Add a section about interpreter isolation. (#400)

pep-0554.rst

@@ -14,8 +14,8 @@ Abstract
 
 This proposal introduces the stdlib ``interpreters`` module.  It exposes
 the basic functionality of subinterpreters that already exists in the
-C-API.  Each subinterpreter runs with its own state, including its own
-copy of all modules, classes, functions, and variables.
+C-API.  Each subinterpreter runs with its own state (see
+``Interpreter Isolation`` below).
 
 
 Rationale
@@ -133,12 +133,41 @@ interpreters via queues.  The minimal solution (running a source string)
 is sufficient for us to get the feature out where it can be explored.
 
 
+Interpreter Isolation
+=====================
+
+CPython's interpreters are intended to be strictly isolated from each
+other.  Each interpreter has its own copy of all modules, classes,
+functions, and variables.  The same applies to state in C, including in
+extension modules.  The CPython C-API docs explain more. [c-api]_
+
+However, there are ways in which interpreters share some state.  First
+of all, some process-global state remains shared, like file descriptors.
+There are no plans to change this.
+
+Second, some isolation is faulty due to bugs or implementations that did
+not take subinterpreters into account.  This includes things like
+at-exit handlers and extension modules that rely on C globals.  In these
+cases bugs should be opened (some are already).
+
+Finally, some potential isolation is missing due to the current design
+of CPython.  This includes the GIL and memory management.  Improvements
+are currently going on to address gaps in this area.
+
+
 Open Questions
 ==============
 
 * Add queues to the proposal anyway?
 
 
+References
+==========
+
+.. [c-api]
+   https://docs.python.org/3/c-api/init.html#bugs-and-caveats
+
+
 Copyright
 =========