fix: on hot reload esm modules were created multiple times (#559)

This was causing frontend code to sometimes use the old version, and
sometimes the new version. With antd and theming sometimes the theme
would not be applied (antd would probably not recognize the theme object
with from the old antd esm).

solara/server/esm.py

@@ -2,15 +2,22 @@
 # in the future, we may want to move the esm features of ipyreact
 # into a separate package, and then we can import it unconditionally
 import logging
+import threading
+from collections import defaultdict
 from pathlib import Path
 from typing import Dict, List, Tuple, Union
 
 import ipyreact.importmap
 import ipyreact.module
 
+from solara.server import kernel_context
+
 logger = logging.getLogger("solara.server.esm")
+lock = threading.Lock()
 
 _modules: Dict[str, Tuple[Union[str, Path], List[str]]] = {}
+_modules_added_per_kernel: Dict[str, Dict[str, ipyreact.module.Module]] = defaultdict(dict)
+_import_map_per_kernel: Dict[str, ipyreact.importmap.ImportMap] = {}
 
 
 # in solara server, we'll monkey patch ipyreact.module with this
@@ -29,10 +36,20 @@ def get_module_names():
 
 
 def create_modules():
+    kernel_id = kernel_context.get_current_context().id
+    _modules_added = _modules_added_per_kernel[kernel_id]
     logger.info("define modules %s", _modules)
     widgets = {}
-    for name, (module, dependencies) in _modules.items():
-        widgets[name] = create_module(name, module, dependencies=dependencies)
+    with lock:
+        for name, (module, dependencies) in _modules.items():
+            if name not in _modules_added:
+                _modules_added[name] = create_module(name, module, dependencies=dependencies)
+                logger.info("create module %s %s %s", name, module, dependencies)
+            else:
+                _modules_added[name].code = module if not isinstance(module, Path) else module.read_text(encoding="utf8")
+                _modules_added[name].dependencies = dependencies
+                logger.info("update module %s %s %s", name, module, dependencies)
+            widgets[name] = _modules_added[name]
     return widgets
 
 
@@ -41,5 +58,12 @@ def create_module(name, module: Union[str, Path], dependencies: List[str]):
 
 
 def create_import_map():
-    logger.info("create import map %s", ipyreact.importmap._effective_import_map)
-    return ipyreact.importmap.ImportMap(import_map=ipyreact.importmap._effective_import_map)
+    kernel_id = kernel_context.get_current_context().id
+    with lock:
+        if kernel_id not in _import_map_per_kernel:
+            _import_map_per_kernel[kernel_id] = ipyreact.importmap.ImportMap(import_map=ipyreact.importmap._effective_import_map)
+            logger.info("create import map %s", ipyreact.importmap._effective_import_map)
+        else:
+            _import_map_per_kernel[kernel_id].import_map = ipyreact.importmap._effective_import_map
+            logger.info("update import map %s", ipyreact.importmap._effective_import_map)
+    return