SSE endpoint for paid invoices.

also move very essential stuff from core/tasks.py to tasks.py so things
are more organized.

lnbits/tasks.py

@@ -0,0 +1,81 @@
+import trio  # type: ignore
+from http import HTTPStatus
+from typing import Optional, Tuple, List, Callable, Awaitable
+from quart import Request, g
+from quart_trio import QuartTrio
+from werkzeug.datastructures import Headers
+
+from lnbits.db import open_db, open_ext_db
+from lnbits.settings import WALLET
+
+from lnbits.core.models import Payment
+from lnbits.core.crud import get_standalone_payment
+
+main_app: Optional[QuartTrio] = None
+
+
+def grab_app_for_later(app: QuartTrio):
+    global main_app
+    main_app = app
+
+
+async def send_push_promise(a, b) -> None:
+    pass
+
+
+async def run_on_pseudo_request(func: Callable, *args):
+    fk = Request(
+        "GET",
+        "http",
+        "/background/pseudo",
+        b"",
+        Headers([("host", "lnbits.background")]),
+        "",
+        "1.1",
+        send_push_promise=send_push_promise,
+    )
+    assert main_app
+
+    async def run():
+        async with main_app.request_context(fk):
+            with open_db() as g.db:  # type: ignore
+                await func(*args)
+
+    async with trio.open_nursery() as nursery:
+        nursery.start_soon(run)
+
+
+invoice_listeners: List[Tuple[str, Callable[[Payment], Awaitable[None]]]] = []
+
+
+def register_invoice_listener(ext_name: str, cb: Callable[[Payment], Awaitable[None]]):
+    """
+    A method intended for extensions to call when they want to be notified about
+    new invoice payments incoming.
+    """
+    print(f"registering {ext_name} invoice_listener callback: {cb}")
+    invoice_listeners.append((ext_name, cb))
+
+
+async def webhook_handler():
+    handler = getattr(WALLET, "webhook_listener", None)
+    if handler:
+        return await handler()
+    return "", HTTPStatus.NO_CONTENT
+
+
+async def invoice_listener():
+    async for checking_id in WALLET.paid_invoices_stream():
+        await run_on_pseudo_request(invoice_callback_dispatcher, checking_id)
+
+
+async def invoice_callback_dispatcher(checking_id: str):
+    payment = get_standalone_payment(checking_id)
+    if payment and payment.is_in:
+        payment.set_pending(False)
+        for ext_name, cb in invoice_listeners:
+            if ext_name == "core":
+                await cb(payment)
+            else:
+                with open_ext_db(ext_name) as g.ext_db:  # type: ignore
+                    await cb(payment)