feat(core): clear out memory space after every workflow

A small fixed list of modules is kept pre-loaded in the GC arena.
These must not keep references to anything else, as all other modules
are unloaded and the memory is cleared.

core/src/main.py

@@ -1,73 +1,25 @@
 # isort:skip_file
+# fmt: off
 
-# unlock the device
-import boot  # noqa: F401
+# Import always-active modules
+import storage
+import storage.device
+from trezor import config, pin, utils  # noqa: F401
 
-# prepare the USB interfaces, but do not connect to the host yet
+# Prepare the USB interfaces first. Do not connect to the host yet.
 import usb
 
-from trezor import loop, utils, wire, workflow
+unimport_manager = utils.unimport()
+
+# unlock the device, unload the boot module afterwards
+with unimport_manager:
+    import boot
+    del boot
 
 # start the USB
-usb.bus.open()
+usb.bus.open(storage.device.get_device_id())
 
-
-def _boot_apps() -> None:
-    # load applications
-    import apps.base
-    import apps.management
-    import apps.bitcoin
-    import apps.misc
-
-    if not utils.BITCOIN_ONLY:
-        import apps.ethereum
-        import apps.lisk
-        import apps.monero
-        import apps.nem
-        import apps.stellar
-        import apps.ripple
-        import apps.cardano
-        import apps.tezos
-        import apps.eos
-        import apps.binance
-        import apps.webauthn
-
-    if __debug__:
-        import apps.debug
-
-    # boot applications
-    apps.base.boot()
-    apps.management.boot()
-    apps.bitcoin.boot()
-    apps.misc.boot()
-    if not utils.BITCOIN_ONLY:
-        apps.ethereum.boot()
-        apps.lisk.boot()
-        apps.monero.boot()
-        apps.nem.boot()
-        apps.stellar.boot()
-        apps.ripple.boot()
-        apps.cardano.boot()
-        apps.tezos.boot()
-        apps.eos.boot()
-        apps.binance.boot()
-        apps.webauthn.boot()
-    if __debug__:
-        apps.debug.boot()
-
-    # run main event loop and specify which screen is the default
-    apps.base.set_homescreen()
-    workflow.start_default()
-
-
-_boot_apps()
-
-# initialize the wire codec
-wire.setup(usb.iface_wire)
-if __debug__:
-    wire.setup(usb.iface_debug, is_debug_session=True)
-
-loop.run()
-
-# loop is empty. That should not happen
-utils.halt("All tasks have died.")
+while True:
+    with unimport_manager:
+        import session  # noqa: F401
+        del session

core/src/session.py

@@ -0,0 +1,28 @@
+from trezor import loop, utils, wire, workflow
+
+# load applications
+import apps.base
+import usb
+
+apps.base.boot()
+
+if not utils.BITCOIN_ONLY and usb.ENABLE_IFACE_WEBAUTHN:
+    import apps.webauthn
+
+    apps.webauthn.boot()
+
+if __debug__:
+    import apps.debug
+
+    apps.debug.boot()
+
+# run main event loop and specify which screen is the default
+apps.base.set_homescreen()
+workflow.start_default()
+
+# initialize the wire codec
+wire.setup(usb.iface_wire)
+if __debug__:
+    wire.setup(usb.iface_debug, is_debug_session=True)
+
+loop.run()

core/src/trezor/utils.py

@@ -27,20 +27,20 @@ if __debug__:
 if False:
     from typing import (
         Any,
-        Iterable,
         Iterator,
         Protocol,
         Union,
         TypeVar,
         Sequence,
+        Set,
     )
 
 
-def unimport_begin() -> Iterable[str]:
+def unimport_begin() -> Set[str]:
     return set(sys.modules)
 
 
-def unimport_end(mods: Iterable[str]) -> None:
+def unimport_end(mods: Set[str], collect: bool = True) -> None:
     for mod in sys.modules:
         if mod not in mods:
             # remove reference from sys.modules
@@ -58,7 +58,23 @@ def unimport_end(mods: Iterable[str]) -> None:
                 # referenced from the parent package. both is fine.
                 pass
     # collect removed modules
-    gc.collect()
+    if collect:
+        gc.collect()
+
+
+class unimport:
+    def __init__(self) -> None:
+        self.mods: Set[str] | None = None
+
+    def __enter__(self) -> None:
+        self.mods = unimport_begin()
+
+    def __exit__(self, _exc_type: Any, _exc_value: Any, _tb: Any) -> None:
+        assert self.mods is not None
+        unimport_end(self.mods, collect=False)
+        self.mods.clear()
+        self.mods = None
+        gc.collect()
 
 
 def ensure(cond: bool, msg: str | None = None) -> None:

core/src/trezor/wire/__init__.py

@@ -284,7 +284,7 @@ class UnexpectedMessageError(Exception):
 
 
 async def handle_session(
-    iface: WireInterface, session_id: int, is_debug_session: bool = True
+    iface: WireInterface, session_id: int, is_debug_session: bool = False
 ) -> None:
     if __debug__ and is_debug_session:
         ctx_buffer = WIRE_BUFFER_DEBUG
@@ -295,6 +295,8 @@ async def handle_session(
     res_msg: protobuf.MessageType | None = None
     req_type = None
     req_msg = None
+
+    modules = utils.unimport_begin()
     while True:
         try:
             if next_msg is None:
@@ -330,7 +332,8 @@ async def handle_session(
 
             # Take a mark of modules that are imported at this point, so we can
             # roll back and un-import any others.  Should not raise.
-            modules = utils.unimport_begin()
+            if is_debug_session:
+                modules = utils.unimport_begin()
 
             # We need to find a handler for this message type.  Should not
             # raise.
@@ -430,6 +433,10 @@ async def handle_session(
             # Unload modules imported by the workflow.  Should not raise.
             utils.unimport_end(modules)
 
+            if not is_debug_session and next_msg is None:  # and msg_type != 0:
+                loop.clear()
+                return
+
         except Exception as exc:
             # The session handling should never exit, just log and continue.
             if __debug__: