x/crypto/ssh: write errors should be returned immediately

[willmo]

Apologies if this is a duplicate or not actually a bug.

What version of Go are you using (go version)?

$ go version
go version go1.18.4 linux/amd64

x/crypto master

Does this issue reproduce with the latest release?

Yes, this behavior hasn't changed since it was introduced in golang/crypto@2e74c77 .

What did you do?

Write to a channel of an SSH connection that's interrupted, e.g. by receiving a TCP RST.

What did you expect to see?

A write error is reported as soon as writing to the underlying net.Conn fails.

What did you see instead?

The error is reported only on subsequent writes. This is not a dire problem, but seems odd/unnecessary.

In particular, handshakeTransport.writePacket sets t.writeError but returns nil. This seems to have been introduced in the aforelinked golang/crypto@2e74c77 .

Is there a missing return err here? If so, I can submit a CL.

[seankhliao]

cc @golang/security