bpo-33592: Document the C API in PEP 567 (contextvars) (GH-7033)

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+.. highlightlang:: c
+
+.. _contextvarsobjects:
+
+Context Variables Objects
+-------------------------
+
+.. versionadded:: 3.7
+
+This section details the public C API for the :mod:`contextvars` module.
+
+.. c:type:: PyContext
+
+   The C structure used to represent a :class:`contextvars.Context`
+   object.
+
+.. c:type:: PyContextVar
+
+   The C structure used to represent a :class:`contextvars.ContextVar`
+   object.
+
+.. c:type:: PyContextToken
+
+   The C structure used to represent a :class:`contextvars.Token` object.
+
+.. c:var:: PyTypeObject PyContext_Type
+
+   The type object representing the *context* type.
+
+.. c:var:: PyTypeObject PyContextVar_Type
+
+   The type object representing the *context variable* type.
+
+.. c:var:: PyTypeObject PyContextToken_Type
+
+   The type object representing the *context variable token* type.
+
+
+Type-check macros:
+
+.. c:function:: int PyContext_CheckExact(PyObject *o)
+
+   Return true if *o* is of type :c:data:`PyContext_Type`. *o* must not be
+   *NULL*.  This function always succeeds.
+
+.. c:function:: int PyContextVar_CheckExact(PyObject *o)
+
+   Return true if *o* is of type :c:data:`PyContextVar_Type`. *o* must not be
+   *NULL*.  This function always succeeds.
+
+.. c:function:: int PyContextToken_CheckExact(PyObject *o)
+
+   Return true if *o* is of type :c:data:`PyContextToken_Type`.
+   *o* must not be *NULL*.  This function always succeeds.
+
+
+Context object management functions:
+
+.. c:function:: PyContext *PyContext_New(void)
+
+   Create a new empty context object.  Returns ``NULL`` if an error
+   has occurred.
+
+.. c:function:: PyContext *PyContext_Copy(PyContext *ctx)
+
+   Create a shallow copy of the passed *ctx* context object.
+   Returns ``NULL`` if an error has occurred.
+
+.. c:function:: PyContext *PyContext_CopyCurrent(void)
+
+   Create a shallow copy of the current thread context.
+   Returns ``NULL`` if an error has occurred.
+
+.. c:function:: int PyContext_Enter(PyContext *ctx)
+
+   Set *ctx* as the current context for the current thread.
+   Returns ``0`` on success, and ``-1`` on error.
+
+.. c:function:: int PyContext_Exit(PyContext *ctx)
+
+   Deactivate the *ctx* context and restore the previous context as the
+   current context for the current thread.  Returns ``0`` on success,
+   and ``-1`` on error.
+
+.. c:function:: int PyContext_ClearFreeList()
+
+   Clear the context variable free list. Return the total number of
+   freed items.  This function always succeeds.
+
+
+Context variable functions:
+
+.. c:function:: PyContextVar *PyContextVar_New(const char *name, PyObject *def)
+
+   Create a new ``ContextVar`` object.  The *name* parameter is used
+   for introspection and debug purposes.  The *def* parameter may optionally
+   specify the default value for the context variable.  If an error has
+   occurred, this function returns ``NULL``.
+
+.. c:function:: int PyContextVar_Get(PyContextVar *var, PyObject *default_value, PyObject **value)
+
+   Get the value of a context variable.  Returns ``-1`` if an error has
+   occurred during lookup, and ``0`` if no error occurred, whether or not
+   a value was found.
+
+   If the context variable was found, *value* will be a pointer to it.
+   If the context variable was *not* found, *value* will point to:
+
+   - *default_value*, if not ``NULL``;
+   - the default value of *var*, if not ``NULL``;
+   - ``NULL``
+
+   If the value was found, the function will create a new reference to it.
+
+.. c:function:: PyContextToken *PyContextVar_Set(PyContextVar *var, PyObject *value)
+
+   Set the value of *var* to *value* in the current context.  Returns a
+   pointer to a :c:type:`PyContextToken` object, or ``NULL`` if an error
+   has occurred.
+
+.. c:function:: int PyContextVar_Reset(PyContextVar *var, PyContextToken *token)
+
+   Reset the state of the *var* context variable to that it was in before
+   :c:func:`PyContextVar_Set` that returned the *token* was called.
+   This function returns ``0`` on success and ``-1`` on error.