gh-116767: fix crash on 'async with' with many context managers (GH-118348)

Account for `add_stopiteration_handler` pushing a block for `async with`.
To allow generator functions that previously almost hit the `CO_MAXBLOCKS`
limit by nesting non-async blocks, the limit is increased by 1.
This increase allows one more block in non-generator functions.

Author: iritkatriel

Include/cpython/code.h

@@ -226,7 +226,7 @@ struct PyCodeObject _PyCode_DEF(1);
 */
 #define PY_PARSER_REQUIRES_FUTURE_KEYWORD
 
-#define CO_MAXBLOCKS 20 /* Max static block nesting within a function */
+#define CO_MAXBLOCKS 21 /* Max static block nesting within a function */
 
 PyAPI_DATA(PyTypeObject) PyCode_Type;
 

Lib/test/test_syntax.py

@@ -2392,13 +2392,40 @@ def bug():
             code += "): yield a"
             return code
 
-        CO_MAXBLOCKS = 20  # static nesting limit of the compiler
+        CO_MAXBLOCKS = 21  # static nesting limit of the compiler
+        MAX_MANAGERS = CO_MAXBLOCKS - 1  # One for the StopIteration block
 
-        for n in range(CO_MAXBLOCKS):
+        for n in range(MAX_MANAGERS):
             with self.subTest(f"within range: {n=}"):
                 compile(get_code(n), "<string>", "exec")
 
-        for n in range(CO_MAXBLOCKS, CO_MAXBLOCKS + 5):
+        for n in range(MAX_MANAGERS, MAX_MANAGERS + 5):
+            with self.subTest(f"out of range: {n=}"):
+                self._check_error(get_code(n), "too many statically nested blocks")
+
+    @support.cpython_only
+    def test_async_with_statement_many_context_managers(self):
+        # See gh-116767
+
+        def get_code(n):
+            code = [ textwrap.dedent("""
+                async def bug():
+                    async with (
+                    a
+                """) ]
+            for i in range(n):
+                code.append(f"    as a{i}, a\n")
+            code.append("): yield a")
+            return "".join(code)
+
+        CO_MAXBLOCKS = 21  # static nesting limit of the compiler
+        MAX_MANAGERS = CO_MAXBLOCKS - 1  # One for the StopIteration block
+
+        for n in range(MAX_MANAGERS):
+            with self.subTest(f"within range: {n=}"):
+                compile(get_code(n), "<string>", "exec")
+
+        for n in range(MAX_MANAGERS, MAX_MANAGERS + 5):
             with self.subTest(f"out of range: {n=}"):
                 self._check_error(get_code(n), "too many statically nested blocks")
 
@@ -2536,7 +2563,8 @@ def test_syntax_error_on_deeply_nested_blocks(self):
                   while 20:
                    while 21:
                     while 22:
-                     break
+                     while 23:
+                      break
 """
         self._check_error(source, "too many statically nested blocks")
 

Misc/NEWS.d/next/Core and Builtins/2024-04-27-16-23-29.gh-issue-116767.z9UFpr.rst

@@ -0,0 +1 @@
+Fix crash in compiler on 'async with' that has many context managers.

Python/compile.c

@@ -113,7 +113,8 @@ compiler IR.
 
 enum fblocktype { WHILE_LOOP, FOR_LOOP, TRY_EXCEPT, FINALLY_TRY, FINALLY_END,
                   WITH, ASYNC_WITH, HANDLER_CLEANUP, POP_VALUE, EXCEPTION_HANDLER,
-                  EXCEPTION_GROUP_HANDLER, ASYNC_COMPREHENSION_GENERATOR };
+                  EXCEPTION_GROUP_HANDLER, ASYNC_COMPREHENSION_GENERATOR,
+                  STOP_ITERATION };
 
 struct fblockinfo {
     enum fblocktype fb_type;
@@ -1503,6 +1504,7 @@ compiler_unwind_fblock(struct compiler *c, location *ploc,
         case EXCEPTION_HANDLER:
         case EXCEPTION_GROUP_HANDLER:
         case ASYNC_COMPREHENSION_GENERATOR:
+        case STOP_ITERATION:
             return SUCCESS;
 
         case FOR_LOOP:
@@ -2232,14 +2234,26 @@ compiler_function_body(struct compiler *c, stmt_ty s, int is_async, Py_ssize_t f
     c->u->u_metadata.u_argcount = asdl_seq_LEN(args->args);
     c->u->u_metadata.u_posonlyargcount = asdl_seq_LEN(args->posonlyargs);
     c->u->u_metadata.u_kwonlyargcount = asdl_seq_LEN(args->kwonlyargs);
+
+    NEW_JUMP_TARGET_LABEL(c, start);
+    USE_LABEL(c, start);
+    bool add_stopiteration_handler = c->u->u_ste->ste_coroutine || c->u->u_ste->ste_generator;
+    if (add_stopiteration_handler) {
+        /* wrap_in_stopiteration_handler will push a block, so we need to account for that */
+        RETURN_IF_ERROR(
+            compiler_push_fblock(c, NO_LOCATION, STOP_ITERATION,
+                                 start, NO_LABEL, NULL));
+    }
+
     for (Py_ssize_t i = first_instr; i < asdl_seq_LEN(body); i++) {
         VISIT_IN_SCOPE(c, stmt, (stmt_ty)asdl_seq_GET(body, i));
     }
-    if (c->u->u_ste->ste_coroutine || c->u->u_ste->ste_generator) {
+    if (add_stopiteration_handler) {
         if (wrap_in_stopiteration_handler(c) < 0) {
             compiler_exit_scope(c);
             return ERROR;
         }
+        compiler_pop_fblock(c, STOP_ITERATION, start);
     }
     PyCodeObject *co = optimize_and_assemble(c, 1);
     compiler_exit_scope(c);