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/test/config.go

@@ -11,7 +11,7 @@ type ConfigTestData[C comparable] struct {
 	Cfg C
 }
 
-func ParseConfigTester[C comparable](t *testing.T, parser func(s string) (C, error), tests []ConfigTestData[C]) {
+func ParseConfigTester[C comparable](t *testing.T, parser func(s string) (*C, error), tests []ConfigTestData[C]) {
 	for i, test := range tests {
 		t.Run(fmt.Sprint(i), func(t *testing.T) {
 			cfg, err := parser(test.S)
@@ -19,9 +19,9 @@ func ParseConfigTester[C comparable](t *testing.T, parser func(s string) (C, err
 				t.Fatalf("%s failed: %v", test.S, err)
 			}
 
-			if !reflect.DeepEqual(cfg, test.Cfg) {
+			if !reflect.DeepEqual(*cfg, test.Cfg) {
 				t.Fatalf("input: %s\n wrong config, want:\n  %#v\ngot:\n  %#v",
-					test.S, test.Cfg, cfg)
+					test.S, test.Cfg, *cfg)
 			}
 		})
 	}

internal/backend/test/suite.go

@@ -13,10 +13,10 @@ import (
 // Suite implements a test suite for restic backends.
 type Suite[C any] struct {
 	// Config should be used to configure the backend.
-	Config C
+	Config *C
 
 	// NewConfig returns a config for a new temporary backend that will be used in tests.
-	NewConfig func() (C, error)
+	NewConfig func() (*C, error)
 
 	// CreateFn is a function that creates a temporary repository for the tests.
 	Create func(cfg C) (restic.Backend, error)
@@ -61,7 +61,7 @@ func (s *Suite[C]) RunTests(t *testing.T) {
 	}
 
 	if s.Cleanup != nil {
-		if err = s.Cleanup(s.Config); err != nil {
+		if err = s.Cleanup(*s.Config); err != nil {
 			t.Fatal(err)
 		}
 	}
@@ -158,13 +158,13 @@ func (s *Suite[C]) RunBenchmarks(b *testing.B) {
 		return
 	}
 
-	if err = s.Cleanup(s.Config); err != nil {
+	if err = s.Cleanup(*s.Config); err != nil {
 		b.Fatal(err)
 	}
 }
 
 func (s *Suite[C]) create(t testing.TB) restic.Backend {
-	be, err := s.Create(s.Config)
+	be, err := s.Create(*s.Config)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -172,7 +172,7 @@ func (s *Suite[C]) create(t testing.TB) restic.Backend {
 }
 
 func (s *Suite[C]) open(t testing.TB) restic.Backend {
-	be, err := s.Open(s.Config)
+	be, err := s.Open(*s.Config)
 	if err != nil {
 		t.Fatal(err)
 	}

internal/backend/test/tests.go

@@ -57,7 +57,7 @@ func (s *Suite[C]) TestCreateWithConfig(t *testing.T) {
 	store(t, b, restic.ConfigFile, []byte("test config"))
 
 	// now create the backend again, this must fail
-	_, err = s.Create(s.Config)
+	_, err = s.Create(*s.Config)
 	if err == nil {
 		t.Fatalf("expected error not found for creating a backend with an existing config file")
 	}