hil: make serial write timeout fatal

pyserial's posix write() raises SerialTimeoutException after partial
progress with the byte count lost, so the retry loop re-sent from the
same offset and could duplicate bytes on the wire — surfacing as bogus
data-mismatch failures that look like device firmware bugs.
write_timeout is already a total per-call deadline, so the loop added
duplication risk without extending the budget: write once and treat a
timeout as fatal. Default bumped 2 -> 10 s to keep the old overall
bound; HIL_SERIAL_WRITE_DEADLINE removed.

The per-character CLI loops keep their existing pacing (the 2 ms sleep
between single-byte writes already spaces them on the wire); no
unbounded ser.flush()/tcdrain is added.

Review follow-up for #3643 (hil_test.py l.257/264 findings).

Co-Authored-By: Claude Fable 5 <[email protected]>

1 files changed, 9 insertions, 20 deletions

diff --git a/test/hil/hil_test.py b/test/hil/hil_test.py
index c2ba34f02..aee59e05a 100755
--- a/test/hil/hil_test.py
+++ b/test/hil/hil_test.py
@@ -145,8 +145,7 @@ class HilConfig(TypedDict):
 CMD_TIMEOUT = int(os.getenv('HIL_CMD_TIMEOUT', '180'))
 POOL_TIMEOUT = int(os.getenv('HIL_POOL_TIMEOUT', '3000'))
 SERIAL_READ_TIMEOUT = float(os.getenv('HIL_SERIAL_READ_TIMEOUT', '5'))
-SERIAL_WRITE_TIMEOUT = float(os.getenv('HIL_SERIAL_WRITE_TIMEOUT', '2'))
-SERIAL_WRITE_DEADLINE = float(os.getenv('HIL_SERIAL_WRITE_DEADLINE', '10'))
+SERIAL_WRITE_TIMEOUT = float(os.getenv('HIL_SERIAL_WRITE_TIMEOUT', '10'))
 
 
 def cmd_stdout_text(out: Any) -> str:
@@ -247,24 +246,14 @@ def open_serial_dev(port: str):
     return ser
 
 
-def serial_write_all(ser: serial.Serial, data: bytes, deadline: float = SERIAL_WRITE_DEADLINE):
-    total = 0
-    end = time.monotonic() + deadline
-
-    while total < len(data):
-        try:
-            written = ser.write(data[total:])
-        except serial.SerialTimeoutException:
-            written = 0
-
-        if written:
-            total += written
-            continue
-
-        if time.monotonic() >= end:
-            raise AssertionError(f'Serial write timeout after {deadline:.1f}s')
-
-        time.sleep(0.01)
+def serial_write_all(ser: serial.Serial, data: bytes):
+    # write_timeout is a total deadline for the whole call (pyserial keeps partial progress
+    # internally). A timeout means the device stopped draining — treat it as fatal: pyserial
+    # loses the partial-write count on raise, so retrying would duplicate bytes on the wire.
+    try:
+        ser.write(data)
+    except serial.SerialTimeoutException:
+        raise AssertionError(f'Serial write timeout after {SERIAL_WRITE_TIMEOUT:.1f}s')
 
 
 def read_disk_file(uid: str, lun: int, fname: str) -> bytes: