feat(core): add mailbox as a simplified version of chan

core/src/trezor/loop.py

@@ -350,6 +350,109 @@ def __iter__(self) -> Task:  # type: ignore [awaitable-is-generator]
             raise
 
 
+class mailbox(Syscall):
+    """
+    Wait to receive a value.
+
+    In terms of synchronization primitives, this is a condition variable that also
+    contains a value. It is a simplification of Go channels, which is one-ended and
+    only has a buffer of size 1.
+
+    The receiving end pauses until a value is received, and then empties the mailbox
+    to wait again.
+
+    The sending end synchronously posts a value. It is impossible to wait until
+    the value is consumed. Trying to post a value when the mailbox is full raises
+    an error, unless `replace=True` is specified
+
+    Example:
+
+    >>> # in task #1:
+    >>> box = loop.mailbox()
+    >>> while True:
+    >>>     result = await box
+    >>>     print("awaited result:", result)
+
+    >>> # in task #2:
+    >>> box.put("Hello from the other task")
+    >>> print("put completed")
+
+    Example Output:
+
+    put completed
+    awaited result: Hello from the other task
+    """
+
+    _NO_VALUE = object()
+
+    def __init__(self, initial_value: Any = _NO_VALUE) -> None:
+        self.value = initial_value
+        self.taker: Task | None = None
+
+    def is_empty(self) -> bool:
+        """Is the mailbox empty?"""
+        return self.value is self._NO_VALUE
+
+    def clear(self) -> None:
+        """Empty the mailbox."""
+        assert self.taker is None
+        self.value = self._NO_VALUE
+
+    def put(self, value: Any, replace: bool = False) -> None:
+        """Put a value into the mailbox.
+
+        If there is another task waiting for the value, it will be scheduled to resume.
+        Otherwise, the mailbox will hold the value until someone consumes it.
+
+        It is an error to call `put()` when there is a value already held, unless
+        `replace` is set to `True`. In such case, the held value is replaced with
+        the new one.
+        """
+        if not self.is_empty() and not replace:
+            raise ValueError("mailbox already has a value")
+
+        self.value = value
+        if self.taker is not None:
+            self._take(self.taker)
+
+    def _take(self, task: Task) -> None:
+        """Take a value and schedule the taker."""
+        self.taker = None
+        schedule(task, self.value)
+        self.clear()
+
+    def handle(self, task: Task) -> None:
+        assert self.taker is None
+        if not self.is_empty():
+            self._take(task)
+        else:
+            self.taker = task
+
+    def __iter__(self) -> Generator:
+        assert self.taker is None
+
+        # short-circuit if there is a value already
+        if not self.is_empty():
+            value = self.value
+            self.clear()
+            return value
+
+        # otherwise, wait for a value
+        try:
+            return (yield self)
+        finally:
+            # Clear the taker even in case of exception. This way stale takers don't
+            # blow up someone calling `maybe_close()`
+            self.taker = None
+
+    def maybe_close(self) -> None:
+        """Shut down the taker if possible."""
+        taker = self.taker
+        self.taker = None
+        if taker is not None and taker is not this_task:
+            taker.close()
+
+
 class chan:
     """
     Two-ended channel.