fix: handle SIGHUP from OpenSSH (#10638)

Fixes an issue where remote forwards are not correctly torn down when using OpenSSH with `coder ssh --stdio`.  OpenSSH sends a disconnect signal, but then also sends SIGHUP to `coder`.  Previously, we just exited when we got SIGHUP, and this raced against properly disconnecting.

Fixes https://github.com/coder/customers/issues/327

cli/clitest/signal.go

@@ -0,0 +1,59 @@
+package clitest
+
+import (
+	"context"
+	"os"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+type FakeSignalNotifier struct {
+	sync.Mutex
+	t       *testing.T
+	ctx     context.Context
+	cancel  context.CancelFunc
+	signals []os.Signal
+	stopped bool
+}
+
+func NewFakeSignalNotifier(t *testing.T) *FakeSignalNotifier {
+	fsn := &FakeSignalNotifier{t: t}
+	return fsn
+}
+
+func (f *FakeSignalNotifier) Stop() {
+	f.Lock()
+	defer f.Unlock()
+	f.stopped = true
+	if f.cancel == nil {
+		f.t.Error("stopped before started")
+		return
+	}
+	f.cancel()
+}
+
+func (f *FakeSignalNotifier) NotifyContext(parent context.Context, signals ...os.Signal) (ctx context.Context, stop context.CancelFunc) {
+	f.Lock()
+	defer f.Unlock()
+	f.signals = signals
+	f.ctx, f.cancel = context.WithCancel(parent)
+	return f.ctx, f.Stop
+}
+
+func (f *FakeSignalNotifier) Notify() {
+	f.Lock()
+	defer f.Unlock()
+	if f.cancel == nil {
+		f.t.Error("notified before started")
+		return
+	}
+	f.cancel()
+}
+
+func (f *FakeSignalNotifier) AssertStopped() {
+	f.Lock()
+	defer f.Unlock()
+	assert.True(f.t, f.stopped)
+}