Allow deeper recursion in mypy daemon, better error reporting (#17707)

Fixes #17706 

Handles recursion error during parse of too complex expressions,
differentiates from internal recursion errors by attempting to unparse
the ast. If builtin ast.unparse fails, then the error is ignored.
Otherwise, re-raises.

Author: kcdodd

mypy/dmypy/client.py

@@ -20,6 +20,7 @@
 from mypy.dmypy_os import alive, kill
 from mypy.dmypy_util import DEFAULT_STATUS_FILE, receive, send
 from mypy.ipc import IPCClient, IPCException
+from mypy.main import RECURSION_LIMIT
 from mypy.util import check_python_version, get_terminal_width, should_force_color
 from mypy.version import __version__
 
@@ -268,6 +269,10 @@ class BadStatus(Exception):
 def main(argv: list[str]) -> None:
     """The code is top-down."""
     check_python_version("dmypy")
+
+    # set recursion limit consistent with mypy/main.py
+    sys.setrecursionlimit(RECURSION_LIMIT)
+
     args = parser.parse_args(argv)
     if not args.action:
         parser.print_usage()

mypy/fastparse.py

@@ -239,6 +239,29 @@ def parse(
             strip_function_bodies=strip_function_bodies,
             path=fnam,
         ).visit(ast)
+
+    except RecursionError as e:
+        # For very complex expressions it is possible to hit recursion limit
+        # before reaching a leaf node.
+        # Should reject at top level instead at bottom, since bottom would already
+        # be at the threshold of the recursion limit, and may fail again later.
+        # E.G. x1+x2+x3+...+xn -> BinOp(left=BinOp(left=BinOp(left=...
+        try:
+            # But to prove that is the cause of this particular recursion error,
+            # try to walk the tree using builtin visitor
+            ast3.NodeVisitor().visit(ast)
+        except RecursionError:
+            errors.report(
+                -1, -1, "Source expression too complex to parse", blocker=False, code=codes.MISC
+            )
+
+            tree = MypyFile([], [], False, {})
+
+        else:
+            # re-raise original recursion error if it *can* be unparsed,
+            # maybe this is some other issue that shouldn't be silenced/misdirected
+            raise e
+
     except SyntaxError as e:
         message = e.msg
         if feature_version > sys.version_info.minor and message.startswith("invalid syntax"):
@@ -406,6 +429,7 @@ def visit(self, node: AST | None) -> Any:
             method = "visit_" + node.__class__.__name__
             visitor = getattr(self, method)
             self.visitor_cache[typeobj] = visitor
+
         return visitor(node)
 
     def set_line(self, node: N, n: AstNode) -> N:

mypy/main.py

@@ -42,6 +42,7 @@
 
 orig_stat: Final = os.stat
 MEM_PROFILE: Final = False  # If True, dump memory profile
+RECURSION_LIMIT: Final = 2**14
 
 
 def stat_proxy(path: str) -> os.stat_result:
@@ -76,7 +77,7 @@ def main(
     util.check_python_version("mypy")
     t0 = time.time()
     # To log stat() calls: os.stat = stat_proxy
-    sys.setrecursionlimit(2**14)
+    sys.setrecursionlimit(RECURSION_LIMIT)
     if args is None:
         args = sys.argv[1:]