Handle USB TX writes that die mid-message (device off the bus)

[patrickrb]

Problem

From the 2026-07-03 POTA activation log: 5 of 265 transmissions silently died — libusb native write FAILED (rc=5 UNKNOWN) ... TX DROPPED, no audio sent this cycle (rig keyed but silent). Two were load-bearing (the first RR73 to N2JFD at 15:26:29 and the first R-13 to W5TRL at 15:33:29 went out as dead air), stretching those QSOs by several cycles. Companion fix to #406.

rc=5 was mislabelled: onOutputComplete (in usb_audio_capture.cpp) stores the failing transfer's positive libusb_transfer_status as nativeWrite's return value, but describeLibusbWriteError only mapped the negative LIBUSB_ERROR_* codes. 5 = LIBUSB_TRANSFER_NO_DEVICE: the USB sound card fell off the bus mid-transmission — classic RF-into-the-USB-cable at TX power in a portable setup (a re-enumeration follows the 15:09:07 drop in the log). Field mitigation: ferrite chokes on the USB leg.

Changes

1. Correct diagnostics — describeLibusbWriteError now names the positive transfer statuses (rc=5 TRANSFER_NO_DEVICE instead of UNKNOWN).
2. No restart-from-zero on air — the UsbRequest fallback rewrites the whole message from byte 0. That's only sane when the native attempt failed before streaming began (its original purpose: kernels where libusb can't run). New shouldFallbackToUsbRequest(rc, elapsedMs) gate: mid-stream deaths (>1 s in), device-gone (NO_DEVICE/TRANSFER_NO_DEVICE), and user-cancelled writes drop the cycle cleanly. Previously this only "worked" because request.initialize() happens to fail fast when the device is gone.
3. Operator visibility — a dropped TX write now shows an on-screen warning ("TX audio failed — no signal was sent this cycle"); at the rig everything looks normal, so the operator otherwise can't tell they transmitted silence. The user's own STOP press does not warn (shouldWarnTxDropped). String added to all 16 locales.

No attempt to salvage the in-flight message: a mid-message gap breaks the FT8 symbol grid, and the protocol-level repeat already handles the retry.

Tests

• UsbAudioWriteErrorTest: transfer-status names (including the previously-asserted-wrong rc=5 UNKNOWN case, now TRANSFER_NO_DEVICE), and the full shouldFallbackToUsbRequest matrix (setup failure allowed, mid-stream/boundary/device-gone/cancelled/success denied).
• FT8TransmitSignalTest: shouldWarnTxDropped matrix.
• Full testDebugUnitTest suite green.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 20.80%. Comparing base (68398ec) to head (02d821a).

Additional details and impacted files

@@            Coverage Diff            @@
##                dev     #407   +/-   ##
=========================================
  Coverage     20.80%   20.80%           
  Complexity      133      133           
=========================================
  Files           148      148           
  Lines         19297    19297           
  Branches       2876     2876           
=========================================
  Hits           4015     4015           
  Misses        15115    15115           
  Partials        167      167           

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android	12.19% <ø> (ø)
native	9.93% <ø> (ø)

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[Copilot]

Pull request overview

Improves robustness and operator visibility when USB TX audio writes fail mid-message (e.g., USB audio device dropping off the bus), by distinguishing libusb setup errors from mid-stream transfer-status failures and preventing unsafe “restart-from-zero” fallbacks.

Changes:

• Extend describeLibusbWriteError() to name positive libusb_transfer_status values (e.g., rc=5 TRANSFER_NO_DEVICE).
• Add shouldFallbackToUsbRequest(rc, elapsedMs) gating to avoid UsbRequest restart after mid-stream/native failures or device-gone/cancelled scenarios.
• Add an operator-facing warning toast for dropped TX audio, plus unit tests covering the new decision matrices.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
ft8af/app/src/main/java/com/k1af/ft8af/wave/UsbAudioDevice.java	Adds native-write elapsed-time gating and maps positive transfer-status codes for clearer diagnostics.
ft8af/app/src/main/java/com/k1af/ft8af/ft8transmit/FT8TransmitSignal.java	Shows on-screen warning when TX audio is dropped (except user-cancel).
ft8af/app/src/test/java/com/k1af/ft8af/wave/UsbAudioWriteErrorTest.java	Adds tests for transfer-status naming and fallback gating matrix.
ft8af/app/src/test/java/com/k1af/ft8af/ft8transmit/FT8TransmitSignalTest.java	Adds tests for the “warn on dropped TX audio” decision matrix.
ft8af/app/src/main/res/values/strings_compose.xml	Adds base tx_audio_dropped string.
ft8af/app/src/main/res/values-ar/strings_compose.xml	Adds Arabic translation for tx_audio_dropped.
ft8af/app/src/main/res/values-cs/strings_compose.xml	Adds Czech translation for tx_audio_dropped.
ft8af/app/src/main/res/values-es/strings_compose.xml	Adds Spanish translation for tx_audio_dropped.
ft8af/app/src/main/res/values-fr/strings_compose.xml	Adds French translation for tx_audio_dropped.
ft8af/app/src/main/res/values-in/strings_compose.xml	Adds Indonesian translation for tx_audio_dropped.
ft8af/app/src/main/res/values-it/strings_compose.xml	Adds Italian translation for tx_audio_dropped.
ft8af/app/src/main/res/values-ja/strings_compose.xml	Adds Japanese translation for tx_audio_dropped.
ft8af/app/src/main/res/values-ko/strings_compose.xml	Adds Korean translation for tx_audio_dropped.
ft8af/app/src/main/res/values-nl/strings_compose.xml	Adds Dutch translation for tx_audio_dropped.
ft8af/app/src/main/res/values-pl/strings_compose.xml	Adds Polish translation for tx_audio_dropped.
ft8af/app/src/main/res/values-ru/strings_compose.xml	Adds Russian translation for tx_audio_dropped.
ft8af/app/src/main/res/values-tr/strings_compose.xml	Adds Turkish translation for tx_audio_dropped.
ft8af/app/src/main/res/values-uk/strings_compose.xml	Adds Ukrainian translation for tx_audio_dropped.
ft8af/app/src/main/res/values-zh-rCN/strings_compose.xml	Adds Simplified Chinese translation for tx_audio_dropped.
ft8af/app/src/main/res/values-zh-rTW/strings_compose.xml	Adds Traditional Chinese translation for tx_audio_dropped.

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