Ensure registration of extension rates in host Cantera library

If a user application is linked statically to the Cantera library,
ExtensibleRate objects need to be registered in this copy of the Cantera
library rather than the one that is embedded in the Python module.
This is achieved by accessing the ReactionRateFactory from the main
application rather than from the Python module.

interfaces/cython/cantera/delegator.pyx

@@ -328,6 +328,11 @@ cdef int assign_delegates(obj, CxxDelegator* delegator) except -1:
 
     return 0
 
+# Specifications for ReactionRate delegators that have not yet been registered with
+# ReactionRateFactory. This list is read by PythonExtensionManager::registerRateBuilders
+# and then cleared.
+_rate_delegators = []
+
 def extension(*, name):
     """
     A decorator for declaring Cantera extensions that should be registered with
@@ -371,8 +376,14 @@ def extension(*, name):
     def decorator(cls):
         if issubclass(cls, ExtensibleRate):
             cls._reaction_rate_type = name
+            # Registering immediately supports the case where the main
+            # application is Python
             CxxPythonExtensionManager.registerPythonRateBuilder(
                 stringify(cls.__module__), stringify(cls.__name__), stringify(name))
+
+            # Deferred registration supports the case where the main application
+            # is not Python
+            _rate_delegators.append((cls.__module__, cls.__name__, name))
         else:
             raise TypeError(f"{cls} is not extensible")
         return cls

src/extensions/PythonExtensionManager.cpp

@@ -144,6 +144,7 @@ void PythonExtensionManager::registerRateBuilders(const string& extensionName)
         throw CanteraError("PythonExtensionManager::registerRateBuilders",
                            "Problem loading module:\n{}", getPythonExceptionInfo());
     }
+    ct_registerReactionDelegators();
 }
 
 void PythonExtensionManager::registerPythonRateBuilder(

src/extensions/pythonExtensions.pyx

@@ -13,6 +13,7 @@ import importlib
 import inspect
 
 import cantera as ct
+from cantera._utils cimport stringify
 from cantera.reaction cimport ExtensibleRate, CxxReactionRate
 from cantera.delegator cimport CxxDelegator, assign_delegates
 
@@ -22,6 +23,12 @@ cdef extern from "cantera/kinetics/ReactionRateDelegator.h" namespace "Cantera":
         CxxReactionRateDelegator()
 
 
+cdef extern from "cantera/extensions/PythonExtensionManager.h" namespace "Cantera":
+    cdef cppclass CxxPythonExtensionManager "Cantera::PythonExtensionManager":
+        @staticmethod
+        void registerPythonRateBuilder(string&, string&, string&)
+
+
 cdef public char* ct_getExceptionString(object exType, object exValue, object exTraceback):
     import traceback
     result = str(exValue) + "\n\n"
@@ -40,3 +47,11 @@ cdef public object ct_newPythonExtensibleRate(CxxReactionRateDelegator* delegato
     cdef ExtensibleRate rate = getattr(mod, class_name.decode())(init=False)
     rate.set_cxx_object(delegator)
     return rate
+
+
+cdef public ct_registerReactionDelegators():
+    for module, cls, name in ct.delegator._rate_delegators:
+        CxxPythonExtensionManager.registerPythonRateBuilder(
+            stringify(module), stringify(cls), stringify(name))
+
+    ct.delegator._rate_delegators.clear()