transport: cleaner Transport list instantiation

Previously if an import of a dependent module (usb1, hid) failed, import
of the whole transport module would fail. This was resolved by catching
ImportErrors in the all_transports method.

This had two drawbacks:
- if something other than ImportError happened - e.g., libusb would
raise OSError if it couldn't find libusb.so - all_transports would crash
anyway
- at the same time, if a legitimately needed dependency
(typing_extensions) was missing, this would be masked by the ImportError
handling.

Instead, we unconditionally import the modules, and inside each one,
wrap dependencies in a try-except.

As an added benefit, it is now possible to disable a transport just by
setting SomeTransport.ENABLED = False

trezorlib/transport/__init__.py

@@ -61,6 +61,7 @@ class Transport:
     """
 
     PATH_PREFIX = None  # type: str
+    ENABLED = False
 
     def __str__(self) -> str:
         return self.get_path()
@@ -100,21 +101,16 @@ class Transport:
 
 
 def all_transports() -> Iterable[Type[Transport]]:
-    transports = set()  # type: Set[Type[Transport]]
+    from .bridge import BridgeTransport
-    for modname in ("bridge", "hid", "udp", "webusb"):
+    from .hid import HidTransport
-        try:
+    from .udp import UdpTransport
-            # Import the module and find the Transport class.
+    from .webusb import WebUsbTransport
-            # To avoid iterating over every item, the module should assign its Transport class
-            # to a constant named TRANSPORT.
-            module = importlib.import_module("." + modname, __name__)
-            try:
-                transports.add(getattr(module, "TRANSPORT"))
-            except AttributeError:
-                LOG.warning("Skipping broken module {}".format(modname))
-        except ImportError as e:
-            LOG.info("Failed to import module {}: {}".format(modname, e))
 
-    return transports
+    return set(
+        cls
+        for cls in (BridgeTransport, HidTransport, UdpTransport, WebUsbTransport)
+        if cls.ENABLED
+    )
 
 
 def enumerate_devices() -> Iterable[Transport]:

trezorlib/transport/bridge.py

@@ -102,6 +102,7 @@ class BridgeTransport(Transport):
     """
 
     PATH_PREFIX = "bridge"
+    ENABLED = True
 
     def __init__(
         self, device: Dict[str, Any], legacy: bool, debug: bool = False
@@ -177,6 +178,3 @@ class BridgeTransport(Transport):
             extra={"protobuf": msg},
         )
         return msg
-
-
-TRANSPORT = BridgeTransport

trezorlib/transport/hid.py

@@ -14,15 +14,23 @@
 # You should have received a copy of the License along with this library.
 # If not, see <https://www.gnu.org/licenses/lgpl-3.0.html>.
 
+import logging
 import sys
 import time
 from typing import Any, Dict, Iterable
 
-import hid
-
 from . import DEV_TREZOR1, UDEV_RULES_STR, TransportException
 from .protocol import ProtocolBasedTransport, ProtocolV1
 
+LOG = logging.getLogger(__name__)
+
+try:
+    import hid
+except Exception as e:
+    LOG.info("HID transport is disabled: {}".format(e))
+    hid = None
+
+
 HidDevice = Dict[str, Any]
 HidDeviceHandle = Any
 
@@ -87,6 +95,7 @@ class HidTransport(ProtocolBasedTransport):
     """
 
     PATH_PREFIX = "hid"
+    ENABLED = hid is not None
 
     def __init__(self, device: HidDevice, hid_handle: HidHandle = None) -> None:
         if hid_handle is None:
@@ -135,6 +144,3 @@ def is_wirelink(dev: HidDevice) -> bool:
 
 def is_debuglink(dev: HidDevice) -> bool:
     return dev["usage_page"] == 0xFF01 or dev["interface_number"] == 1
-
-
-TRANSPORT = HidTransport

trezorlib/transport/udp.py

@@ -30,6 +30,7 @@ class UdpTransport(ProtocolBasedTransport):
     DEFAULT_HOST = "127.0.0.1"
     DEFAULT_PORT = 21324
     PATH_PREFIX = "udp"
+    ENABLED = True
 
     def __init__(self, device: str = None) -> None:
         if not device:
@@ -123,6 +124,3 @@ class UdpTransport(ProtocolBasedTransport):
         if len(chunk) != 64:
             raise TransportException("Unexpected chunk size: %d" % len(chunk))
         return bytearray(chunk)
-
-
-TRANSPORT = UdpTransport

trezorlib/transport/webusb.py

@@ -15,15 +15,22 @@
 # If not, see <https://www.gnu.org/licenses/lgpl-3.0.html>.
 
 import atexit
+import logging
 import sys
 import time
 from typing import Iterable, Optional
 
-import usb1
-
 from . import TREZORS, UDEV_RULES_STR, TransportException
 from .protocol import ProtocolBasedTransport, ProtocolV1
 
+LOG = logging.getLogger(__name__)
+
+try:
+    import usb1
+except Exception as e:
+    LOG.warning("WebUSB transport is disabled: {}".format(e))
+    usb1 = None
+
 if False:
     # mark Optional as used, otherwise it only exists in comments
     Optional
@@ -84,6 +91,7 @@ class WebUsbTransport(ProtocolBasedTransport):
     """
 
     PATH_PREFIX = "webusb"
+    ENABLED = usb1 is not None
     context = None
 
     def __init__(
@@ -150,6 +158,3 @@ def dev_to_str(dev: usb1.USBDevice) -> str:
     return ":".join(
         str(x) for x in ["%03i" % (dev.getBusNumber(),)] + dev.getPortNumberList()
     )
-
-
-TRANSPORT = WebUsbTransport