add internal asyncio implementation docs

InternalDocs/README.md

@@ -41,3 +41,9 @@ Program Execution
 - [Garbage Collector Design](garbage_collector.md)
 
 - [Exception Handling](exception_handling.md)
+
+
+Modules
+---
+
+- [asyncio](asyncio.md)

InternalDocs/asyncio.md

@@ -0,0 +1,123 @@
+asyncio
+=======
+
+
+This document describes the working and implementation details of C implementation of the
+[`asyncio`](https://docs.python.org/3/library/asyncio.html) module.
+
+
+## Pre-Python 3.14 implementation
+
+Until Python 3.13, the C implementation of `asyncio` used a [`WeakSet`](https://docs.python.org/3/library/weakref.html#weakref.WeakSet) to store all the tasks created by the event loop. `WeakSet` was used so that the event loop
+doesn't hold strong references to the tasks, allowing them to be garbage collected when they are no longer needed.
+The current task of the event loop was stored in a dict mapping the event loop to the current task.
+
+```c
+    /* Dictionary containing tasks that are currently active in
+       all running event loops.  {EventLoop: Task} */
+    PyObject *current_tasks;
+
+    /* WeakSet containing all tasks scheduled to run on event loops. */
+    PyObject *scheduled_tasks;
+```
+
+This implementation had a few drawbacks:
+1. **Performance**: Using a `WeakSet` for storing tasks is inefficient as it requires maintaining a full set of weak references to tasks along with corresponding weakref callback to cleanup the tasks when they are garbage collected.
+This increases the work done by the garbage collector and in applications with a large number of tasks,  this becomes a bottle neck, with increased memory usage and lower performance. Looking up the current task was slow as it required a dictionary lookup on the `current_tasks` dict.
+
+2. **Thread safety**: Until Python 3.14, concurrent iterations over `WeakSet` was not thread safe[^1]. This meant calling APIs like `asyncio.all_tasks()` could lead to inconsistent results or even `RuntimeError` if used in multiple threads[^2].
+
+3. **Poor scaling in free-threading**: Using global `WeakSet` for storing all tasks across all threads lead to contention when adding and removing tasks from the set which is a frequent operation. As such it performed poorly in free-threading and did not scale well with the number of threads. Similarly accessing the current task in multiple threads did not scale due to contention on the global `current_tasks` dictionary.
+
+## Python 3.14 implementation
+
+To address these issues, Python 3.14 implements several changes to improve the performance and thread safety of tasks management.
+
+- **Per-thread double linked list for tasks**: Python 3.14 introduces a per-thread circular double linked list implementation for storing tasks. This allows each thread to maintain its own list of tasks and allows for lock free addition and removal of tasks. This is designed to be efficient, and thread-safe and scales well with the number of threads in free-threading. This also allows external introspection tools such as `python -m asyncio pstree` to inspect tasks running in all threads and was implemented as part of [Audit asyncio thread safety](https://github.com/python/cpython/issues/128002).
+
+- **Per-thread current task**: Python 3.14 stores the current task on the current thread state instead of a global dictionary. This allows for faster access to the current task without the need for a dictionary lookup. Each thread maintains its own current task, which is stored in the `PyThreadState` structure. This was implemented in https://github.com/python/cpython/issues/129898.
+
+Storing the current task and list of all tasks per-thread instead of storing it per-loop was chosen primarily to support external introspection tools such as `python -m asyncio pstree` as looking up arbitrary attributes on the loop object
+is not possible externally. Storing data per-thread also makes it easy to support third party event loop implementations such as `uvloop` and is more efficient for single threaded asyncio use-case as it avoids the overhead of attribute lookups on the loop object and several other calls on the performance critical path of adding and removing tasks from the per-loop task list.
+
+
+## Per-thread double linked list for tasks
+
+This implementation uses a circular doubly linked list to store tasks on the thread states.  This is used for all tasks which are instances of `asyncio.Task` or subclasses of it, for third-party tasks a fallback `WeakSet` implementation is used. The linked list is implemented using an embedded `llist_node` structure within each `TaskObj`. By embedding the list node directly into the task object, the implementation avoids additional memory allocations for linked list nodes.
+
+The `PyThreadState` structure gained a new field `asyncio_tasks_head`, which serves as the head of the circular linked list of tasks. This allows for lock free addition and removal of tasks from the list.
+
+ It is possible that when a thread state is deallocated, there are lingering tasks in its list; this can happen if another thread has references to the tasks of this thread. Therefore, the `PyInterpreterState` structure also gains a new `asyncio_tasks_head` field to store any lingering tasks. When a thread state is deallocated, any remaining lingering tasks are moved to the interpreter state tasks list, and the thread state tasks list is cleared.
+The `asyncio_tasks_lock` is used protect the interpreter's tasks list from concurrent modifications.
+
+
+```c
+typedef struct TaskObj {
+    ...
+    struct llist_node asyncio_node;
+} TaskObj;
+
+typedef struct PyThreadState {
+    ...
+    struct llist_node asyncio_tasks_head;
+} PyThreadState;
+
+typedef struct PyInterpreterState {
+    ...
+    struct llist_node asyncio_tasks_head;
+    PyMutex asyncio_tasks_lock;
+} PyInterpreterState;
+```
+
+When a task is created, it is added to the current thread's list of tasks by the `register_task` function. When the task is done, it is removed from the list by the `unregister_task` function. In free-threading, the thread id of the thread which
+created the task is stored in `task_tid` field of the `TaskObj`. This is used to check if the task is being removed from the correct thread's task list. If the current thread is same as the thread which created it then no locking is required, otherwise in free-threading, the `stop-the-world` pause is used to pause all other threads and then safely remove the task from the tasks list.
+
+```mermaid
+
+flowchart TD
+    subgraph one["Executing Thread"]
+        A["task = asyncio.create_task(coro())"] -->B("register_task(task)")
+        B --> C{"task->task_state?"}
+        C -->|pending| D["task_step(task)"]
+        C -->|done| F["unregister_task(task)"]
+        C -->|cancelled| F["unregister_task(task)"]
+        D --> C
+        F --> G{"free-threading?"}
+        G --> |false| H["unregister_task_safe(task)"]
+        G --> |true| J{"correct thread? <br>task->task_tid == _Py_ThreadId()"}
+        J --> |true| H
+        J --> |false| I["stop the world <br> pause all threads"]
+        I --> H["unregister_task_safe(task)"]
+    end
+    subgraph two["Thread deallocating"]
+        A1{"thread's task list empty? <br> llist_empty(tstate->asyncio_tasks_head)"}
+        A1 --> |true| B1["deallocate thread<br>free_threadstate(tstate)"]
+        A1 --> |false| C1["add tasks to interpreter's task list<br> llist_concat(&tstate->interp->asyncio_tasks_head,tstate->asyncio_tasks_head)"]
+        C1 --> B1
+    end
+
+    one --> two
+```
+
+`asyncio.all_tasks` now iterates over the per-thread task lists of all threads and the interpreter's task list to get all the tasks. In free-threading this is done by pausing all the threads using the `stop-the-world` pause to ensure that no tasks are being added or removed while iterating over the lists. This allows for a consistent view of all task lists across all threads and is thread safe.
+
+This design allows for lock free execution and scales well in free-threading with multiple event loops running in different threads.
+
+## Per-thread current task
+This implementation stores the current task in the `PyThreadState` structure, which allows for faster access to the current task without the need for a dictionary lookup.
+
+```c
+typedef struct PyThreadState {
+    ...
+    PyObject *asyncio_current_loop;
+    PyObject *asyncio_current_task;
+} PyThreadState;
+```
+
+When a task is entered or left, the current task is updated in the thread state using `enter_task` and `leave_task` functions. When `current_task(loop)` is called where `loop` is the current running event loop of the current thread, no locking is required as the current task is stored in the thread state and is returned directly (general case). Otherwise, if the `loop` is not current running event loop, the `stop-the-world` pause is used to pause all threads in free-threading and then by iterating over all the thread states and checking if the `loop` matches with `tstate->asyncio_current_loop`, the current task is found and returned. If no matching thread state is found, `None` is returned.
+
+In free-threading, it avoids contention on a global dictionary as threads can access the current task of thier running loop without any locking.
+
+
+[^1]: https://github.com/python/cpython/issues/123089
+[^2]: https://github.com/python/cpython/issues/80788