fix: make handleManifest always signal dependents (#13141)

Fixes #13139

Using a bare channel to signal dependent goroutines means that we can only signal success, not failure, which leads to deadlock if we fail in a way that doesn't cause the whole `apiConnRoutineManager` to tear down routines.

Instead, we use a new object called a `checkpoint` that signals success or failure, so that dependent routines get unblocked if the routine they depend on fails.

agent/checkpoint_internal_test.go

@@ -0,0 +1,49 @@
+package agent
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"golang.org/x/xerrors"
+
+	"cdr.dev/slog/sloggers/slogtest"
+	"github.com/coder/coder/v2/testutil"
+)
+
+func TestCheckpoint_CompleteWait(t *testing.T) {
+	t.Parallel()
+	logger := slogtest.Make(t, nil)
+	ctx := testutil.Context(t, testutil.WaitShort)
+	uut := newCheckpoint(logger)
+	err := xerrors.New("test")
+	uut.complete(err)
+	got := uut.wait(ctx)
+	require.Equal(t, err, got)
+}
+
+func TestCheckpoint_CompleteTwice(t *testing.T) {
+	t.Parallel()
+	logger := slogtest.Make(t, &slogtest.Options{IgnoreErrors: true})
+	ctx := testutil.Context(t, testutil.WaitShort)
+	uut := newCheckpoint(logger)
+	err := xerrors.New("test")
+	uut.complete(err)
+	uut.complete(nil) // drops CRITICAL log
+	got := uut.wait(ctx)
+	require.Equal(t, err, got)
+}
+
+func TestCheckpoint_WaitComplete(t *testing.T) {
+	t.Parallel()
+	logger := slogtest.Make(t, nil)
+	ctx := testutil.Context(t, testutil.WaitShort)
+	uut := newCheckpoint(logger)
+	err := xerrors.New("test")
+	errCh := make(chan error, 1)
+	go func() {
+		errCh <- uut.wait(ctx)
+	}()
+	uut.complete(err)
+	got := testutil.RequireRecvCtx(ctx, t, errCh)
+	require.Equal(t, err, got)
+}