SCP uploads hang in TotalCommander

[levshutov]

Dropbear running on Openwrt has a strange issue in SCP mode. It is "incompatible" with TotalCommander SFTP plugin.
The issue is described here https://www.ghisler.ch/board/viewtopic.php?p=424348#p424348

The issue is caused by commit 8e6f73e (second part, svr-chansession.c).

[mkj]

I'll have a look.

Pasting the forum report below for future.

TC SFTP has some kind of conflict with the latest Openwrt SSH server Dropbear v2022.xx.
It works fine with remote devices (several ms latency).
But it hangs in some situations with local devices (less 1ms latency).

Problem criteria:

1. Local network device with low latency (less 1ms)
2. Modern device with multiple cores
3. Dropbear v2022.xx SSH server

SFTP upload operation of large file (several MB) may freeze TC for about a minute.

Openwrt 22.03.2 running in VirtualBox can be used for problem emulation.

[kodidu]

@mkj did you already look into this issue

we are also suffering from this change:

return chansess->exit.exitpid != -1;

to

return chansess->pid == 0 || chansess->exit.exitpid != -1;

we are using some random java ssh library which keeps the session open and executes multiple nopty exec command via channels.
Previously the library received exit codes and now randomly the library does not get an exit code from dropbear anymore. e.g. dropbear did not wait for the child process to exit.

maybe see also #258