Strip fx call-chain noise from startup errors; clarify file:// error

internal/cli/app.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"os/signal"
 	"path/filepath"
+	"strings"
 	"syscall"
 	"time"
 
@@ -68,6 +69,24 @@ func NewApp(opts AppOptions) *fx.App {
 	return fx.New(allOptions...)
 }
 
+// cleanStartupError strips fx's dependency-injection call-chain noise from
+// startup errors. fx wraps the underlying error with messages like
+//
+//	could not build arguments for function "X" (file:line): failed to build T:
+//	could not build arguments for function "Y" (file:line): failed to build U:
+//	received non-nil error from function "Z" (file:line): <real error>
+//
+// Users care about the real error, not the DI plumbing. We strip everything
+// up through the last "): " (which is always the close-paren of an fx
+// function-location annotation followed by the wrapped error).
+func cleanStartupError(err error) error {
+	msg := err.Error()
+	if idx := strings.LastIndex(msg, "): "); idx >= 0 {
+		msg = msg[idx+3:]
+	}
+	return errors.New(msg)
+}
+
 // RunApp starts and stops the fx application within the given context.
 // It handles graceful shutdown on interrupt signals (SIGINT, SIGTERM) and
 // ensures the application stops cleanly. The function blocks until the
@@ -83,7 +102,7 @@ func RunApp(ctx context.Context, app *fx.App) error {
 
 	// Start the app
 	if err := app.Start(ctx); err != nil {
-		return fmt.Errorf("failed to start app: %w", err)
+		return cleanStartupError(err)
 	}
 
 	// Handle shutdown

internal/cli/app_test.go

@@ -0,0 +1,39 @@
+package cli
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestCleanStartupError(t *testing.T) {
+	tests := []struct {
+		name string
+		in   string
+		want string
+	}{
+		{
+			name: "real fx error chain",
+			in:   `could not build arguments for function "sneak.berlin/go/vaultik/internal/cli".newSnapshotCreateCommand.func1.1 (/Users/user/dev/vaultik/internal/cli/snapshot.go:71): failed to build *vaultik.Vaultik: could not build arguments for function "sneak.berlin/go/vaultik/internal/vaultik".New (/Users/user/dev/vaultik/internal/vaultik/vaultik.go:59): failed to build storage.Storer: received non-nil error from function "sneak.berlin/go/vaultik/internal/storage".NewStorer (/Users/user/dev/vaultik/internal/storage/module.go:23): creating base path: mkdir /Volumes/BACKUPS: permission denied`,
+			want: `creating base path: mkdir /Volumes/BACKUPS: permission denied`,
+		},
+		{
+			name: "no fx wrapping",
+			in:   "plain error",
+			want: "plain error",
+		},
+		{
+			name: "single fx wrapping",
+			in:   `received non-nil error from function "foo" (file.go:1): underlying problem`,
+			want: "underlying problem",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := cleanStartupError(errors.New(tt.in)).Error()
+			if got != tt.want {
+				t.Errorf("got %q, want %q", got, tt.want)
+			}
+		})
+	}
+}