func FindDefaultCredentialsWithParams()

in google/default.go [111:156]
32 lines of code
7 McCabe index (conditional complexity)

func FindDefaultCredentialsWithParams(ctx context.Context, params CredentialsParams) (*Credentials, error) {
	// Make defensive copy of the slices in params.
	params = params.deepCopy()

	// First, try the environment variable.
	const envVar = "GOOGLE_APPLICATION_CREDENTIALS"
	if filename := os.Getenv(envVar); filename != "" {
		creds, err := readCredentialsFile(ctx, filename, params)
		if err != nil {
			return nil, fmt.Errorf("google: error getting credentials using %v environment variable: %v", envVar, err)
		}
		return creds, nil
	}

	// Second, try a well-known file.
	filename := wellKnownFile()
	if creds, err := readCredentialsFile(ctx, filename, params); err == nil {
		return creds, nil
	} else if !os.IsNotExist(err) {
		return nil, fmt.Errorf("google: error getting credentials using well-known file (%v): %v", filename, err)
	}

	// Third, if we're on a Google App Engine standard first generation runtime (<= Go 1.9)
	// use those credentials. App Engine standard second generation runtimes (>= Go 1.11)
	// and App Engine flexible use ComputeTokenSource and the metadata server.
	if appengineTokenFunc != nil {
		return &DefaultCredentials{
			ProjectID:   appengineAppIDFunc(ctx),
			TokenSource: AppEngineTokenSource(ctx, params.Scopes...),
		}, nil
	}

	// Fourth, if we're on Google Compute Engine, an App Engine standard second generation runtime,
	// or App Engine flexible, use the metadata server.
	if metadata.OnGCE() {
		id, _ := metadata.ProjectID()
		return &DefaultCredentials{
			ProjectID:   id,
			TokenSource: ComputeTokenSource("", params.Scopes...),
		}, nil
	}

	// None are found; return helpful error.
	const url = "https://developers.google.com/accounts/docs/application-default-credentials"
	return nil, fmt.Errorf("google: could not find default credentials. See %v for more information.", url)
}