httpd threads hang using mod_cluster when JBoss is killed and restarted

Issue

• We use httpd with mod_cluster. After killing JBoss, we see some requests that appear to be stalled on httpd through the mod_status page. pstack from the time seems to show some recursive call in mod_proxy_cluster:

#0  0x00007ff8eb547533 in select () from /lib64/libc.so.6
#1  0x00007ff8eba39185 in apr_sleep () from /usr/lib64/libapr-1.so.0
#2  0x00007ff8e84be0d1 in ?? () from /etc/httpd/modules/mod_proxy_cluster.so
...
#160 0x00007ff8e84beb9f in ?? () from /etc/httpd/modules/mod_proxy_cluster.so
#161 0x00007ff8e88d2116 in proxy_run_pre_request () from /etc/httpd/modules/mod_proxy.so
#162 0x00007ff8e88d9186 in ap_proxy_pre_request () from /etc/httpd/modules/mod_proxy.so
#163 0x00007ff8e88d63c2 in ?? () from /etc/httpd/modules/mod_proxy.so