"Cancel Execution and Clear Results" doesn't work

[andrewbasterfield]

When I click "Cancel Execution and Clear Results" in query view while a long query is running it pops up a dialogue saying "Are you sure you want to cancel ?" - if you click yes it does nothing

If you try to close the window it says "Waiting for Query Threads to terminate" in the bottom status area

If you kill the query in Server Administration the UI will hang with the following spat to stderr

X Error: BadLength (poly request too large or internal Xlib length error) 16
  Major opcode:  59
  Minor opcode:  0
  Resource id:  0x2200662
X Error: BadLength (poly request too large or internal Xlib length error) 16
  Major opcode:  57
  Minor opcode:  0
  Resource id:  0x2200662