usb: Add critical section to the USB Buffer write path.

This isn't needed if the same thread is always the writer, but is needed if
multiple threads (or main thread plus callback irq) try to write the
buffer.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <[email protected]>

Author: projectgus

micropython/usb/usb-device/usb/device/core.py

@@ -780,11 +780,13 @@ def pend_write(self, wmax=None):
         #
         # If wmax is set then the memoryview is pre-sliced to be at most
         # this many bytes long.
-        #
-        # (No critical section needed as self._w is only updated by the producer.)
-        self._w = self._n
-        end = (self._w + wmax) if wmax else len(self._b)
-        return self._b[self._w : end]
+        ist = machine.disable_irq()
+        try:
+            self._w = self._n
+            end = (self._w + wmax) if wmax else len(self._b)
+            return self._b[self._w : end]
+        finally:
+            machine.enable_irq(ist)
 
     def finish_write(self, nbytes):
         # Called by the producer to indicate it wrote nbytes into the buffer.