Method _send

Lib/multiprocessing/resource_tracker.py:282–313

(self, cmd, name, rtype)

        assert nbytes == len(msg), f"{nbytes=} != {len(msg)=}"

    def _send(self, cmd, name, rtype):
        if self._use_simple_format and '\n' not in name:
            msg = f"{cmd}:{name}:{rtype}\n".encode("ascii")
            if len(msg) > 512:
                # posix guarantees that writes to a pipe of less than PIPE_BUF
                # bytes are atomic, and that PIPE_BUF >= 512
                raise ValueError('msg too long')
            self._ensure_running_and_write(msg)
            return

        # POSIX guarantees that writes to a pipe of less than PIPE_BUF (512 on Linux)
        # bytes are atomic. Therefore, we want the message to be shorter than 512 bytes.
        # POSIX shm_open() and sem_open() require the name, including its leading slash,
        # to be at most NAME_MAX bytes (255 on Linux)
        # With json.dump(..., ensure_ascii=True) every non-ASCII byte becomes a 6-char
        # escape like \uDC80.
        # As we want the overall message to be kept atomic and therefore smaller than 512,
        # we encode encode the raw name bytes with URL-safe Base64 - so a 255 long name
        # will not exceed 340 bytes.
        b = name.encode('utf-8', 'surrogateescape')
        if len(b) > 255:
            raise ValueError('shared memory name too long (max 255 bytes)')
        b64 = base64.urlsafe_b64encode(b).decode('ascii')

        payload = {"cmd": cmd, "rtype": rtype, "base64_name": b64}
        msg = (json.dumps(payload, ensure_ascii=True, separators=(",", ":")) + "\n").encode("ascii")

        # The entire JSON message is guaranteed < PIPE_BUF (512 bytes) by construction.
        assert len(msg) <= 512, f"internal error: message too long ({len(msg)} bytes)"
        assert msg.startswith(b'{')

        self._ensure_running_and_write(msg)

_resource_tracker = ResourceTracker()
ensure_running = _resource_tracker.ensure_running