Use Go rules for Windows filename components

internal/engine/discovery.go

@@ -258,27 +258,16 @@ func validFilenameComponent(value string) bool {
 		return false
 	}
 	if runtime.GOOS == "windows" {
-		return validWindowsFilenameComponent(value)
-	}
-	return true
-}
-
-func validWindowsFilenameComponent(value string) bool {
-	if strings.ContainsAny(value, `<>:"|?*`) || strings.HasSuffix(value, ".") || strings.HasSuffix(value, " ") {
-		return false
-	}
-	for _, character := range value {
-		if character < 32 {
+		if !filepath.IsLocal(value) || strings.ContainsAny(value, `<>:"|?*`) || strings.HasSuffix(value, ".") || strings.HasSuffix(value, " ") {
 			return false
 		}
+		for _, character := range value {
+			if character < 32 {
+				return false
+			}
+		}
 	}
-	base := strings.ToUpper(strings.TrimSuffix(value, filepath.Ext(value)))
-	switch base {
-	case "CON", "PRN", "AUX", "NUL", "COM1", "COM2", "COM3", "COM4", "COM5", "COM6", "COM7", "COM8", "COM9", "COM¹", "COM²", "COM³", "LPT1", "LPT2", "LPT3", "LPT4", "LPT5", "LPT6", "LPT7", "LPT8", "LPT9", "LPT¹", "LPT²", "LPT³":
-		return false
-	default:
-		return true
-	}
+	return true
 }
 
 var errUnixSystemPresetUnavailable = errors.New("Unix system directories are unavailable on this platform")

internal/engine/discovery_test.go

@@ -129,31 +129,33 @@ func TestFilenameComponentValidation(t *testing.T) {
 		}
 	}
 	if runtime.GOOS == "windows" {
-		for _, value := range []string{"CON", "nul.txt", "name.", "bad:name"} {
+		for _, value := range []string{"CON", "CONIN$", "CONOUT$", "name.", "bad:name", "bad\x01name"} {
 			if validFilenameComponent(value) {
 				t.Errorf("Windows component %q unexpectedly accepted", value)
 			}
 		}
-	} else {
-		for _, value := range []string{"name with spaces", "naïve"} {
-			if !validFilenameComponent(value) {
-				t.Errorf("host-valid component %q rejected", value)
+		// Device names with extensions depend on the Windows version. Match
+		// Go's platform-aware decision instead of imposing a blanket ban.
+		for _, value := range []string{"nul.txt", "COM¹.log", "LPT³.txt", "CON.extra.yml"} {
+			if got, want := validFilenameComponent(value), filepath.IsLocal(value); got != want {
+				t.Errorf("Windows component %q validity = %v, want Go's %v", value, got, want)
 			}
 		}
+	} else {
+		for _, value := range []string{"CONIN$", "CONOUT$", "nul.txt"} {
+			if !validFilenameComponent(value) {
+				t.Errorf("Windows component restriction applied to this host: %q", value)
+			}
+		}
+	}
+	for _, value := range []string{"name with spaces", "naïve"} {
+		if !validFilenameComponent(value) {
+			t.Errorf("host-valid component %q rejected", value)
+		}
 	}
 	if diagnostics := validateSearch(SearchDirectory("configs", DirectorySearchOptions{Filenames: []FilenameGroup{CustomFilenames("bad/name")}}), ""); len(diagnostics) == 0 {
 		t.Fatal("invalid custom filename was not retained for validation")
 	}
-	for _, value := range []string{"CON", "nul.txt", "COM¹.log", "LPT³.txt", "name.", "bad:name", "bad\x01name"} {
-		if validWindowsFilenameComponent(value) {
-			t.Errorf("Windows component %q unexpectedly accepted", value)
-		}
-	}
-	for _, value := range []string{"name with spaces", "naïve"} {
-		if !validWindowsFilenameComponent(value) {
-			t.Errorf("valid Windows component %q rejected", value)
-		}
-	}
 }
 
 func TestSearchCandidatesPreserveDirectoryAndGroupOrder(t *testing.T) {

internal/engine/discovery_windows_test.go

@@ -0,0 +1,32 @@
+//go:build windows
+
+package engine
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestWindowsSearchRejectsReservedComponentsBeforeAcquisition(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	first := writeImportFile(t, root, "first.yml", "value: first\n")
+	for _, name := range []string{"CONIN$", "CONOUT$"} {
+		for _, source := range []Source{
+			SearchConfig(name, Searches(Paths(first), WorkingDirectory())),
+			SearchConfig("", Searches(Paths(first), SearchDirectory(root, DirectorySearchOptions{
+				Filenames: []FilenameGroup{CustomFilenames(name)},
+			}))),
+		} {
+			result, err := testLoad[discoveryLoadConfig](source, Options[discoveryLoadConfig]{})
+			var failure *LoadError
+			if result != nil || !errors.As(err, &failure) || len(failure.Diagnostics()) != 1 || failure.Diagnostics()[0].Code != CodeOptionsInvalid {
+				t.Fatalf("reserved component %q accepted: %#v, %v", name, result, err)
+			}
+			if len(failure.Sources()) != 0 {
+				t.Fatalf("reserved component %q was checked after acquisition: %#v", name, failure.Sources())
+			}
+		}
+	}
+}