feat(fxtest): Add WithTestLogger option (#1159)

fxtest.New has this really nice behavior where by default,
it uses the `testing.TB` as the destination for log output.
Unfortunately, fxtest is not good for testing failure cases
because it fails the test if the container failed.

    app := fxtest.New(t,
        fx.Invoke(func() error { return errors.New("fail") }),
    )

    err := app.Start(ctx)
    // We never get here because fxtest.New has already failed the test.

So the expectation there is to use `fx.New(..)` which, by default,
logs to stderr.
This can be addressed by using the `fx.WithLogger` option
in combination with `fxtest.NewTestLogger`,
but it ends up being a mouthful:

    app := fx.New(
        fx.Invoke(func() error { return errors.New("fail") }),
fx.WithLogger(func() fxevent.Logger { return fxtest.NewTestLogger(t) }),
    )

This PR is a proposal and implementation of a new
`fxtest.WithTestLogger` option that shortens the above to:

    app := fx.New(
        fx.Invoke(func() error { return errors.New("fail") }),
        fxtest.WithTestLogger(t),
    )

As an example, a couple tests in Fx itself become more readable
with this new API.

Co-authored-by: Jacob Oaks <joaks@uber.com>

CHANGELOG.md

@@ -10,8 +10,11 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## Unrelease
-- No changes yet.
+## Unreleased
+
+### Added
+- fxtest: Add WithTestLogger option that uses a `testing.TB` as the
+  Fx event logger.
 
 ## [1.20.1](https://github.com/uber-go/fx/compare/v1.20.0...v1.20.1) - 2023-10-17
 

app_test.go

@@ -53,7 +53,7 @@ func NewForTest(tb testing.TB, opts ...Option) *App {
 		// Provide both: Logger and WithLogger so that if the test
 		// WithLogger fails, we don't pollute stderr.
 		Logger(fxtest.NewTestPrinter(tb)),
-		WithLogger(func() fxevent.Logger { return fxtest.NewTestLogger(tb) }),
+		fxtest.WithTestLogger(tb),
 	}
 	opts = append(testOpts, opts...)
 
@@ -73,7 +73,7 @@ func validateTestApp(tb testing.TB, opts ...Option) error {
 		// Provide both: Logger and WithLogger so that if the test
 		// WithLogger fails, we don't pollute stderr.
 		Logger(fxtest.NewTestPrinter(tb)),
-		WithLogger(func() fxevent.Logger { return fxtest.NewTestLogger(tb) }),
+		fxtest.WithTestLogger(tb),
 	}
 	opts = append(testOpts, opts...)
 

fxtest/app.go

@@ -24,7 +24,6 @@ import (
 	"context"
 
 	"go.uber.org/fx"
-	"go.uber.org/fx/fxevent"
 )
 
 // App is a wrapper around fx.App that provides some testing helpers. By
@@ -38,7 +37,7 @@ type App struct {
 // New creates a new test application.
 func New(tb TB, opts ...fx.Option) *App {
 	allOpts := make([]fx.Option, 0, len(opts)+1)
-	allOpts = append(allOpts, fx.WithLogger(func() fxevent.Logger { return NewTestLogger(tb) }))
+	allOpts = append(allOpts, WithTestLogger(tb))
 	allOpts = append(allOpts, opts...)
 
 	app := fx.New(allOpts...)

fxtest/printer.go

@@ -32,6 +32,14 @@ func NewTestLogger(t TB) fxevent.Logger {
 	return fxlog.DefaultLogger(testutil.WriteSyncer{T: t})
 }
 
+// WithTestLogger returns an fx.Option that uses the provided TB
+// as the destination for Fx's log output.
+func WithTestLogger(t TB) fx.Option {
+	return fx.WithLogger(func() fxevent.Logger {
+		return NewTestLogger(t)
+	})
+}
+
 type testPrinter struct {
 	TB
 }