Bug: compressEnd does not free up the MCCP buffer if data fails to send.
Danger: Medium - If you have a lot of people using MCCP disconnecting regularly and not able to receive data, it leaks 4096 bytes every time that happens.
Discovered in: AFKMud 2.1.3
Found by: Nick Gammon
Fixed by: Nick Gammon

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features.cpp, descriptor_data::compressEnd

Locate:

   if( deflate( mccp->out_compress, Z_FINISH ) != Z_STREAM_END )
      return false;

   if( !process_compressed(  ) ) /* try to send any residual data */
      return false;

Change to:

   if( deflate( mccp->out_compress, Z_FINISH ) == Z_STREAM_END )
      process_compressed();   /* try to send any residual data */

As explained by Nick, if data can't be written to the descriptor when shutting down compression, a chunk of 4096 bytes will be left leaking.

Enough players doing this by disconnecting and reconnecting a lot can lead to substantial memory loss and lead to the need for premature reboots.