Mosh shouldn't abort if closing an `fd` fails due to non-critical reasons

[yashvsingh]

At https://github.com/mobile-shell/mosh/blob/master/src/network/network.cc#L605 there is fatal_assert() even when the error is ignorable (e.g. EIO, see https://man7.org/linux/man-pages/man2/close.2.html )
We should check errono and if the error is ignorable (e.g. I/O failed) mosh shouldn't abort. (We should also log the errono).

I am encountering this assert just 2 minutes after disconnecting from the network.

[achernya]

Can you provide more details about the environment in which you are getting EIO? That seems more interesting to me to figure out how to resolve this.

I disagree with the proposed solution of ignoring the failures -- this is a C++ destructor, which is not allowed to fail. If close() becomes something that needs retrying then we need more infrastructure to handle this as to not leak the FD.

[yashvsingh]

I can't tell whether I am getting EIO as the error is not printed by mosh. But I am hitting the above mentioned assert. Note that I hit it only when disconnected from VPN but not from WiFi. If I disconnect from WiFi then it doesn't hit the assert. The server address can be reached only over (company) VPN

[achernya]

What system are you on? If you're on Linux you can strace a running process. Do you have detailed steps on how to reproduce this behavior? In general I wouldn't expect mosh to try to close a socket during its regular operation.

[yashvsingh]

Sorry I should have mentioned, I am on Mac OS 15.6.1

1. Enable VPN.
2. mosh <server IP>. The IP can be reached only over the VPN.
3. Disconnect VPN. Wait for 2 minutes. (mosh prints "Last contact ... ago")
4. Mosh aborts with the above assert

[achernya]

Which version of mosh is this? Is this IPv4 or IPv6? Is the connection fully established and you have a shell before you disconnect?

[yashvsingh]

mosh 1.4.0 [build mosh 1.4.0]
Copyright 2012 Keith Winstein <mosh-devel@mit.edu>
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

It's using IPv4. Yes the connection gets established with a shell

[achernya]

It would be good to see the routing tables before and after the VPN disconnect (netstar -rn on macOS, I believe). My best guess right now is macOS is doing something different than we expect when the destination address is not reputable rather than the usual "No route to host". But I would also still need to trace the code to see why mosh is attempting a shutdown, that doesn't make sense to me.

[yashvsingh]

Hmm, if I ping the IP after disconnecting VPN, ping doesn't say "no route to host" but just "request timeout". When I disconnect Wifi it says "no route to host"

[yashvsingh]

Also, interestingly, mosh always crashes at exactly 1 minutes 54 seconds after VPN disconnection with the following message

Fatal assertion failure in function ~Socket at network.cc:636
Failed test: close( _fd ) == 0