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/local/config.go

@@ -27,12 +27,12 @@ func init() {
 }
 
 // ParseConfig parses a local backend config.
-func ParseConfig(s string) (Config, error) {
+func ParseConfig(s string) (*Config, error) {
 	if !strings.HasPrefix(s, "local:") {
-		return Config{}, errors.New(`invalid format, prefix "local" not found`)
+		return nil, errors.New(`invalid format, prefix "local" not found`)
 	}
 
 	cfg := NewConfig()
 	cfg.Path = s[6:]
-	return cfg, nil
+	return &cfg, nil
 }

internal/backend/local/config_test.go

@@ -0,0 +1,18 @@
+package local
+
+import (
+	"testing"
+
+	"github.com/restic/restic/internal/backend/test"
+)
+
+var configTests = []test.ConfigTestData[Config]{
+	{S: "local:/some/path", Cfg: Config{
+		Path:        "/some/path",
+		Connections: 2,
+	}},
+}
+
+func TestParseConfig(t *testing.T) {
+	test.ParseConfigTester(t, ParseConfig, configTests)
+}

internal/backend/local/local_test.go

@@ -15,7 +15,7 @@ import (
 func newTestSuite(t testing.TB) *test.Suite[local.Config] {
 	return &test.Suite[local.Config]{
 		// NewConfig returns a config for a new temporary backend that will be used in tests.
-		NewConfig: func() (local.Config, error) {
+		NewConfig: func() (*local.Config, error) {
 			dir, err := os.MkdirTemp(rtest.TestTempDir, "restic-test-local-")
 			if err != nil {
 				t.Fatal(err)
@@ -23,7 +23,7 @@ func newTestSuite(t testing.TB) *test.Suite[local.Config] {
 
 			t.Logf("create new backend at %v", dir)
 
-			cfg := local.Config{
+			cfg := &local.Config{
 				Path:        dir,
 				Connections: 2,
 			}