[cups] CUPS randomly freezing when returning list of printers to the client

[Alex Korobkin]

I have a weird problem with a busy CUPS 1.7.5 running on Ubuntu server
12.04 x64.

It would freeze several times per hour, last message in the log always
being about cupsdWriteClient, when replying to a random Linux client while
returning the list of printers.

D [28/Oct/2014:20:18:28 +0000] [Client 152] Returning IPP successful-ok for
CUPS-Get-Printers (no URI) from 172.16.60.111
D [28/Oct/2014:20:18:28 +0000] [Client 152] Content-Length: 442727
D [28/Oct/2014:20:18:28 +0000] [Client 152] cupsdWriteClient error=0,
used=0, state=HTTP_STATE_POST_SEND, data_encoding=HTTP_ENCODING_LENGTH,
data_remaining=442727, response=0x7fb93e163360(IPP_IDLE), pipe_pid=0,
file=-1

strace shows the process being stuck at sendto() as well.

20:18:29 sendto(152,
"\27\3\3\10\20\361\233\275\26\243,\226\243\204\346\201\275\3\301\315\321\217\336\6\367\271hV\"\231kI:\312F\214k5\324y\311_\234\227\2049\3034\311\217\27\242a/\7\244\332C\274:\302\335\"\n"...,
2069, 0, NULL, 0) = 2069
20:18:29 getrusage(RUSAGE_SELF, {ru_utime={72, 598642}, ru_stime={19,
631219}, ...}) = 0
20:18:29 times({tms_utime=7259, tms_stime=1963, tms_cutime=750,
tms_cstime=242}) = 1835914399
20:18:29 getrusage(RUSAGE_SELF, {ru_utime={72, 598642}, ru_stime={19,
631219}, ...}) = 0
20:18:29 times({tms_utime=7259, tms_stime=1963, tms_cutime=750,
tms_cstime=242}) = 1835914399
20:18:29 sendto(152,
"\27\3\3\10@\31F\256\3R\215[\323\212W\255a\225/\214@\3433?\21HX{.\3207>k\260q\"+\0174\361\34\215\37\261+)L$\326\360\nw|1\25\36\207.<\27\365\366\305`"...,
2117, 0, NULL, 0 <unfinished ...>

No other log indicates any problems: the machine has lots of TCP sockets,
high ulimit for open files, etc.

Any suggestions on what to look for?
-- 
-Alex