moving to scripts

asq-env/lib/python3.9/site-packages/trio/_signals.py

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+import signal
+from contextlib import contextmanager
+from collections import OrderedDict
+
+import trio
+from ._util import signal_raise, is_main_thread, ConflictDetector
+
+# Discussion of signal handling strategies:
+#
+# - On Windows signals barely exist. There are no options; signal handlers are
+#   the only available API.
+#
+# - On Linux signalfd is arguably the natural way. Semantics: signalfd acts as
+#   an *alternative* signal delivery mechanism. The way you use it is to mask
+#   out the relevant signals process-wide (so that they don't get delivered
+#   the normal way), and then when you read from signalfd that actually counts
+#   as delivering it (despite the mask). The problem with this is that we
+#   don't have any reliable way to mask out signals process-wide -- the only
+#   way to do that in Python is to call pthread_sigmask from the main thread
+#   *before starting any other threads*, and as a library we can't really
+#   impose that, and the failure mode is annoying (signals get delivered via
+#   signal handlers whether we want them to or not).
+#
+# - on macOS/*BSD, kqueue is the natural way. Semantics: kqueue acts as an
+#   *extra* signal delivery mechanism. Signals are delivered the normal
+#   way, *and* are delivered to kqueue. So you want to set them to SIG_IGN so
+#   that they don't end up pending forever (I guess?). I can't find any actual
+#   docs on how masking and EVFILT_SIGNAL interact. I did see someone note
+#   that if a signal is pending when the kqueue filter is added then you
+#   *don't* get notified of that, which makes sense. But still, we have to
+#   manipulate signal state (e.g. setting SIG_IGN) which as far as Python is
+#   concerned means we have to do this from the main thread.
+#
+# So in summary, there don't seem to be any compelling advantages to using the
+# platform-native signal notification systems; they're kinda nice, but it's
+# simpler to implement the naive signal-handler-based system once and be
+# done. (The big advantage would be if there were a reliable way to monitor
+# for SIGCHLD from outside the main thread and without interfering with other
+# libraries that also want to monitor for SIGCHLD. But there isn't. I guess
+# kqueue might give us that, but in kqueue we don't need it, because kqueue
+# can directly monitor for child process state changes.)
+
+
+@contextmanager
+def _signal_handler(signals, handler):
+    original_handlers = {}
+    try:
+        for signum in set(signals):
+            original_handlers[signum] = signal.signal(signum, handler)
+        yield
+    finally:
+        for signum, original_handler in original_handlers.items():
+            signal.signal(signum, original_handler)
+
+
+class SignalReceiver:
+    def __init__(self):
+        # {signal num: None}
+        self._pending = OrderedDict()
+        self._lot = trio.lowlevel.ParkingLot()
+        self._conflict_detector = ConflictDetector(
+            "only one task can iterate on a signal receiver at a time"
+        )
+        self._closed = False
+
+    def _add(self, signum):
+        if self._closed:
+            signal_raise(signum)
+        else:
+            self._pending[signum] = None
+            self._lot.unpark()
+
+    def _redeliver_remaining(self):
+        # First make sure that any signals still in the delivery pipeline will
+        # get redelivered
+        self._closed = True
+
+        # And then redeliver any that are sitting in pending. This is done
+        # using a weird recursive construct to make sure we process everything
+        # even if some of the handlers raise exceptions.
+        def deliver_next():
+            if self._pending:
+                signum, _ = self._pending.popitem(last=False)
+                try:
+                    signal_raise(signum)
+                finally:
+                    deliver_next()
+
+        deliver_next()
+
+    # Helper for tests, not public or otherwise used
+    def _pending_signal_count(self):
+        return len(self._pending)
+
+    def __aiter__(self):
+        return self
+
+    async def __anext__(self):
+        if self._closed:
+            raise RuntimeError("open_signal_receiver block already exited")
+        # In principle it would be possible to support multiple concurrent
+        # calls to __anext__, but doing it without race conditions is quite
+        # tricky, and there doesn't seem to be any point in trying.
+        with self._conflict_detector:
+            if not self._pending:
+                await self._lot.park()
+            else:
+                await trio.lowlevel.checkpoint()
+            signum, _ = self._pending.popitem(last=False)
+            return signum
+
+
+@contextmanager
+def open_signal_receiver(*signals):
+    """A context manager for catching signals.
+
+    Entering this context manager starts listening for the given signals and
+    returns an async iterator; exiting the context manager stops listening.
+
+    The async iterator blocks until a signal arrives, and then yields it.
+
+    Note that if you leave the ``with`` block while the iterator has
+    unextracted signals still pending inside it, then they will be
+    re-delivered using Python's regular signal handling logic. This avoids a
+    race condition when signals arrives just before we exit the ``with``
+    block.
+
+    Args:
+      signals: the signals to listen for.
+
+    Raises:
+      TypeError: if no signals were provided.
+
+      RuntimeError: if you try to use this anywhere except Python's main
+          thread. (This is a Python limitation.)
+
+    Example:
+
+      A common convention for Unix daemons is that they should reload their
+      configuration when they receive a ``SIGHUP``. Here's a sketch of what
+      that might look like using :func:`open_signal_receiver`::
+
+         with trio.open_signal_receiver(signal.SIGHUP) as signal_aiter:
+             async for signum in signal_aiter:
+                 assert signum == signal.SIGHUP
+                 reload_configuration()
+
+    """
+    if not signals:
+        raise TypeError("No signals were provided")
+
+    if not is_main_thread():
+        raise RuntimeError(
+            "Sorry, open_signal_receiver is only possible when running in "
+            "Python interpreter's main thread"
+        )
+    token = trio.lowlevel.current_trio_token()
+    queue = SignalReceiver()
+
+    def handler(signum, _):
+        token.run_sync_soon(queue._add, signum, idempotent=True)
+
+    try:
+        with _signal_handler(signals, handler):
+            yield queue
+    finally:
+        queue._redeliver_remaining()