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/b2/b2_test.go

@@ -30,10 +30,10 @@ func newB2TestSuite(t testing.TB) *test.Suite[b2.Config] {
 		WaitForDelayedRemoval: 10 * time.Second,
 
 		// NewConfig returns a config for a new temporary backend that will be used in tests.
-		NewConfig: func() (b2.Config, error) {
+		NewConfig: func() (*b2.Config, error) {
 			cfg, err := b2.ParseConfig(os.Getenv("RESTIC_TEST_B2_REPOSITORY"))
 			if err != nil {
-				return b2.Config{}, err
+				return nil, err
 			}
 
 			cfg.AccountID = os.Getenv("RESTIC_TEST_B2_ACCOUNT_ID")

internal/backend/b2/config.go

@@ -59,15 +59,15 @@ func checkBucketName(name string) error {
 // ParseConfig parses the string s and extracts the b2 config. The supported
 // configuration format is b2:bucketname/prefix. If no prefix is given the
 // prefix "restic" will be used.
-func ParseConfig(s string) (Config, error) {
+func ParseConfig(s string) (*Config, error) {
 	if !strings.HasPrefix(s, "b2:") {
-		return Config{}, errors.New("invalid format, want: b2:bucket-name[:path]")
+		return nil, errors.New("invalid format, want: b2:bucket-name[:path]")
 	}
 
 	s = s[3:]
 	bucket, prefix, _ := strings.Cut(s, ":")
 	if err := checkBucketName(bucket); err != nil {
-		return Config{}, err
+		return nil, err
 	}
 
 	if len(prefix) > 0 {
@@ -78,7 +78,7 @@ func ParseConfig(s string) (Config, error) {
 	cfg.Bucket = bucket
 	cfg.Prefix = prefix
 
-	return cfg, nil
+	return &cfg, nil
 }
 
 // ApplyEnvironment saves values from the environment to the config.