IniConfigNode: avoid writing unmodified data

Sometimes the new content of the file is exactly the same as the old
one, for example when the tracking node gets reset and
recreated. Detect that for all cases at the lower level by reading the
existing file first and comparing it against the new content.

src/syncevo/IniConfigNode.cpp

@@ -45,8 +45,20 @@ void IniBaseConfigNode::flush()
         throw std::runtime_error(m_data->getName() + ": internal error: flushing read-only config node not allowed");
     }
 
-    boost::shared_ptr<std::ostream> file = m_data->write();
-    toFile(*file);
+    // Our m_modified check is not perfect: sometimes changes are made
+    // that once complete, lead to the exact same file content. Catch
+    // that with a brute-force memory compare and avoid rewriting the
+    // file unless something changed.
+    std::stringstream temp;
+    toFile(temp);
+    std::string newcontent = temp.str();
+    boost::shared_ptr<std::istream> oldfile = m_data->read();
+    std::string oldcontent;
+    if (!ReadFile(*oldfile, oldcontent) ||
+        oldcontent != newcontent) {
+        boost::shared_ptr<std::ostream> newfile = m_data->write();
+        *newfile << newcontent;
+    }
 
     m_modified = false;
 }