[3.9] bpo-39934: Account for control blocks in 'except' in compiler. (GH-22395) (GH-23303)

* bpo-39934: backport PR 22395 to 3.9

Author: iritkatriel

Lib/test/test_syntax.py

@@ -946,6 +946,15 @@ def test_empty_line_after_linecont(self):
  except SyntaxError:
  self.fail("Empty line after a line continuation character is valid.")
 
+ @support.cpython_only
+ def test_nested_named_except_blocks(self):
+ code = ""
+ for i in range(12):
+ code += f"{' '*i}try:\n"
+ code += f"{' '*(i+1)}raise Exception\n"
+ code += f"{' '*i}except Exception as e:\n"
+ code += f"{' '*4*12}pass"
+ self._check_error(code, "too many statically nested blocks")
 
  def test_barry_as_flufl_with_syntax_errors(self):
  # The "barry_as_flufl" rule can produce some "bugs-at-a-distance" if

Misc/NEWS.d/next/Core and Builtins/2020-09-24-12-15-45.bpo-39934.YVHTCF.rst

@@ -0,0 +1,3 @@
+Correctly count control blocks in 'except' in compiler. Ensures that a
+syntax error, rather a fatal error, occurs for deeply nested, named
+exception handlers.

Python/compile.c

@@ -84,8 +84,8 @@ It's called a frame block to distinguish it from a basic block in the
 compiler IR.
 */
 
-enum fblocktype { WHILE_LOOP, FOR_LOOP, EXCEPT, FINALLY_TRY, FINALLY_END,
- WITH, ASYNC_WITH, HANDLER_CLEANUP, POP_VALUE };
+enum fblocktype { WHILE_LOOP, FOR_LOOP, TRY_EXCEPT, FINALLY_TRY, FINALLY_END,
+ WITH, ASYNC_WITH, HANDLER_CLEANUP, POP_VALUE, EXCEPTION_HANDLER };
 
 struct fblockinfo {
  enum fblocktype fb_type;
@@ -1624,9 +1624,7 @@ compiler_push_fblock(struct compiler *c, enum fblocktype t, basicblock *b,
 {
  struct fblockinfo *f;
  if (c->u->u_nfblocks >= CO_MAXBLOCKS) {
- PyErr_SetString(PyExc_SyntaxError,
- "too many statically nested blocks");
- return 0;
+ return compiler_error(c, "too many statically nested blocks");
  }
  f = &c->u->u_fblock[c->u->u_nfblocks++];
  f->fb_type = t;
@@ -1666,6 +1664,7 @@ compiler_unwind_fblock(struct compiler *c, struct fblockinfo *info,
 {
  switch (info->fb_type) {
  case WHILE_LOOP:
+ case EXCEPTION_HANDLER:
  return 1;
 
  case FOR_LOOP:
@@ -1676,7 +1675,7 @@ compiler_unwind_fblock(struct compiler *c, struct fblockinfo *info,
  ADDOP(c, POP_TOP);
  return 1;
 
- case EXCEPT:
+ case TRY_EXCEPT:
  ADDOP(c, POP_BLOCK);
  return 1;
 
@@ -3064,14 +3063,17 @@ compiler_try_except(struct compiler *c, stmt_ty s)
  return 0;
  ADDOP_JREL(c, SETUP_FINALLY, except);
  compiler_use_next_block(c, body);
- if (!compiler_push_fblock(c, EXCEPT, body, NULL, NULL))
+ if (!compiler_push_fblock(c, TRY_EXCEPT, body, NULL, NULL))
  return 0;
  VISIT_SEQ(c, stmt, s->v.Try.body);
  ADDOP(c, POP_BLOCK);
- compiler_pop_fblock(c, EXCEPT, body);
+ compiler_pop_fblock(c, TRY_EXCEPT, body);
  ADDOP_JREL(c, JUMP_FORWARD, orelse);
  n = asdl_seq_LEN(s->v.Try.handlers);
  compiler_use_next_block(c, except);
+ /* Runtime will push a block here, so we need to account for that */
+ if (!compiler_push_fblock(c, EXCEPTION_HANDLER, NULL, NULL, NULL))
+ return 0;
  for (i = 0; i < n; i++) {
  excepthandler_ty handler = (excepthandler_ty)asdl_seq_GET(
  s->v.Try.handlers, i);
@@ -3156,6 +3158,7 @@ compiler_try_except(struct compiler *c, stmt_ty s)
  }
  compiler_use_next_block(c, except);
  }
+ compiler_pop_fblock(c, EXCEPTION_HANDLER, NULL);
  ADDOP(c, RERAISE);
  compiler_use_next_block(c, orelse);
  VISIT_SEQ(c, stmt, s->v.Try.orelse);