adding comments to strange functions used in parallel parsing

dbms/src/IO/ReadHelpers.cpp

@@ -1053,7 +1053,6 @@ void skipToUnescapedNextLineOrEOF(ReadBuffer & buf)
     }
 }
 
-/// TODO (akuzm) - write comments for this and next function.
 void saveUpToPosition(ReadBuffer & in, DB::Memory<> & memory, char * current)
 {
     assert(current >= in.position());

dbms/src/IO/ReadHelpers.h

@@ -924,8 +924,24 @@ if (method == DB::CompressionMethod::Gzip)
 return std::make_unique<TReadBuffer>(args...);
 }
 
-/// TODO (akuzm) - write comments for this and next function.
+/** This function just copies the data from buffer's internal position (in.position())
+  * to current position (from arguments) into memory.
+  * @param buf - original buffer to read from.
+  *        memory - where to put data from buf
+  *        current - defines the upper bound of copyble data.
+  */
 void saveUpToPosition(ReadBuffer & in, DB::Memory<> & memory, char * current);
+
+/** This function is negative to eof().
+  * In fact it returns whether the data was loaded to internal ReadBuffers's buffer or not.
+  * And saves data from buffer's position to current if there is no pending data in buffer.
+  * Why we have to use this strange function? Consider we have buffer's internal position in the middle
+  * of our buffer and the current cursor in the end of the buffer. When we call eof() it calls next().
+  * And this function can fill the buffer with new data, so we will lose the data from previous buffer state.
+  * @param buf - original buffer to read from.
+  *        memory - where to put data from buf
+  *        current - defines the upper bound of copyble data.
+  */
 bool loadAtPosition(ReadBuffer & in, DB::Memory<> & memory, char * & current);
 
 }