prevent running key exchange if hello-only flag set

[thegwan]

Fixed hello-only bug to prevent key exchange sequence from entering at all if the hello-only flag is set

[thegwan]

It looks like the continuous integration test is failing on cannot find package "github.com/cespare/xxhash/v2", but I don't think I'm using this?

[dadrian]

@thegwan if you cherry-pick 8e713fa onto this PR, it should fix CI

[codyprime]

The code looks correct, but my only concern is the duplication of the cleanup to unblock writers, and the mutex lock taken before a conditional and then unlocked in the if/else branches of the conditional.

Could the same thing be accomplished by inverting the HelloOnly check, like so:

	t.mu.Lock()

	firstKex := t.sessionID == nil
	if !t.config.HelloOnly {
		err = t.enterKeyExchangeLocked(p)
		if err != nil {
			// drop connection
			t.conn.Close()
			t.writeError = err
		}

		if debugHandshake {
			log.Printf("%s exited key exchange (first %v), err %v", t.id(), firstKex, err)
		}
	}
	// Unblock writers.
	t.sentInitMsg = nil
	t.sentInitPacket = nil
	t.cond.Broadcast()
	t.writtenSinceKex = 0
	t.mu.Unlock()

	if err != nil {
		return nil, err
	}

This does create an unnecessary err check at the end iif HelloOnly is true, but reduces code duplication and keeps the mutex lock/unlock balanced outside conditionals.

[thegwan]

sounds good, making the change