Fix multithreading crash (bug #9366)

This minimal fix makes the crash go away on my machine. A much more complete
fix is in master (and will be futher extended), but for 2.1.x we'll do only
minimal changes.

src/Language.cpp

@@ -38,6 +38,12 @@ Language const * latex_language = 0;
 Language const * reset_language = 0;
 
 
+std::string const Language::babel() const
+{
+	return deep_copy(babel_);
+}
+
+
 bool Language::isPolyglossiaExclusive() const
 {
 	return babel().empty() && !polyglossia().empty() && requires().empty();

src/Language.h

@@ -35,7 +35,7 @@ public:
 	/// LyX language name
 	std::string const & lang() const { return lang_; }
 	/// Babel language name
-	std::string const & babel() const { return babel_; }
+	std::string const babel() const;
 	/// polyglossia language name
 	std::string const & polyglossia() const { return polyglossia_name_; }
 	/// polyglossia language options

src/support/lstrings.h

@@ -24,6 +24,19 @@
 namespace lyx {
 namespace support {
 
+/// Helper to enforce creating a deep copy of lyx::docstring or std::string
+/// even if std::basic_string uses copy-on-write as in GNU libstdc++.
+/// \sa https://gcc.gnu.org/bugzilla/show_bug.cgi?id=21334
+template<typename Char, typename Traits, typename Alloc>
+typename std::basic_string<Char, Traits, Alloc>
+deep_copy(typename std::basic_string<Char, Traits, Alloc> const & src)
+{
+	typedef typename std::basic_string<Char, Traits, Alloc> String;
+	// Use constructor with two arguments to support strings with embedded
+	// \0 characters
+	return String(src.c_str(), src.length());
+}
+
 /// Compare \p s and \p s2, ignoring the case.
 /// Does not depend on the locale.
 int compare_no_case(docstring const & s, docstring const & s2);

status.21x

@@ -158,6 +158,9 @@ What's new
 - Fix logic of "Maintain aspect ratio" checkbox in the graphics dialog
   (bug 9357).
 
+- Fix most frequent reason for crash while editing with open view source
+  window (bug 9336).
+
 
 * INTERNALS