Loop in casDGClient::processDG() after zero-length PV

[Andrew Johnson]

From Shuei Yamada in http://www.aps.anl.gov/epics/tech-talk/2017/msg00714.php:

I have a problem with CA gateway that it runs away and all PVs
subscribing via ca-gateway become disconnected. The problem occurred 4
times in past 6 month and each time I had to restart CA gateway. I'm
using CA gataway 2.1.0 with EPICS base 3.14.12.3 on Scientific Linux
6.8 (RHEL 6.8 clone).

The log file contains following message:
====
CAS Request: ? on jkjnuc31.ccr.jkcont:40325: cmd=6 cid=76 typ=5 cnt=11
psz=32 avail=4c
CAS:
Apr 20 06:29:30 !!! Errlog message received (message is above)
zero length PV name in UDP search request?
====
The machine in the logfile, jkjnuc31.ccr.jkcont (we're using private
network for accelerator control), is one of our PCs running
CS-Studio. I replaced the machine with different one, upgraded CSS
from 4.1.1 to 4.4.1, but jkjnuc31 is sure to be involved every time.

I attached to the process during the inspection and got following back-trace:
====
(gdb) bt
#0 0x00007f5a180a74e0 in outBuf::commitMsg() () from
/svjk/jk/epics/R3.14.12/base-3.14.12.3-CSA/lib/linux-x86_64/libcas.so.3.14
#1 0x00007f5a18099512 in casDGClient::sendVersion() () from
/svjk/jk/epics/R3.14.12/base-3.14.12.3-CSA/lib/linux-x86_64/libcas.so.3.14
#2 0x00007f5a1809abb3 in casDGClient::processDG() () from
/svjk/jk/epics/R3.14.12/base-3.14.12.3-CSA/lib/linux-x86_64/libcas.so.3.14
#3 0x00007f5a180aa16b in casDGIntfOS::recvCB(inBufClient::fillParameter) ()
   from /svjk/jk/epics/R3.14.12/base-3.14.12.3-CSA/lib/linux-x86_64/libcas.so.3.14
#4 0x00007f5a17388fa6 in fdManager::process(double) () from
/svjk/jk/epics/R3.14.12/base-3.14.12.3-CSA/lib/linux-x86_64/libCom.so.3.14
#5 0x0000000000412f0e in gateServer::mainLoop (this=0x7a2440) at
../gateServer.cc:280
#6 0x0000000000406830 in startEverything (prefix=0x7ffcd6f010f5
"MRCO:GW:MR-CCR01") at ../gateway.cc:685
#7 0x0000000000408c16 in main (argc=26, argv=0x7ffcd6effbf8) at
../gateway.cc:1353
====
It seems that gateway falls into an infinite loop within
casDGClient::processDG().

Related (fix in CS-Studio): https://github.com/ControlSystemStudio/cs-studio/issues/2199