mod_cluster deadlocks, leading to httpd thread exhaustion

Issue

• We see httpd stall on windows using mod_cluster after all requests are exhausted. It stays like this permanently until restart. It looks like mod_cluster hit a deadlock as many threads are stalled here:

kernel32.dll!LockFileEx+0x1b
libapr-1.dll!apr_file_lock+0x69 <-- here
mod_slotmem.so+0x1318 <-- here
mod_manager.so+0x2a11 <-- here
mod_proxy_cluster.so+0x679e
mod_proxy.so!proxy_run_post_request+0x4e
mod_proxy.so!proxy_run_request_status+0x924
libhttpd.dll!ap_run_handler+0x35
libhttpd.dll!ap_invoke_handler+0x114
libhttpd.dll!ap_die+0x2ea
libhttpd.dll!ap_psignature+0x1ae8
libhttpd.dll!ap_run_process_connection+0x35
libhttpd.dll!ap_process_connection+0x3b