backend: let ParseConfig return a Config pointer

In order to change the backend initialization in `global.go` to be able
to generically call cfg.ApplyEnvironment() for supported backends, the
`interface{}` returned by `ParseConfig` must contain a pointer to the
configuration.

An alternative would be to use reflection to convert the type from
`interface{}(Config)` to `interface{}(*Config)` (from value to pointer
type). However, this would just complicate the type mess further.

internal/backend/s3/config.go

@@ -45,7 +45,7 @@ func init() {
 // supported configuration formats are s3://host/bucketname/prefix and
 // s3:host/bucketname/prefix. The host can also be a valid s3 region
 // name. If no prefix is given the prefix "restic" will be used.
-func ParseConfig(s string) (Config, error) {
+func ParseConfig(s string) (*Config, error) {
 	switch {
 	case strings.HasPrefix(s, "s3:http"):
 		// assume that a URL has been specified, parse it and
@@ -53,11 +53,11 @@ func ParseConfig(s string) (Config, error) {
 		// bucket name and prefix
 		url, err := url.Parse(s[3:])
 		if err != nil {
-			return Config{}, errors.WithStack(err)
+			return nil, errors.WithStack(err)
 		}
 
 		if url.Path == "" {
-			return Config{}, errors.New("s3: bucket name not found")
+			return nil, errors.New("s3: bucket name not found")
 		}
 
 		bucket, path, _ := strings.Cut(url.Path[1:], "/")
@@ -67,7 +67,7 @@ func ParseConfig(s string) (Config, error) {
 	case strings.HasPrefix(s, "s3:"):
 		s = s[3:]
 	default:
-		return Config{}, errors.New("s3: invalid format")
+		return nil, errors.New("s3: invalid format")
 	}
 	// use the first entry of the path as the endpoint and the
 	// remainder as bucket name and prefix
@@ -76,9 +76,9 @@ func ParseConfig(s string) (Config, error) {
 	return createConfig(endpoint, bucket, prefix, false)
 }
 
-func createConfig(endpoint, bucket, prefix string, useHTTP bool) (Config, error) {
+func createConfig(endpoint, bucket, prefix string, useHTTP bool) (*Config, error) {
 	if endpoint == "" {
-		return Config{}, errors.New("s3: invalid format, host/region or bucket name not found")
+		return nil, errors.New("s3: invalid format, host/region or bucket name not found")
 	}
 
 	if prefix != "" {
@@ -90,7 +90,7 @@ func createConfig(endpoint, bucket, prefix string, useHTTP bool) (Config, error)
 	cfg.UseHTTP = useHTTP
 	cfg.Bucket = bucket
 	cfg.Prefix = prefix
-	return cfg, nil
+	return &cfg, nil
 }
 
 // ApplyEnvironment saves values from the environment to the config.

internal/backend/s3/s3_test.go

@@ -128,7 +128,7 @@ func newMinioTestSuite(ctx context.Context, t testing.TB) *test.Suite[MinioTestC
 
 	return &test.Suite[MinioTestConfig]{
 		// NewConfig returns a config for a new temporary backend that will be used in tests.
-		NewConfig: func() (MinioTestConfig, error) {
+		NewConfig: func() (*MinioTestConfig, error) {
 			cfg := MinioTestConfig{}
 
 			cfg.tempdir = rtest.TempDir(t)
@@ -142,7 +142,7 @@ func newMinioTestSuite(ctx context.Context, t testing.TB) *test.Suite[MinioTestC
 			cfg.Config.UseHTTP = true
 			cfg.Config.KeyID = key
 			cfg.Config.Secret = options.NewSecretString(secret)
-			return cfg, nil
+			return &cfg, nil
 		},
 
 		// CreateFn is a function that creates a temporary repository for the tests.
@@ -224,10 +224,10 @@ func newS3TestSuite(t testing.TB) *test.Suite[s3.Config] {
 		MinimalData: true,
 
 		// NewConfig returns a config for a new temporary backend that will be used in tests.
-		NewConfig: func() (s3.Config, error) {
+		NewConfig: func() (*s3.Config, error) {
 			cfg, err := s3.ParseConfig(os.Getenv("RESTIC_TEST_S3_REPOSITORY"))
 			if err != nil {
-				return s3.Config{}, err
+				return nil, err
 			}
 
 			cfg.KeyID = os.Getenv("RESTIC_TEST_S3_KEY")