Keep only the last rerender reasons, two by default

reacton/core.py

@@ -7,11 +7,13 @@
 
 """
 
+import collections
 import contextlib
 import copy
 import functools
 import inspect
 import logging
+import os
 import sys
 import threading
 import traceback
@@ -24,6 +26,7 @@
     Any,
     Callable,
     ContextManager,
+    Deque,
     Dict,
     Generic,
     List,
@@ -1274,7 +1277,11 @@ def __init__(self, element: Element, container: widgets.Widget = None, children_
         self.last_root_widget: widgets.Widget = None
         self._is_rendering = False
         self._rerender_needed = False
-        self._rerender_needed_reasons: List[RerenderReason] = []
+        # the reasons are only read for the "too many renders" error message, and a reason
+        # holds the previous and next state value: keeping all of them kept every old state
+        # value alive until close(). REACTON_RERENDER_REASONS keeps more, for debugging.
+        max_reasons = max(1, int(os.environ.get("REACTON_RERENDER_REASONS", "2")))
+        self._rerender_needed_reasons: Deque[RerenderReason] = collections.deque(maxlen=max_reasons)
         self.thread_lock = threading.Lock()
         self._closing = False
         self.tracebacks: List[TracebackType] = []
@@ -1654,10 +1661,10 @@ def format(reason: RerenderReason):
                                         f += f"Triggered at: {''.join(reason.trigger_stack)}\n"
                                     return f
 
-                                self._rerender_needed_reasons[-1]
                                 msg = f"Too many renders triggered, your render loop does not stop\nLast reason: {format(self._rerender_needed_reasons[-1])}\n"
                                 if len(self._rerender_needed_reasons) >= 2:
-                                    msg += f"Previous reasons: {format(self._rerender_needed_reasons[-2])}\n"
+                                    previous = reversed(list(self._rerender_needed_reasons)[:-1])
+                                    msg += f"Previous reasons: {''.join(format(reason) for reason in previous)}\n"
                                 raise RuntimeError(msg)
                             logger.info("Entering nested render phase: %r", self._rerender_needed_reasons[-1])
                             self._rerender_needed = False
@@ -3022,8 +3029,6 @@ def _remove_stale_root_elements(self, parent_key):
 
 
 def _render_context_class():
-    import os
-
     return _RenderContextFast if os.environ.get("REACTON_FAST") == "1" else _RenderContext
 
 

reacton/core_test.py

@@ -3110,11 +3110,31 @@ def Infinite():
         rc.render(rc.element)
 
     assert "Too many renders triggered" in str(e)
-    assert len(rc._rerender_needed_reasons) >= 50
+    assert "Last reason: Reason: state changed" in str(e)
+    assert "Previous reasons: Reason: state changed" in str(e)
+    assert Infinite.render_count >= 50  # type: ignore
 
     rc.close()
 
 
+def test_debug_infinite_loop_more_reasons(monkeypatch):
+    monkeypatch.setenv("REACTON_RERENDER_REASONS", "5")
+
+    @reacton.component
+    def Infinite():
+        state, set_state = reacton.use_state(0)
+        set_state(state + 1)
+        return w.Button(description=str(state))
+
+    with pytest.raises(RuntimeError) as e:
+        rc = core._RenderContext(Infinite(), handle_error=False)
+        rc.render(rc.element)
+
+    assert rc._rerender_needed_reasons.maxlen == 5
+    assert str(e.value).count("Reason: state changed") == 5
+    rc.close()
+
+
 def test_batch_update():
     set_state = lambda x: None  # noqa
 
@@ -3247,6 +3267,36 @@ def test_no_reference_cycles_after_close(component):
         gc.enable()
 
 
+def test_state_changes_do_not_keep_old_values_alive():
+    # every state change used to append a RerenderReason with the previous and next
+    # value to the render context, so a long-lived page kept every old state value
+    # alive until it was closed
+    class Big:
+        pass
+
+    set_value: Callable[[Big], None] = lambda x: None  # noqa
+
+    @react.component
+    def Test():
+        nonlocal set_value
+        value, set_value = react.use_state(Big())
+        return w.Button(description=str(id(value)))
+
+    box, rc = react.render(Test(), handle_error=False)
+    refs = []
+    for i in range(20):
+        big = Big()
+        refs.append(weakref.ref(big))
+        set_value(big)
+        del big
+    gc.collect()
+    alive = sum(ref() is not None for ref in refs)
+    # the current value, and the values of the last two rerender reasons
+    assert alive <= 3
+    assert len(rc._rerender_needed_reasons) <= 2
+    rc.close()
+
+
 def test_fragment():
     @reacton.component
     def Children():