hplip-3.17.4-1.1.x86_64

Trying to stop/start printer using hp-toolbox tab "Printer Control", the Qt GUI
starts hanging. "cupsd" is seen in top with 75% CPU. strace shows that
hp-toolbox is constantly trying to send commands via the cups socket and gets 
"HTTP/1.1 401 Unauthorized" errors. 

> 21127 14:56:41.581250 sendto(20, "POST /admin/ HTTP/1.1\r\nAuthorization: Basic cm9vdDo=\r\nContent-Length: 154\r\nContent-Type: application/ipp\r\nHost: localhost:0\r\nUser-Agent: CUPS/2.2.3 (Linux 4.11.8-1-default; x86_64) IPP/2.0\r\nExpect: 100-continue\r\n\r\n", 214, 0, NULL, 0) = 214
> 21127 14:56:41.581272 sendto(20, "\2\0\0\20\0\0\0\1\1G\0\22attributes-charset\0\5utf-8H\0\33attributes-natural-language\0\5en-usE\0\vprinter-uri\0\"ipp://localhost/printers/ENVY_5530B\0\24requesting-user-name\0\4root\3", 154, 0, NULL, 0) = 154
> 21127 14:56:41.581419 recvfrom(20, "HTTP/1.1 401 Unauthorized\r\nConnection: close\r\nContent-Language: en_US\r\nContent-Length: 521\r\nContent-Type: text/html; charset=utf-8\r\nDate: Tue, 11 Jul 2017 12:56:41 GMT\r\nWWW-Authenticate: Basic realm=\"CUPS\", trc=\"y\"\r\nAccept-Encoding: gzip, deflate, identity"..., 2048, 0, NULL, NULL) = 879
> ...
> 21127 14:56:41.581972 sendto(20, "POST /admin/ HTTP/1.1\r\nAuthorization: Basic cm9vdDo=\r\nContent-Length: 154\r\nContent-Type: application/ipp\r\nHost: localhost:0\r\nUser-Agent: CUPS/2.2.3 (Linux 4.11.8-1-default; x86_64) IPP/2.0\r\nExpect: 100-continue\r\n\r\n", 214, 0, NULL, 0) = 214
> 21127 14:56:41.581992 sendto(20, "\2\0\0\20\0\0\0\1\1G\0\22attributes-charset\0\5utf-8H\0\33attributes-natural-language\0\5en-usE\0\vprinter-uri\0\"ipp://localhost/printers/ENVY_5530B\0\24requesting-user-name\0\4root\3", 154, 0, NULL, 0) = 154
> 21127 14:56:41.582132 recvfrom(20, "HTTP/1.1 401 Unauthorized\r\nConnection: close\r\nContent-Language: en_US\r\nContent-Length: 521\r\nContent-Type: text/html; charset=utf-8\r\nDate: Tue, 11 Jul 2017 12:56:41 GMT\r\nWWW-Authenticate: Basic realm=\"CUPS\", trc=\"y\"\r\nAccept-Encoding: gzip, deflate, identity"..., 2048, 0, NULL, NULL) = 879

This is a default cups setup, no special permissions configured. cupsd.conf:

   # All printer operations require a printer operator to authenticate...
  <Limit Pause-Printer Resume-Printer Enable-Printer Disable-Printer
Pause-Printer-After-Current-Job Hold-New-Jobs Release-Held-New-Jobs
Deactivate-Printer Activate-Printer Restart-Printer Shutdown-Printer
Startup-Printer Promote-Job Schedule-Job-After Cancel-Jobs CUPS-Accept-Jobs
CUPS-Reject-Jobs>
    AuthType Default
    Require user @SYSTEM
    Order deny,allow
  </Limit>

So it's not surprising that the permission isn't granted, but it's wrong to
keep trying.