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Nick Coghlan committed 90f1a0c Draft

Handle transfer of config to subinterpreters

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 To keep the implementation complexity under control, this PEP does *not*
 propose wholesale changes to the way the interpreter state is accessed at
-runtime, nor does it propose changes to the way subinterpreters are
-created after the main interpreter has already been initialized (beyond
-any changes needed to make sure they continue working as expected). Changing
-the order in which the existing initialization steps occur in order to make
+runtime. Changing the order in which the existing initialization steps
+occur in order to make
 the startup sequence easier to maintain is already a substantial change, and
 attempting to make those other changes at the same time will make the
 change significantly more invasive and much harder to review. However, such
 once this PEP has been implemented.
 
 
+
 Background
 ==========
 
 modified by the interpreter during runtime (except as noted above).
 
 
+Creating and Configuring Subinterpreters
+----------------------------------------
+
+As the new configuration settings are stored in the interpreter state, they
+need to be initialised when a new subinterpreter is created. This turns out
+to be trickier than one might think due to ``PyThreadState_Swap(NULL);``
+(which is fortunately exercised by CPython's own embedding tests, allowing
+this problem to be detected during development).
+
+To provide a straightforward solution for this case, the PEP proposes to
+add a new API::
+
+    Py_InterpreterState *Py_InterpreterState_Main();
+
+This will be a counterpart to Py_InterpreterState_Head(), reporting the
+oldest currently existing interpreter rather than the newest. If
+``Py_NewInterpreter()`` is called from a thread with an existing thread
+state, then the interpreter configuration for that thread will be
+used when initialising the new subinterpreter. If there is no current
+thread state, the configuration from ``Py_InterpreterState_Main()``
+will be used.
+
+While the existing ``Py_InterpreterState_Head()`` API could be used instead,
+that reference changes as subinterpreters are created and destroyed, while
+``PyInterpreterState_Main()`` will always refer to the initial interpreter
+state created in ``Py_BeginInitialization()``.
+
+A new constraint is also added to the embedding API: attempting to delete
+the main interpreter while subinterpreters still exist will now be a fatal
+error.
+
+
 Stable ABI
 ----------