util/agents/usb_hid_relay: fix concurrent access

Since c9fc5bf, it was no longer possible to use the same USB relay
device from multiple labgrid processes, as the USB device was kept open
and claimed.

To fix this, we use a context manager which first claims the USB
interface (with retry while busy) and releases it after the transaction.

With this fix, multiple processes can toggle outputs in a busy loop
without causing 'USBError(16, 'Resource busy')' failures.

Fixes: c9fc5bf ("labgrid/util/agents/usb_hid_relay: keep the USB device open")
Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
(cherry picked from commit 659f7df)
Signed-off-by: Jan Luebbe <jlu@pengutronix.de>

Author: jluebbe

labgrid/util/agents/usb_hid_relay.py

@@ -11,6 +11,10 @@
 - Turn digital output on and off
 """
 
+import errno
+from contextlib import contextmanager
+from time import monotonic, sleep
+
 import usb.core
 import usb.util
 
@@ -26,18 +30,35 @@ def __init__(self, **args):
             raise ValueError("Device not found")
 
         if self._dev.idVendor == 0x16C0:
-            self.set_output = self.set_output_dcttech
-            self.get_output = self.get_output_dcttech
+            self._set_output = self._set_output_dcttech
+            self._get_output = self._get_output_dcttech
         elif self._dev.idVendor == 0x5131:
-            self.set_output = self.set_output_lcus
-            self.get_output = self.get_output_lcus
+            self._set_output = self._set_output_lcus
+            self._get_output = self._get_output_lcus
         else:
             raise ValueError(f"Unknown vendor/protocol for VID {self._dev.idVendor:x}")
 
         if self._dev.is_kernel_driver_active(0):
             self._dev.detach_kernel_driver(0)
 
-    def set_output_dcttech(self, number, status):
+    @contextmanager
+    def _claimed(self):
+        timeout = monotonic() + 1.0
+        while True:
+            try:
+                usb.util.claim_interface(self._dev, 0)
+                break
+            except usb.core.USBError as e:
+                if monotonic() > timeout:
+                    raise e
+                if e.errno == errno.EBUSY:
+                    sleep(0.01)
+                else:
+                    raise e
+        yield
+        usb.util.release_interface(self._dev, 0)
+
+    def _set_output_dcttech(self, number, status):
         assert 1 <= number <= 8
         req = [0xFF if status else 0xFD, number]
         self._dev.ctrl_transfer(
@@ -48,7 +69,7 @@ def set_output_dcttech(self, number, status):
             req,  # payload
         )
 
-    def get_output_dcttech(self, number):
+    def _get_output_dcttech(self, number):
         assert 1 <= number <= 8
         resp = self._dev.ctrl_transfer(
             usb.util.CTRL_TYPE_CLASS | usb.util.CTRL_RECIPIENT_DEVICE | usb.util.ENDPOINT_IN,
@@ -59,7 +80,7 @@ def get_output_dcttech(self, number):
         )
         return bool(resp[7] & (1 << (number - 1)))
 
-    def set_output_lcus(self, number, status):
+    def _set_output_lcus(self, number, status):
         assert 1 <= number <= 8
         ep_in = self._dev[0][(0, 0)][0]
         ep_out = self._dev[0][(0, 0)][1]
@@ -68,13 +89,18 @@ def set_output_lcus(self, number, status):
         ep_out.write(req)
         ep_in.read(64)
 
-    def get_output_lcus(self, number):
+    def _get_output_lcus(self, number):
         assert 1 <= number <= 8
         # we have no information on how to read the current value
         return False
 
-    def __del__(self):
-        usb.util.release_interface(self._dev, 0)
+    def set_output(self, number, status):
+        with self._claimed():
+            self._set_output(number, status)
+
+    def get_output(self, number):
+        with self._claimed():
+            self._get_output(number)
 
 
 _relays = {}