fix: on_kernel_start callbacks acculumated after hot reload

Introduced in #471
We should remove the on_kernel_start callbacks on a hot reload, but
not remove the ones added by the hot reload itself.

solara/server/app.py

@@ -249,6 +249,31 @@ def reload(self):
 
             solara.lab.toestand.ConnectionStore._type_counter.clear()
 
+            # we need to remove callbacks that are added in the app code
+            # which will be re-executed after the reload and we do not
+            # want to keep executing the old ones.
+            for kc in kernel_context._on_kernel_start_callbacks.copy():
+                callback, path, module, cleanup = kc
+                will_reload = False
+                if module is not None:
+                    module_name = module.__name__
+                    if module_name in reload.reloader.get_reload_module_names():
+                        will_reload = True
+                elif path is not None:
+                    if str(path.resolve()).startswith(str(self.directory)):
+                        will_reload = True
+                    else:
+                        logger.warning(
+                            "script %s is not in the same directory as the app %s but is using on_kernel_start, "
+                            "this might lead to multiple entries, and might indicate a bug.",
+                            path,
+                            self.directory,
+                        )
+
+                if will_reload:
+                    logger.info("reload: Removing on_kernel_start callback: %s (since it will be added when reloaded)", callback)
+                    cleanup()
+
             context_values = list(kernel_context.contexts.values())
             # save states into the context so the hot reload will
             # keep the same state

solara/server/kernel_context.py

@@ -1,13 +1,15 @@
 import asyncio
 import dataclasses
 import enum
+import inspect
 import logging
 import os
 import pickle
 import threading
 import time
 from pathlib import Path
-from typing import Any, Callable, Dict, List, Optional, cast
+from types import FrameType, ModuleType
+from typing import Any, Callable, Dict, List, NamedTuple, Optional, cast
 
 import ipywidgets as widgets
 import reacton
@@ -36,11 +38,46 @@ class PageStatus(enum.Enum):
     CLOSED = "closed"
 
 
-_on_kernel_start_callbacks: List[Callable[[], Optional[Callable[[], None]]]] = []
+class _on_kernel_callback_entry(NamedTuple):
+    callback: Callable[[], Optional[Callable[[], None]]]
+    callpoint: Optional[Path]
+    module: Optional[ModuleType]
+    cleanup: Callable[[], None]
 
 
-def on_kernel_start(f: Callable[[], Optional[Callable[[], None]]]):
-    _on_kernel_start_callbacks.append(f)
+_on_kernel_start_callbacks: List[_on_kernel_callback_entry] = []
+
+
+def _find_root_module_frame() -> Optional[FrameType]:
+    # basically the module where the call stack origined from
+    current_frame = inspect.currentframe()
+    root_module_frame = None
+
+    while current_frame is not None:
+        if current_frame.f_code.co_name == "<module>":
+            root_module_frame = current_frame
+            break
+        current_frame = current_frame.f_back
+
+    return root_module_frame
+
+
+def on_kernel_start(f: Callable[[], Optional[Callable[[], None]]]) -> Callable[[], None]:
+    root = _find_root_module_frame()
+    path: Optional[Path] = None
+    module: Optional[ModuleType] = None
+    if root is not None:
+        path_str = inspect.getsourcefile(root)
+        module = inspect.getmodule(root)
+        if path_str is not None:
+            path = Path(path_str)
+
+    def cleanup():
+        return _on_kernel_start_callbacks.remove(kce)
+
+    kce = _on_kernel_callback_entry(f, path, module, cleanup)
+    _on_kernel_start_callbacks.append(kce)
+    return cleanup
 
 
 @dataclasses.dataclass
@@ -74,7 +111,7 @@ class VirtualKernelContext:
 
     def __post_init__(self):
         with self:
-            for f in _on_kernel_start_callbacks:
+            for (f, *_) in _on_kernel_start_callbacks:
                 cleanup = f()
                 if cleanup:
                     self.on_close(cleanup)

solara/server/reload.py

@@ -152,13 +152,13 @@ def start(self):
             self._first = False
 
     def _on_change(self, name):
-        # used for testing
-        self.reload_event_next.set()
         # flag that we need to reload all modules next time
         self.requires_reload = True
         # and forward callback
         if self.on_change:
             self.on_change(name)
+        # used for testing
+        self.reload_event_next.set()
 
     def close(self):
         self.watcher.close()

solara/website/pages/documentation/api/utilities/on_kernel_start.py

@@ -4,15 +4,21 @@
 Run a function when a virtual kernel (re)starts and optionally run a cleanup function on shutdown.
 
 ```python
-def on_kernel_start(f: Callable[[], Optional[Callable[[], None]]]):
+def on_kernel_start(f: Callable[[], Optional[Callable[[], None]]]) -> Callable[[], None]:
+    ...
 ```
 
 `f` will be called on each virtual kernel (re)start. This (usually) happens each time a browser tab connects to the server
 [see solara server for more details](https://solara.dev/docs/understanding/solara-server).
 The (optional) function returned by `f` will be called on kernel shutdown.
 
 Note that the cleanup functions are called in reverse order with respect to the order in which they were registered
-(e.g. the cleanup function of the last call to `on_kernel_start` will be called first on kernel shutdown)
+(e.g. the cleanup function of the last call to `on_kernel_start` will be called first on kernel shutdown).
+
+The return value of on_kernel_start is a cleanup function that will remove the callback from the list of callbacks to be called on kernel start.
+
+During hot reload, the callbacks that are added from scripts or modules that will be reloaded will be removed before the app is loaded
+again. This can cause the order of the callbacks to be different than at first run.
 """
 
 from solara.website.components import NoPage

tests/unit/reload_test.py

@@ -0,0 +1,56 @@
+import shutil
+from pathlib import Path
+
+import pytest
+
+import solara.lab
+import solara.server.kernel_context
+from solara.server import reload
+from solara.server.app import AppScript
+
+HERE = Path(__file__).parent
+
+kernel_start_path = HERE / "solara_test_apps" / "kernel_start.py"
+
+
+@pytest.mark.parametrize("as_module", [False, True])
+def test_script_reload_component(tmpdir, kernel_context, extra_include_path, no_kernel_context, as_module):
+
+    target = Path(tmpdir) / "kernel_start.py"
+    shutil.copy(kernel_start_path, target)
+    with extra_include_path(str(tmpdir)):
+        on_kernel_start_callbacks = solara.server.kernel_context._on_kernel_start_callbacks.copy()
+        callbacks_start = [k.callback for k in solara.server.kernel_context._on_kernel_start_callbacks]
+        if as_module:
+            app = AppScript(f"{target.stem}")
+        else:
+            app = AppScript(f"{target}")
+        try:
+            app.run()
+            callback = app.routes[0].module.test_callback  # type: ignore
+            callbacks = [k.callback for k in solara.server.kernel_context._on_kernel_start_callbacks]
+            assert callbacks == [*callbacks_start, callback]
+            prev = callbacks.copy()
+            reload.reloader.reload_event_next.clear()
+            target.touch()
+            # wait for the event to trigger
+            reload.reloader.reload_event_next.wait()
+            app.run()
+            callback = app.routes[0].module.test_callback  # type: ignore
+            callbacks = [k[0] for k in solara.server.kernel_context._on_kernel_start_callbacks]
+            assert callbacks != prev
+            assert callbacks == [*callbacks_start, callback]
+        finally:
+            app.close()
+            solara.server.kernel_context._on_kernel_start_callbacks.clear()
+            solara.server.kernel_context._on_kernel_start_callbacks.extend(on_kernel_start_callbacks)
+
+
+def test_on_kernel_start_cleanup(kernel_context, no_kernel_context):
+    def test_callback_cleanup():
+        pass
+
+    cleanup = solara.lab.on_kernel_start(test_callback_cleanup)
+    assert test_callback_cleanup in [k.callback for k in solara.server.kernel_context._on_kernel_start_callbacks]
+    cleanup()
+    assert test_callback_cleanup not in [k.callback for k in solara.server.kernel_context._on_kernel_start_callbacks]

tests/unit/solara_test_apps/kernel_start.py

@@ -0,0 +1,14 @@
+import solara
+import solara.lab
+
+
+def test_callback():
+    pass
+
+
+solara.lab.on_kernel_start(test_callback)
+
+
+@solara.component
+def Page():
+    solara.Text("Hello, World!")