udev: assume that the recv buffer size of the netlink socket is already configured when the socket is passed in

This makes ReceiveBuffer= option in systemd-udevd-kernel.socket unit useful.

src/udev/udevd.c

@@ -1577,7 +1577,11 @@ static int manager_new(Manager **ret, int fd_ctrl, int fd_uevent, const char *cg
         if (r < 0)
                 return log_error_errno(r, "Failed to initialize device monitor: %m");
 
-        (void) sd_device_monitor_set_receive_buffer_size(manager->monitor, 128 * 1024 * 1024);
+        /* Bump receiver buffer, but only if we are not called via socket activation, as in that
+         * case systemd sets the receive buffer size for us, and the value in the .socket unit
+         * should take full effect. */
+        if (fd_uevent < 0)
+                (void) sd_device_monitor_set_receive_buffer_size(manager->monitor, 128 * 1024 * 1024);
 
         r = device_monitor_enable_receiving(manager->monitor);
         if (r < 0)

test/test-network/systemd-networkd-tests.py

@@ -2211,7 +2211,10 @@ class NetworkdNetworkTests(unittest.TestCase, Utilities):
         output = check_output('tc qdisc show dev dummy98')
         print(output)
         self.assertRegex(output, 'qdisc fq')
-        self.assertRegex(output, 'limit 1000p flow_limit 200p buckets 512 orphan_mask 511 quantum 1500 initial_quantum 13000 maxrate 1Mbit')
+        self.assertRegex(output, 'limit 1000p flow_limit 200p buckets 512 orphan_mask 511')
+        self.assertRegex(output, 'quantum 1500')
+        self.assertRegex(output, 'initial_quantum 13000')
+        self.assertRegex(output, 'maxrate 1Mbit')
         self.assertRegex(output, 'qdisc codel')
         self.assertRegex(output, 'limit 2000p target 10.0ms ce_threshold 100.0ms interval 50.0ms ecn')