Ensure that null C strings do not break logging (#15255)

src/log.h

@@ -32,21 +32,60 @@ class StreamProxy {
 public:
 	StreamProxy(std::ostream *os) : m_os(os) { }
 
+	static void fix_stream_state(std::ostream &os);
+
 	template<typename T>
-	StreamProxy& operator<<(T&& arg) {
+	StreamProxy& operator<<(T&& arg)
+	{
 		if (m_os) {
+			if (!m_os->good())
+				fix_stream_state(*m_os);
 			*m_os << std::forward<T>(arg);
 		}
 		return *this;
 	}
 
-	StreamProxy& operator<<(std::ostream& (*manip)(std::ostream&)) {
+	StreamProxy& operator<<(std::ostream& (*manip)(std::ostream&))
+	{
 		if (m_os) {
+			if (!m_os->good())
+				fix_stream_state(*m_os);
 			*m_os << manip;
 		}
 		return *this;
 	}
 
+private:
+	template<typename T>
+	StreamProxy& emit_with_null_check(T&& arg)
+	{
+		// These calls explicitly use the templated version of operator<<,
+		// so that they won't use the overloads created by ADD_NULL_CHECK.
+		if (arg == nullptr)
+			return this->operator<< <const char*> ("(null)");
+		else
+			return this->operator<< <T>(std::forward<T>(arg));
+	}
+
+public:
+	// Add specific overrides for operator<< which check for NULL string
+	// pointers. This is undefined behavior in the C++ spec, so emit "(null)"
+	// instead. These are method overloads, rather than template specializations.
+#define ADD_NULL_CHECK(_type) \
+	StreamProxy& operator<<(_type arg) \
+	{ \
+		return emit_with_null_check(std::forward<_type>(arg)); \
+	}
+
+	ADD_NULL_CHECK(char*)
+	ADD_NULL_CHECK(unsigned char*)
+	ADD_NULL_CHECK(signed char*)
+	ADD_NULL_CHECK(const char*)
+	ADD_NULL_CHECK(const unsigned char*)
+	ADD_NULL_CHECK(const signed char*)
+
+#undef ADD_NULL_CHECK
+
 private:
 	std::ostream *m_os;
 };