bpo-37947: Avoid double-decrement in symtable recursion counting (GH-15593)

With `symtable_visit_expr` now correctly adjusting the recursion depth for named
expressions, `symtable_handle_namedexpr` should be leaving it alone.
Also adds a new check to `PySymtable_BuildObject` that raises `SystemError`
if a successful first symbol analysis pass fails to keep the stack depth
accounting clean.
(cherry picked from commit 0614523)
Co-authored-by: Nick Coghlan <ncoghlan@gmail.com>

Author: miss-islington

Python/symtable.c

@@ -266,6 +266,7 @@ PySymtable_BuildObject(mod_ty mod, PyObject *filename, PyFutureFeatures *future)
 inti;
 PyThreadState*tstate;
 intrecursion_limit=Py_GetRecursionLimit();
+intstarting_recursion_depth;
 
 if (st==NULL)
 returnNULL;
@@ -284,8 +285,9 @@ PySymtable_BuildObject(mod_ty mod, PyObject *filename, PyFutureFeatures *future)
 returnNULL;
  }
 /* Be careful here to prevent overflow. */
-st->recursion_depth= (tstate->recursion_depth<INT_MAX / COMPILER_STACK_FRAME_SCALE) ?
+starting_recursion_depth= (tstate->recursion_depth<INT_MAX / COMPILER_STACK_FRAME_SCALE) ?
 tstate->recursion_depth*COMPILER_STACK_FRAME_SCALE : tstate->recursion_depth;
+st->recursion_depth=starting_recursion_depth;
 st->recursion_limit= (recursion_limit<INT_MAX / COMPILER_STACK_FRAME_SCALE) ?
 recursion_limit*COMPILER_STACK_FRAME_SCALE : recursion_limit;
 
@@ -329,6 +331,14 @@ PySymtable_BuildObject(mod_ty mod, PyObject *filename, PyFutureFeatures *future)
 PySymtable_Free(st);
 returnNULL;
  }
+/* Check that the recursion depth counting balanced correctly */
+if (st->recursion_depth!=starting_recursion_depth) {
+PyErr_Format(PyExc_SystemError,
+"symtable analysis recursion depth mismatch (before=%d, after=%d)",
+starting_recursion_depth, st->recursion_depth);
+PySymtable_Free(st);
+returnNULL;
+ }
 /* Make the second symbol analysis pass */
 if (symtable_analyze(st))
 returnst;
@@ -1501,16 +1511,16 @@ symtable_handle_namedexpr(struct symtable *st, expr_ty e)
 PyErr_SyntaxLocationObject(st->st_filename,
 e->lineno,
 e->col_offset);
-VISIT_QUIT(st, 0);
+return0;
  }
 if (st->st_cur->ste_comprehension) {
 /* Inside a comprehension body, so find the right target scope */
 if (!symtable_extend_namedexpr_scope(st, e->v.NamedExpr.target))
-VISIT_QUIT(st, 0);
+return0;
  }
 VISIT(st, expr, e->v.NamedExpr.value);
 VISIT(st, expr, e->v.NamedExpr.target);
-VISIT_QUIT(st, 1);
+return1;
 }
 
 staticint