vssh: do not busy-loop in blocking state machine without a timeout

[aeroyorch]

Avoid busy loop in SSH state machines (both libssh and libssh2) when Curl_timeleft_ms() returns 0 (i.e. no timeout).

[bagder]

Ideally of course we should not use these blocking functions at all, but while we do I think this is a correct fix.

[Copilot]

Copilot review overview

🟢 Approval recommended

Both SSH backends correctly avoid zero-timeout polling without affecting finite timeout behavior.

Review effort: Lite
Findings: None

What changed in this PR

Prevents libssh and libssh2 blocking state machines from busy-looping when no transfer timeout is configured.

Changes:

• Uses a 1-second wait when Curl_timeleft_ms() returns zero.
• Preserves existing timeout capping behavior.

File	Description
lib/​vssh/​libssh2.c	Fixes socket wait timeout handling.
lib/​vssh/​libssh.c	Fixes readable-socket wait timeout handling.

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[aeroyorch]

Ideally of course we should not use these blocking functions at all, but while we do I think this is a correct fix.

Indeed. I ran into this while using the multi interface with async python wrapper for SFTP transfers and realized the SFTP paths still block. Is anyone working on making them non-blocking? If not, I'd be happy to contribute.

[bagder]

Is anyone working on making them non-blocking? If not, I'd be happy to contribute.

I'm not aware of anyone working on these, no.

[bagder]

Thanks!