Serial log can be truncated at end of wokwi-cli run

[egnor]

System info

% uname -a
Linux roc 6.17.0-6-generic #6-Ubuntu SMP PREEMPT_DYNAMIC Tue Oct  7 13:34:17 UTC 2025 x86_64 GNU/Linux
% arduino-cli version
arduino-cli  Version: 1.3.1 Commit: 08ff7e2b Date: 2025-08-28T13:51:45Z
% wokwi-cli --version
Wokwi CLI v0.19.0 (95ac9a33d1df)

Reproduction steps

1. install arduino-cli and wokwi-cli per above
2. unzip the test sketch: wokwi_bug.zip
3. cd wokwi_bug
4. arduino-cli compile --export-binaries
5. wokwi-cli --scenario scenario.yaml --serial-log-file serial.txt && tail serial.txt

Expected result

serial.txt contains the whole serial log as printed on stdout, including 100 nana- followed by batman!

% tail serial.txt
nana-
nana-
nana-
nana-
nana-
nana-
nana-
nana-
nana-
batman

Actual result

NOTE this often takes multiple tries to reproduce!! Re-run the last command as needed. When I ran it 10 times, I had 5 successes and 5 failures. On failure the actual amount of output varies.

% tail serial.txt
nana-
nana-
nana-
nana-
nana-
nana-
nana-
nana-
nana-
nan

Discussion

The full serial output is always printed to stdout, it just doesn't make it into the log file for some reason. Feels like the program exits before fully writing the serial log file?

[egnor]

Looking at the code, I THINK the problem MAY be that serialLogStream, which is created by fs.createWriteStream, is written to without waiting for completion:

wokwi-cli/packages/cli/src/main.ts

Line 344 in 8108a85

serialLogStream?.write(Buffer.from(bytes));

However, stream write() buffers the chunk and offers a completion callback to the caller. So the chunk may not be actually written if the program exits.

One solution could be to add to the finally block at the end of main()...

wokwi-cli/packages/cli/src/main.ts

Line 392 in 8108a85

} finally {

...code to fully drain the log stream if one is present:

  if (serialLogStream) {
    await new Promise(cb => { serialLogStream.on('finish', cb); serialLogStream.end(); });
  }

However, I'm not fluent in nodejs and there may be a more idiomatic way to do things??

[urish]

Thanks for reporting (and trying to find a solution), looking into this

[urish]

I wasn't able to reproduce here, but I've applied a fix (7d706b4) that I hope will solve the issue. Can you please try again with v0.19.1 and report if it helped?