twister: fix serial connection for flash_before

The --flash-before flag allows devices to be flashed before the serial
connection is established. However, the implementation was incomplete and
only worked if the port address already existed at the start of the run.
This is incompatible with devices that implement the USB in software
(eg: USB-CDC).

This commit fixes the implementation to delay setting up the connection
until after the device is flashed, and to retry the connection for two
seconds after flashing to give the device time to enumerate as a USB
device.

Signed-off-by: Mike Szczys <mike@golioth.io>

scripts/pylib/pytest-twister-harness/src/twister_harness/device/device_adapter.py

@@ -14,6 +14,7 @@ import threading
 import time
 from datetime import datetime
 from pathlib import Path
+from serial import SerialException
 
 from twister_harness.exceptions import (
     TwisterHarnessException,
@@ -69,14 +70,23 @@ class DeviceAdapter(abc.ABC):
 
         if self.device_config.type != 'hardware':
             self._flash_and_run()
+            self._device_run.set()
+            self._start_reader_thread()
+            self.connect()
+            return
 
         self._device_run.set()
         self._start_reader_thread()
-        self.connect()
 
-        if self.device_config.type == 'hardware':
+        if self.device_config.flash_before:
+            # For hardware devices with shared USB or software USB, connect after flashing.
+            # Retry for up to 10 seconds for USB-CDC based devices to enumerate.
+            self._flash_and_run()
+            self.connect(retry_s = 10)
+        else:
             # On hardware, flash after connecting to COM port, otherwise some messages
             # from target can be lost.
+            self.connect()
             self._flash_and_run()
 
     def close(self) -> None:
@@ -89,7 +99,7 @@ class DeviceAdapter(abc.ABC):
         self._device_run.clear()
         self._join_reader_thread()
 
-    def connect(self) -> None:
+    def connect(self, retry_s: int = 0) -> None:
         """Connect to device - allow for output gathering."""
         if self.is_device_connected():
             logger.debug('Device already connected')
@@ -98,7 +108,20 @@ class DeviceAdapter(abc.ABC):
             msg = 'Cannot connect to not working device'
             logger.error(msg)
             raise TwisterHarnessException(msg)
-        self._connect_device()
+
+        if retry_s > 0:
+            retry_cycles = retry_s * 10
+            for i in range(retry_cycles):
+                try:
+                    self._connect_device()
+                    break
+                except SerialException:
+                    if i == retry_cycles - 1:
+                        raise
+                    time.sleep(0.1)
+        else:
+            self._connect_device()
+
         self._device_connected.set()
 
     def disconnect(self) -> None:

scripts/pylib/pytest-twister-harness/src/twister_harness/plugin.py

@@ -84,6 +84,13 @@ def pytest_addoption(parser: pytest.Parser):
         '--device-serial-pty',
         help='Script for controlling pseudoterminal.'
     )
+    twister_harness_group.addoption(
+        '--flash-before',
+        type=bool,
+        help='Flash device before attaching to serial port'
+             'This is useful for devices that share the same port for programming'
+             'and serial console, or use soft-USB, where flash must come first.'
+    )
     twister_harness_group.addoption(
         '--west-flash-extra-args',
         help='Extend parameters for west flash. '

scripts/pylib/pytest-twister-harness/src/twister_harness/twister_harness_config.py

@@ -26,6 +26,7 @@ class DeviceConfig:
     id: str = ''
     product: str = ''
     serial_pty: str = ''
+    flash_before: bool = False
     west_flash_extra_args: list[str] = field(default_factory=list, repr=False)
     name: str = ''
     pre_script: Path | None = None
@@ -62,6 +63,7 @@ class TwisterHarnessConfig:
             id=config.option.device_id,
             product=config.option.device_product,
             serial_pty=config.option.device_serial_pty,
+            flash_before=bool(config.option.flash_before),
             west_flash_extra_args=west_flash_extra_args,
             pre_script=_cast_to_path(config.option.pre_script),
             post_script=_cast_to_path(config.option.post_script),

scripts/pylib/twister/twisterlib/environment.py

@@ -178,7 +178,7 @@ Artificially long but functional example:
     parser.add_argument("--flash-before", action="store_true", default=False,
                         help="""Flash device before attaching to serial port.
                         This is useful for devices that share the same port for programming
-                        and serial console, where flash must come first.
+                        and serial console, or use soft-USB, where flash must come first.
                         """)
 
     test_or_build.add_argument(

scripts/pylib/twister/twisterlib/harness.py

@@ -408,6 +408,9 @@ class Pytest(Harness):
         if hardware.post_script:
             command.append(f'--post-script={hardware.post_script}')
 
+        if hardware.flash_before:
+            command.append(f'--flash-before={hardware.flash_before}')
+
         return command
 
     def run_command(self, cmd, timeout):