Merging upstream version 3.0.21.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 91 insertions, 0 deletions

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+Concurrency-related challenges regarding embedding of ptpython in asyncio code
+==============================================================================
+
+Things we want to be possible
+-----------------------------
+
+- Embed blocking ptpython in non-asyncio code (the normal use case).
+- Embed blocking ptpython in asyncio code (the event loop will block).
+- Embed awaitable ptpython in asyncio code (the loop will continue).
+- React to resize events (SIGWINCH).
+- Support top-level await.
+- Be able to patch_stdout, so that logging messages from another thread will be
+  printed above the prompt.
+- It should be possible to handle `KeyboardInterrupt` during evaluation of an
+  expression.
+- The "eval" should happen in the same thread from where embed() was called.
+
+
+Limitations of asyncio/python
+-----------------------------
+
+- We can only listen to SIGWINCH signal (resize) events in the main thread.
+
+- Usage of Control-C for triggering a `KeyboardInterrupt` only works for code
+  running in the main thread. (And only if the terminal was not set in raw
+  input mode).
+
+- Spawning a new event loop from within a coroutine, that's being executed in
+  an existing event loop is not allowed in asyncio. We can however spawn any
+  event loop in a separate thread, and wait for that thread to finish.
+
+- For patch_stdout to work correctly, we have to know what prompt_toolkit
+  application is running on the terminal, then tell that application to print
+  the output and redraw itself.
+
+
+Additional challenges for IPython
+---------------------------------
+
+IPython supports integration of 3rd party event loops (for various GUI
+toolkits). These event loops are supposed to continue running while we are
+prompting for input. In an asyncio environment, it means that there are
+situations where we have to juggle three event loops:
+
+- The asyncio loop in which the code was embedded.
+- The asyncio loop from the prompt.
+- The 3rd party GUI loop. 
+
+Approach taken in ptpython 3.0.11
+---------------------------------
+
+For ptpython, the most reliable solution is to to run the prompt_toolkit input
+prompt in a separate background thread. This way it can use its own asyncio
+event loop without ever having to interfere with whatever runs in the main
+thread.
+
+Then, depending on how we embed, we do the following:
+When a normal blocking embed is used:
+    * We start the UI thread for the input, and do a blocking wait on
+      `thread.join()` here.
+    * The "eval" happens in the main thread.
+    * The "print" happens also in the main thread. Unless a pager is shown,
+      which is also a prompt_toolkit application, then the pager itself is runs
+      also in another thread, similar to the way we do the input.
+
+When an awaitable embed is used, for embedding in a coroutine, but having the
+event loop continue:
+    * We run the input method from the blocking embed in an asyncio executor
+      and do an `await loop.run_in_excecutor(...)`.
+    * The "eval" happens again in the main thread.
+    * "print" is also similar, except that the pager code (if used) runs in an
+      executor too.
+
+This means that the prompt_toolkit application code will always run in a
+different thread. It means it won't be able to respond to SIGWINCH (window
+resize events), but prompt_toolkit's 3.0.11 has now terminal size polling which
+solves this.
+
+Control-C key presses won't interrupt the main thread while we wait for input,
+because the prompt_toolkit application turns the terminal in raw mode, while
+it's reading, which means that it will receive control-c key presses as raw
+data in its own thread.
+
+Top-level await works in most situations as expected.
+- If a blocking embed is used. We execute ``loop.run_until_complete(code)``.
+  This assumes that the blocking embed is not used in a coroutine of a running
+  event loop, otherwise, this will attempt to start a nested event loop, which
+  asyncio does not support. In that case we will get an exception.
+- If an awaitable embed is used. We literally execute ``await code``. This will
+  integrate nicely in the current event loop.