This is the diagnostic resulting from uv_poll_init() when the return value is EEXIST.

IO_TASK: POLL_PROXY -  Polling Initialization Error -17, EEXIST

The stack trace is as follows.

Program terminated with signal SIGABRT, Aborted.
#0  0x00007fb53a39b267 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:55
55	../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt
#0  0x00007fb53a39b267 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:55
#1  0x00007fb53a39ceca in __GI_abort () at abort.c:89
#2  0x000000000040d70f in fatal_error (log_prio=2, 
    fmt=0x425028 "IO_TASK: POLL_PROXY -  Polling Initialization Error %d, %s") at misc.c:135
#3  0x0000000000405ba3 in poll_proxy (handle=0x642f80 <IO_Task_Async_Handle>) at network_io.c:287
#4  0x00000000004117bb in uv__async_io (loop=0xc27800 <Poll_Loop>, w=<optimized out>, events=<optimized out>)
    at src/unix/async.c:163
#5  0x000000000041d545 in uv__io_poll (loop=loop@entry=0xc27800 <Poll_Loop>, timeout=<optimized out>)
    at src/unix/linux-core.c:462
#6  0x0000000000411f38 in uv_run (loop=0xc27800 <Poll_Loop>, mode=UV_RUN_DEFAULT) at src/unix/core.c:385
#7  0x0000000000405891 in IO_Task (arg=0x0) at network_io.c:100
#8  0x00007fb53a7376aa in start_thread (arg=0x7fb535d0d700) at pthread_create.c:333
#9  0x00007fb53a46ceed in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109

In this case fd = 30 and Poll_Loop.watchers[30] is NULL.