gh-135944: Add a "Runtime Components" Section to the Execution Model Docs (gh-135945)

The section provides a brief overview of the Python runtime's execution environment.  It is meant to be implementation agnostic,
(cherry picked from commit 46a1f0a)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>

Author: ericsnowcurrently

Doc/reference/executionmodel.rst

@@ -391,6 +391,192 @@ See also the description of the :keyword:`try` statement in section :ref:`try`
 and :keyword:`raise` statement in section :ref:`raise`.
 
 
+.. _execcomponents:
+
+Runtime Components
+==================
+
+General Computing Model
+-----------------------
+
+Python's execution model does not operate in a vacuum.  It runs on
+a host machine and through that host's runtime environment, including
+its operating system (OS), if there is one.  When a program runs,
+the conceptual layers of how it runs on the host look something
+like this:
+
+   | **host machine**
+   |   **process** (global resources)
+   |     **thread** (runs machine code)
+
+Each process represents a program running on the host.  Think of each
+process itself as the data part of its program.  Think of the process'
+threads as the execution part of the program.  This distinction will
+be important to understand the conceptual Python runtime.
+
+The process, as the data part, is the execution context in which the
+program runs.  It mostly consists of the set of resources assigned to
+the program by the host, including memory, signals, file handles,
+sockets, and environment variables.
+
+Processes are isolated and independent from one another.  (The same
+is true for hosts.)  The host manages the process' access to its
+assigned resources, in addition to coordinating between processes.
+
+Each thread represents the actual execution of the program's machine
+code, running relative to the resources assigned to the program's
+process.  It's strictly up to the host how and when that execution
+takes place.
+
+From the point of view of Python, a program always starts with exactly
+one thread.  However, the program may grow to run in multiple
+simultaneous threads.  Not all hosts support multiple threads per
+process, but most do.  Unlike processes, threads in a process are not
+isolated and independent from one another.  Specifically, all threads
+in a process share all of the process' resources.
+
+The fundamental point of threads is that each one does *run*
+independently, at the same time as the others.  That may be only
+conceptually at the same time ("concurrently") or physically
+("in parallel").  Either way, the threads effectively run
+at a non-synchronized rate.
+
+.. note::
+
+   That non-synchronized rate means none of the process' memory is
+   guaranteed to stay consistent for the code running in any given
+   thread.  Thus multi-threaded programs must take care to coordinate
+   access to intentionally shared resources.  Likewise, they must take
+   care to be absolutely diligent about not accessing any *other*
+   resources in multiple threads; otherwise two threads running at the
+   same time might accidentally interfere with each other's use of some
+   shared data.  All this is true for both Python programs and the
+   Python runtime.
+
+   The cost of this broad, unstructured requirement is the tradeoff for
+   the kind of raw concurrency that threads provide.  The alternative
+   to the required discipline generally means dealing with
+   non-deterministic bugs and data corruption.
+
+Python Runtime Model
+--------------------
+
+The same conceptual layers apply to each Python program, with some
+extra data layers specific to Python:
+
+   | **host machine**
+   |   **process** (global resources)
+   |     Python global runtime (*state*)
+   |       Python interpreter (*state*)
+   |         **thread** (runs Python bytecode and "C-API")
+   |           Python thread *state*
+
+At the conceptual level: when a Python program starts, it looks exactly
+like that diagram, with one of each.  The runtime may grow to include
+multiple interpreters, and each interpreter may grow to include
+multiple thread states.
+
+.. note::
+
+   A Python implementation won't necessarily implement the runtime
+   layers distinctly or even concretely.  The only exception is places
+   where distinct layers are directly specified or exposed to users,
+   like through the :mod:`threading` module.
+
+.. note::
+
+   The initial interpreter is typically called the "main" interpreter.
+   Some Python implementations, like CPython, assign special roles
+   to the main interpreter.
+
+   Likewise, the host thread where the runtime was initialized is known
+   as the "main" thread.  It may be different from the process' initial
+   thread, though they are often the same.  In some cases "main thread"
+   may be even more specific and refer to the initial thread state.
+   A Python runtime might assign specific responsibilities
+   to the main thread, such as handling signals.
+
+As a whole, the Python runtime consists of the global runtime state,
+interpreters, and thread states.  The runtime ensures all that state
+stays consistent over its lifetime, particularly when used with
+multiple host threads.
+
+The global runtime, at the conceptual level, is just a set of
+interpreters.  While those interpreters are otherwise isolated and
+independent from one another, they may share some data or other
+resources.  The runtime is responsible for managing these global
+resources safely.  The actual nature and management of these resources
+is implementation-specific.  Ultimately, the external utility of the
+global runtime is limited to managing interpreters.
+
+In contrast, an "interpreter" is conceptually what we would normally
+think of as the (full-featured) "Python runtime".  When machine code
+executing in a host thread interacts with the Python runtime, it calls
+into Python in the context of a specific interpreter.
+
+.. note::
+
+   The term "interpreter" here is not the same as the "bytecode
+   interpreter", which is what regularly runs in threads, executing
+   compiled Python code.
+
+   In an ideal world, "Python runtime" would refer to what we currently
+   call "interpreter".  However, it's been called "interpreter" at least
+   since introduced in 1997 (`CPython:a027efa5b`_).
+
+   .. _CPython:a027efa5b: https://github.com/python/cpython/commit/a027efa5b
+
+Each interpreter completely encapsulates all of the non-process-global,
+non-thread-specific state needed for the Python runtime to work.
+Notably, the interpreter's state persists between uses.  It includes
+fundamental data like :data:`sys.modules`.  The runtime ensures
+multiple threads using the same interpreter will safely
+share it between them.
+
+A Python implementation may support using multiple interpreters at the
+same time in the same process.  They are independent and isolated from
+one another.  For example, each interpreter has its own
+:data:`sys.modules`.
+
+For thread-specific runtime state, each interpreter has a set of thread
+states, which it manages, in the same way the global runtime contains
+a set of interpreters.  It can have thread states for as many host
+threads as it needs.  It may even have multiple thread states for
+the same host thread, though that isn't as common.
+
+Each thread state, conceptually, has all the thread-specific runtime
+data an interpreter needs to operate in one host thread.  The thread
+state includes the current raised exception and the thread's Python
+call stack.  It may include other thread-specific resources.
+
+.. note::
+
+   The term "Python thread" can sometimes refer to a thread state, but
+   normally it means a thread created using the :mod:`threading` module.
+
+Each thread state, over its lifetime, is always tied to exactly one
+interpreter and exactly one host thread.  It will only ever be used in
+that thread and with that interpreter.
+
+Multiple thread states may be tied to the same host thread, whether for
+different interpreters or even the same interpreter.  However, for any
+given host thread, only one of the thread states tied to it can be used
+by the thread at a time.
+
+Thread states are isolated and independent from one another and don't
+share any data, except for possibly sharing an interpreter and objects
+or other resources belonging to that interpreter.
+
+Once a program is running, new Python threads can be created using the
+:mod:`threading` module (on platforms and Python implementations that
+support threads).  Additional processes can be created using the
+:mod:`os`, :mod:`subprocess`, and :mod:`multiprocessing` modules.
+Interpreters can be created and used with the
+:mod:`~concurrent.interpreters` module.  Coroutines (async) can
+be run using :mod:`asyncio` in each interpreter, typically only
+in a single thread (often the main thread).
+
+
 .. rubric:: Footnotes
 
 .. [#] This limitation occurs because the code that is executed by these operations