coredevice: Re-enable TCP keepalive

This partially reverts commit b5e1bd3fa2,
which had removed keepalive. This, however, led to experiments
hanging forever if the core device had dropped the connection
(e.g. to a kernel CPU panic, or the device being rebooted).

The chosen keepalive settings are fairly conservative (with the
10 s timeout) to avoid any possible interaction with smoltcp's
3 s ARP try interval (see GitHub issue #1150), even though this
should be a non-issue now due to the larger ARP cache.

artiq/coredevice/comm.py

@@ -0,0 +1,28 @@
+import sys
+import socket
+import logging
+
+logger = logging.getLogger(__name__)
+
+
+def set_keepalive(sock, after_idle, interval, max_fails):
+    if sys.platform.startswith("linux"):
+        sock.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)
+        sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPIDLE, after_idle)
+        sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPINTVL, interval)
+        sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPCNT, max_fails)
+    elif sys.platform.startswith("win") or sys.platform.startswith("cygwin"):
+        # setting max_fails is not supported, typically ends up being 5 or 10
+        # depending on Windows version
+        sock.ioctl(socket.SIO_KEEPALIVE_VALS,
+                   (1, after_idle * 1000, interval * 1000))
+    else:
+        logger.warning("TCP keepalive not supported on platform '%s', ignored",
+                       sys.platform)
+
+
+def initialize_connection(host, port):
+    sock = socket.create_connection((host, port))
+    set_keepalive(sock, 10, 10, 3)
+    logger.debug("connected to %s:%d", host, port)
+    return sock

artiq/coredevice/comm_kernel.py

@@ -8,6 +8,7 @@ from fractions import Fraction
 from collections import namedtuple
 
 from artiq.coredevice import exceptions
+from artiq.coredevice.comm import initialize_connection
 from artiq import __version__ as software_version
 
 
@@ -184,8 +185,7 @@ class CommKernel:
     def open(self):
         if hasattr(self, "socket"):
             return
-        self.socket = socket.create_connection((self.host, self.port))
-        logger.debug("connected to %s:%d", self.host, self.port)
+        self.socket = initialize_connection(self.host, self.port)
         self.socket.sendall(b"ARTIQ coredev\n")
         endian = self._read(1)
         if endian == b"e":

artiq/coredevice/comm_mgmt.py

@@ -1,8 +1,9 @@
 from enum import Enum
 import logging
-import socket
 import struct
 
+from artiq.coredevice.comm import initialize_connection
+
 
 logger = logging.getLogger(__name__)
 
@@ -60,8 +61,7 @@ class CommMgmt:
     def open(self):
         if hasattr(self, "socket"):
             return
-        self.socket = socket.create_connection((self.host, self.port))
-        logger.debug("connected to %s:%d", self.host, self.port)
+        self.socket = initialize_connection(self.host, self.port)
         self.socket.sendall(b"ARTIQ management\n")
         endian = self._read(1)
         if endian == b"e":