shell: say what a long command line's timeout probably means

[ale-rinaldi]

Closes #3.

The problem

Past a limit of its own, a remote shell truncates the typed command line and says nothing at all. The exit-code marker sits at the end of that line, so it goes with it, and -c waits for output that cannot come — then reports:

dwshell: timed out waiting for command to finish

which explains nothing. That is what the reporter of #1 hit, and why they spent time bisecting.

Why not something better

The limits belong to the remote shell and differ by shell — roughly a megabyte with bash/zsh, 4094 characters under sh/dash/ash (the tty cutting a canonical-mode read), 8190 on cmd.exe. dwshell cannot know which shell will answer, so it cannot predict this, and guessing would deny commands the remote would have run.

Detecting it properly was tried and reverted: the probe in v1.3.9 typed an extra line at the remote, which any command reading stdin consumed — dwshell host -c "cat > file" wrote dwshell's own marker into the user's file. Trading a hang for silent data corruption is the worse outcome.

What this does instead

Nothing is added to the wire and nothing is typed at the remote. The timeout reads two things dwshell already has locally — the length of the line it sent, and whether anything ran — and past 4000 characters, the lowest point any measured shell truncates at, names truncation as a possible cause:

dwshell: timed out waiting for command to finish; the command line was 12000
characters, and past a limit of its own the remote shell truncates the line
silently, losing the exit-code marker — if that is what happened here, upload
it with `dwshell put` and run it by path

It stays a possibility, never a diagnosis. A command that is simply still running looks identical from here, which is why the hint is gated on a length that could actually have been cut. Below that length the message is unchanged.

Verification

Case	Result
12000-character line, Windows agent	hint, naming the length and the workaround
sleep 60 against an 8 s timeout	plain message — no misleading hint
Unit tests	short line, long line, and nothing-ran

gofmt, go vet, go test -race ./... green.

🤖 Generated with Claude Code

https://claude.ai/code/session_01MvidAFW9a2r4hTgHPW9ywG