genesys: Add a way to log messages using DBG_HELPER

backend/genesys/error.cpp

@@ -114,7 +114,11 @@ void SaneException::set_msg(const char* format, std::va_list vlist)
     const char* status_msg = sane_strstatus(status_);
     std::size_t status_msg_len = std::strlen(status_msg);
 
-    int msg_len = std::vsnprintf(nullptr, 0, format, vlist);
+    std::va_list vlist2;
+    va_copy(vlist2, vlist);
+    int msg_len = std::vsnprintf(nullptr, 0, format, vlist2);
+    va_end(vlist2);
+
     if (msg_len < 0) {
         const char* formatting_error_msg = "(error formatting arguments)";
         msg_.reserve(std::strlen(formatting_error_msg) + 3 + status_msg_len);
@@ -178,4 +182,34 @@ void DebugMessageHelper::vstatus(const char* format, ...)
     va_end(args);
 }
 
+void DebugMessageHelper::log(unsigned level, const char* msg)
+{
+    DBG(level, "%s: %s\n", func_, msg);
+}
+
+void DebugMessageHelper::vlog(unsigned level, const char* format, ...)
+{
+    std::string msg;
+
+    std::va_list args;
+
+    va_start(args, format);
+    int msg_len = std::vsnprintf(nullptr, 0, format, args);
+    va_end(args);
+
+    if (msg_len < 0) {
+        DBG(level, "%s: error formatting error message: %s\n", func_, format);
+        return;
+    }
+    msg.resize(msg_len + 1, ' ');
+
+    va_start(args, format);
+    std::vsnprintf(&msg.front(), msg.size(), format, args);
+    va_end(args);
+
+    msg.resize(msg_len, ' '); // strip the null character
+
+    DBG(level, "%s: %s\n", func_, msg.c_str());
+}
+
 } // namespace genesys

backend/genesys/error.h

@@ -124,6 +124,13 @@ public:
 
     void clear() { msg_[0] = '\n'; }
 
+    void log(unsigned level, const char* msg);
+    void vlog(unsigned level, const char* format, ...)
+    #ifdef __GNUC__
+        __attribute__((format(printf, 3, 4)))
+    #endif
+    ;
+
 private:
     const char* func_ = nullptr;
     char msg_[MAX_BUF_SIZE];