error_goto(self.pos))
         code.putln(

docs/src/userguide/external_C_code.rst

@@ -549,14 +549,18 @@ You can release the GIL around a section of code using the
     with nogil:
         <code to be executed with the GIL released>
 
-Code in the body of the with-statement must not raise exceptions or
-manipulate Python objects in any way, and must not call anything that
-manipulates Python objects without first re-acquiring the GIL.  Cython
-validates these operations at compile time, but cannot look into
-external C functions, for example.  They must be correctly declared
-as requiring or not requiring the GIL (see below) in order to make
+Code in the body of the with-statement must not manipulate Python objects
+in any way, and must not call anything that manipulates Python objects without
+first re-acquiring the GIL.  Cython validates these operations at compile time,
+but cannot look into external C functions, for example.  They must be correctly
+declared as requiring or not requiring the GIL (see below) in order to make
 Cython's checks effective.
 
+Since Cython 3.0, some simple Python statements can be used inside of ``nogil``
+sections: ``raise``, ``assert`` and ``print`` (the Py2 statement, not the function).
+Since they tend to be lone Python statements, Cython will automatically acquire
+and release the GIL around them for convenience.
+
 .. _gil:
 
 Acquiring the GIL

tests/errors/e_assert.pyx

+# mode: error
+# tag: assert
+
+def nontrivial_assert_in_nogil(int a, obj):
+    with nogil:
+        # NOK
+        assert obj
+        assert a*obj
+        assert a, f"123{a}xyz"
+
+        # OK
+        assert a
+        assert a*a
+        assert a, "abc"
+        assert a, u"abc"
+
+
+_ERRORS = """
+7:15: Truth-testing Python object not allowed without gil
+8:15: Converting to Python object not allowed without gil
+8:16: Operation not allowed without gil
+8:16: Truth-testing Python object not allowed without gil
+9:18: String concatenation not allowed without gil
+9:18: String formatting not allowed without gil
+"""

tests/run/with_gil_automatic.pyx

@@ -5,6 +5,8 @@
 cimport cython
 
 
+#### print
+
 @cython.test_assert_path_exists(
     "//GILStatNode",
     "//GILStatNode//GILStatNode",
@@ -23,6 +25,9 @@ def test_print_in_nogil_section(x):
     "//GILStatNode",
     "//GILStatNode//PrintStatNode",
 )
+@cython.test_fail_if_path_exists(
+    "//GILStatNode//GILStatNode",
+)
 cpdef int test_print_in_nogil_func(x) nogil except -1:
     """
     >>> _ = test_print_in_nogil_func(123)
@@ -31,6 +36,8 @@ cpdef int test_print_in_nogil_func(x) nogil except -1:
     print f"--{x}--"
 
 
+#### raise
+
 @cython.test_assert_path_exists(
     "//GILStatNode",
     "//GILStatNode//GILStatNode",
@@ -51,11 +58,82 @@ def test_raise_in_nogil_section(x):
     "//GILStatNode",
     "//GILStatNode//RaiseStatNode",
 )
+@cython.test_fail_if_path_exists(
+    "//GILStatNode//GILStatNode",
+)
 cpdef int test_raise_in_nogil_func(x) nogil except -1:
     """
-    >>> try: test_raise_in_nogil_func(123)
-    ... except ValueError as exc: print(exc)
-    ... else: print("NOT RAISED !")
-    --123--
+    >>> test_raise_in_nogil_func(123)
+    Traceback (most recent call last):
+    ValueError: --123--
     """
     raise ValueError(f"--{x}--")
+
+
+#### assert
+
+@cython.test_assert_path_exists(
+    "//GILStatNode",
+    "//GILStatNode//AssertStatNode",
+)
+@cython.test_fail_if_path_exists(
+    "//GILStatNode//GILStatNode",
+)
+def assert_in_nogil_section(int x):
+    """
+    >>> assert_in_nogil_section(123)
+    >>> assert_in_nogil_section(0)
+    Traceback (most recent call last):
+    AssertionError
+    """
+    with nogil:
+        assert x
+
+
+@cython.test_assert_path_exists(
+    "//GILStatNode",
+    "//GILStatNode//AssertStatNode",
+)
+@cython.test_fail_if_path_exists(
+    "//GILStatNode//GILStatNode",
+)
+def assert_in_nogil_section_ustring(int x):
+    """
+    >>> assert_in_nogil_section_string(123)
+    >>> assert_in_nogil_section_string(0)
+    Traceback (most recent call last):
+    AssertionError: failed!
+    """
+    with nogil:
+        assert x, u"failed!"
+
+
+@cython.test_assert_path_exists(
+    "//GILStatNode",
+    "//GILStatNode//AssertStatNode",
+)
+@cython.test_fail_if_path_exists(
+    "//GILStatNode//GILStatNode",
+)
+def assert_in_nogil_section_string(int x):
+    """
+    >>> assert_in_nogil_section_string(123)
+    >>> assert_in_nogil_section_string(0)
+    Traceback (most recent call last):
+    AssertionError: failed!
+    """
+    with nogil:
+        assert x, "failed!"
+
+
+@cython.test_fail_if_path_exists(
+    "//GILStatNode",
+)
+cpdef int assert_in_nogil_func(int x) nogil except -1:
+    """
+    >>> _ = assert_in_nogil_func(123)
+    >>> assert_in_nogil_func(0)
+    Traceback (most recent call last):
+    AssertionError: failed!
+    """
+    assert x, "failed!"