Fix config merge regression with root slices (e.g. disableKinds)

Fixes #11089

config/allconfig/allconfig.go

@@ -191,6 +191,22 @@ type configCompiler interface {
 func (c Config) cloneForLang() *Config {
 	x := c
 	x.C = nil
+	copyStringSlice := func(in []string) []string {
+		if in == nil {
+			return nil
+		}
+		out := make([]string, len(in))
+		copy(out, in)
+		return out
+	}
+
+	// Copy all the slices to avoid sharing.
+	x.DisableKinds = copyStringSlice(x.DisableKinds)
+	x.DisableLanguages = copyStringSlice(x.DisableLanguages)
+	x.MainSections = copyStringSlice(x.MainSections)
+	x.IgnoreErrors = copyStringSlice(x.IgnoreErrors)
+	x.IgnoreFiles = copyStringSlice(x.IgnoreFiles)
+	x.Theme = copyStringSlice(x.Theme)
 
 	// Collapse all static dirs to one.
 	x.StaticDir = x.staticDirs()
@@ -787,12 +803,14 @@ func fromLoadConfigResult(fs afero.Fs, logger loggers.Logger, res config.LoadCon
 
 			// Create a copy of the complete config and replace the root keys with the language specific ones.
 			clone := all.cloneForLang()
+
 			if err := decodeConfigFromParams(fs, bcfg, mergedConfig, clone, differentRootKeys); err != nil {
 				return nil, fmt.Errorf("failed to decode config for language %q: %w", k, err)
 			}
 			if err := clone.CompileConfig(logger); err != nil {
 				return nil, err
 			}
+
 			langConfigMap[k] = clone
 		case maps.ParamsMergeStrategy:
 		default:

hugolib/config_test.go

@@ -750,6 +750,74 @@ author: {{ site.Author }}
 
 }
 
+// Issue #11089
+func TestHugoConfigSliceOverrides(t *testing.T) {
+	t.Parallel()
+
+	filesTemplate := `
+-- hugo.toml --
+disableKinds = ["section"]
+[languages]
+[languages.en]
+disableKinds = []
+title = "English"
+weigHt = WEIGHT_EN
+[languages.sv]
+title = "Swedish"
+wEight =  WEIGHT_SV
+disableKinds = ["page"]
+-- layouts/index.html --
+Home: {{ .Lang}}|{{ len site.RegularPages }}|
+-- layouts/_default/single.html --
+Single.
+-- content/p1.en.md --
+-- content/p2.en.md --
+-- content/p1.sv.md --
+-- content/p2.sv.md --
+
+`
+
+	t.Run("En first", func(t *testing.T) {
+		files := strings.ReplaceAll(filesTemplate, "WEIGHT_EN", "1")
+		files = strings.ReplaceAll(files, "WEIGHT_SV", "2")
+
+		cfg := config.New()
+		b, err := NewIntegrationTestBuilder(
+			IntegrationTestConfig{
+				T:           t,
+				TxtarString: files,
+				BaseCfg:     cfg,
+			},
+		).BuildE()
+
+		b.Assert(err, qt.IsNil)
+		b.AssertFileContent("public/index.html", "Home: en|2|")
+		b.AssertFileContent("public/sv/index.html", "Home: sv|0|")
+
+	})
+
+	t.Run("Sv first", func(t *testing.T) {
+		files := strings.ReplaceAll(filesTemplate, "WEIGHT_EN", "2")
+		files = strings.ReplaceAll(files, "WEIGHT_SV", "1")
+
+		for i := 0; i < 20; i++ {
+			cfg := config.New()
+			b, err := NewIntegrationTestBuilder(
+				IntegrationTestConfig{
+					T:           t,
+					TxtarString: files,
+					BaseCfg:     cfg,
+				},
+			).BuildE()
+
+			b.Assert(err, qt.IsNil)
+			b.AssertFileContent("public/index.html", "Home: en|2|")
+			b.AssertFileContent("public/sv/index.html", "Home: sv|0|")
+		}
+	})
+
+}
+
 func TestConfigOutputFormatDefinedInTheme(t *testing.T) {
 	t.Parallel()