Preserve literal characters in shared file paths

internal/validation/tree.go

@@ -210,7 +210,7 @@ func insideVendor(moduleRelative string) bool {
 }
 
 func normalizeSharedFile(name string) (string, error) {
-	if name == "" || path.IsAbs(name) || strings.Contains(name, "\\") {
+	if name == "" || path.IsAbs(name) {
 		return "", fmt.Errorf("shared file path must be a repository-relative file path")
 	}
 	clean := path.Clean(name)

internal/validation/tree_test.go

@@ -231,6 +231,36 @@ func TestReadCandidateTreeDefaultsAreOptionalAndExplicitMissingFilesFail(t *test
 	}
 }
 
+func TestSharedFileSelectionPreservesLiteralBackslash(t *testing.T) {
+	if filepath.Separator == '\\' {
+		t.Skip("backslash is a filesystem separator on this platform")
+	}
+	repository := newTreeRepository(t)
+	writeTreeFile(t, repository, "module/go.mod", []byte("module example.com/module\n"))
+	const name = `docs/notice\copy.txt`
+	contents := []byte("literal filename\n")
+	writeTreeFile(t, repository, name, contents)
+	commit := commitCandidateTree(t, repository)
+	moduleDir := filepath.Join(repository, "module")
+	tree, err := ReadCandidateTreeForScope(t.Context(), repository, commit, TreeScope{ModuleDir: moduleDir, SharedFiles: []string{name, name}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	cleanupCandidateTree(t, tree)
+	if got := inputPaths(tree); strings.Join(got, "\n") != name+"\nmodule/go.mod" {
+		t.Fatalf("literal selection = %q", got)
+	}
+	read, err := tree.ReadFile(t.Context(), name)
+	if err != nil || string(read) != string(contents) {
+		t.Fatalf("literal file content = %q, error = %v", read, err)
+	}
+	for _, invalid := range []string{"../outside", "/absolute", "docs"} {
+		if _, err := ReadCandidateTreeForScope(t.Context(), repository, commit, TreeScope{ModuleDir: moduleDir, SharedFiles: []string{invalid}}); err == nil {
+			t.Fatalf("shared selection accepted %q", invalid)
+		}
+	}
+}
+
 func TestRunBuildsInputsOnlyForChecksOrExplicitSharedFiles(t *testing.T) {
 	repository := newTreeRepository(t)
 	writeTreeFile(t, repository, "go.mod", []byte("module example.com/validation-tree\n\ngo 1.25.0\n"))