add an async read method to EventFD

Adding an async write does not work for values larger than 1, because
the fd becomes writable once a value of 1 can be written, but a larger
value might still cause EAGAIN putting us into a busy loop. Hitting the
limit with writing ones is implausible, so async code can just use the
synchronous write method.

1 files changed, 31 insertions, 0 deletions

diff --git a/linuxnamespaces/syscalls.py b/linuxnamespaces/syscalls.py
index 338e602..06b5025 100644
--- a/linuxnamespaces/syscalls.py
+++ b/linuxnamespaces/syscalls.py
@@ -5,9 +5,11 @@
 for Linux namespaces including the new mount API.
 """
 
+import asyncio
 import ctypes
 import dataclasses
 import enum
+import errno
 import os
 import typing
 
@@ -419,6 +421,35 @@ class EventFD:
         call_libc("eventfd_read", self.fd, ctypes.byref(cvalue))
         return cvalue.value
 
+    def __handle_readable(self, fd: int, fut: asyncio.Future[int]) -> None:
+        """Internal helper of aread."""
+        try:
+            if fd != self.fd:
+                raise RuntimeError("EventFD file descriptor changed")
+            try:
+                result = self.read()
+            except OSError as err:
+                if err.errno == errno.EAGAIN:
+                    return
+                raise
+        except Exception as exc:
+            fut.get_loop().remove_reader(fd)
+            fut.set_exception(exc)
+        else:
+            fut.get_loop().remove_reader(fd)
+            fut.set_result(result)
+
+    def aread(self) -> typing.Awaitable[int]:
+        """Decrease the value of the eventfd asynchronously. It mst have been
+        constructed using EventFDFlags.NONBLOCK.
+        """
+        if self.fd < 0:
+            raise ValueError("attempt to read from closed eventfd")
+        loop = asyncio.get_running_loop()
+        fut: asyncio.Future[int] = loop.create_future()
+        loop.add_reader(self.fd, self.__handle_readable, self.fd, fut)
+        return fut
+
     def write(self, value: int = 1) -> None:
         """Add the given value to the eventfd using eventfd_write."""
         if self.fd < 0: