Some bisecting pointed out the CoreFoundation backend mode. It seems it gets spurious poll events.

Test case:

int main(int argc, array argv) { Pike.DefaultBackend.enable_core_foundation(1); for (int i = 1; i <= 20; i++) { object client = Protocols.DNS.async_client ("212.247.28.2"); werror ("Loop: %d\n", i);

client->ip_to_host (sprintf ("212.247.28.%d", i), lambda (mixed ... args) { werror ("%O\n", args); }); } return -1; }

Output with POLL_DEBUG below. It seems fd 12 gets a second READ event and then it hangs in the read callback because there's nothing to read.

--- cut some stuff ---

[1962238720]BACKEND[0]: Creating wakeup pipe... [1962238720]BACKEND[0]: Initializing wakeup pipe... update_fd_set(0x100602d10, 28, 0x00000000, 0x00000001) [1962238720]BACKEND[0]: pdb_update_fd_set(.., 28, 0, 1, 0): [1962238720]BACKEND[0]: fd:28 READ, ADD [1962238720]BACKEND[0]: change_fd_for_box: fd from -1 to 28 [1962238720]BACKEND[0]: Wakeup pipe is [29, 1129270852] [1962238720]BACKEND[0]: Doing poll on fds: => 32 (timeout was: 4) [1962238720]BACKEND[0]: fd:25166112 filter:-15688 flags:0x0000001d UNKNOWN(25166144) [1962238720]BACKEND[0]: fd:32 filter:-4016 flags:0x00005fbf UNKNOWN[ERROR](1606414128) [1962238720]BACKEND[0]: fd:13109 filter:4 flags:0x00000000 UNKNOWN(1950392) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(653005601) [1962238720]BACKEND[0]: fd:6148 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:1 flags:0x00000000 UNKNOWN(1606414224) [1962238720]BACKEND[0]: fd:1606414544 filter:-14156 flags:0x00000019 UNKNOWN(16849152) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(1606414288) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(3) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:156897360 filter:0 flags:0x00000000 UNKNOWN(1428914176) [1962238720]BACKEND[0]: fd:-2147483622 filter:26 flags:0x00008000 UNKNOWN[EOF](1) [1962238720]BACKEND[0]: fd:159 filter:159 flags:0x00000000 UNKNOWN(-2147483648) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:12 filter:-1 flags:0x00000001 EVFILT_READ(204) [1962238720]BACKEND[0]: POLLRDNORM|POLLIN on 12 [1962238720]BACKEND[0]: hooking in box for fd 12 [1962238720]BACKEND[0]: fd:11 filter:-1 flags:0x00000001 EVFILT_READ(102) [1962238720]BACKEND[0]: POLLRDNORM|POLLIN on 11 [1962238720]BACKEND[0]: hooking in box for fd 11 [1962238720]BACKEND[0]: fd:10 filter:-1 flags:0x00000001 EVFILT_READ(201) [1962238720]BACKEND[0]: POLLRDNORM|POLLIN on 10 [1962238720]BACKEND[0]: hooking in box for fd 10 [1962238720]BACKEND[0]: fd:9 filter:-1 flags:0x00000001 EVFILT_READ(201) [1962238720]BACKEND[0]: POLLRDNORM|POLLIN on 9 [1962238720]BACKEND[0]: hooking in box for fd 9 [1962238720]BACKEND[0]: fd:8 filter:-1 flags:0x00000001 EVFILT_READ(217) [1962238720]BACKEND[0]: POLLRDNORM|POLLIN on 8 [1962238720]BACKEND[0]: hooking in box for fd 8 [1962238720]BACKEND[0]: Examining box for fd 8 revents:0x0001 [1962238720]BACKEND[0]: read_callback(8, 0x1008d8a80) ({ /* 2 elements */ "212.247.28.1", "lin-fw-internal.roxen.com" }) [1962238720]BACKEND[0]: Examining box for fd 9 revents:0x0001 [1962238720]BACKEND[0]: read_callback(9, 0x1008d8c00) ({ /* 2 elements */ "212.247.28.2", "ns3.roxen.com" }) [1962238720]BACKEND[0]: Examining box for fd 10 revents:0x0001 [1962238720]BACKEND[0]: read_callback(10, 0x1008d94a0) ({ /* 2 elements */ "212.247.28.3", "ns4.roxen.com" }) [1962238720]BACKEND[0]: Examining box for fd 11 revents:0x0001 [1962238720]BACKEND[0]: read_callback(11, 0x1008d9200) ({ /* 2 elements */ "212.247.28.4", 0 }) [1962238720]BACKEND[0]: Examining box for fd 12 revents:0x0001 [1962238720]BACKEND[0]: read_callback(12, 0x1008d8ed0) ({ /* 2 elements */ "212.247.28.5", "yp.roxen.com" }) [1962238720]BACKEND[0]: Examining box for fd -1 revents:0x0000 [1962238720]BACKEND[0]: Doing poll on fds: => 32 (timeout was: 4) [1962238720]BACKEND[0]: fd:25166112 filter:-15688 flags:0x0000001d UNKNOWN(25166144) [1962238720]BACKEND[0]: fd:32 filter:-4016 flags:0x00005fbf UNKNOWN[ERROR](1606414128) [1962238720]BACKEND[0]: fd:13109 filter:4 flags:0x00000000 UNKNOWN(1950392) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(653005601) [1962238720]BACKEND[0]: fd:6148 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:1 flags:0x00000000 UNKNOWN(1606414224) [1962238720]BACKEND[0]: fd:1606414544 filter:-14156 flags:0x00000019 UNKNOWN(16849152) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(1606414288) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(3) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:156897360 filter:0 flags:0x00000000 UNKNOWN(1428914176) [1962238720]BACKEND[0]: fd:-2147483622 filter:26 flags:0x00008000 UNKNOWN[EOF](1) [1962238720]BACKEND[0]: fd:159 filter:159 flags:0x00000000 UNKNOWN(-2147483648) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:0 filter:0 flags:0x00000000 UNKNOWN(0) [1962238720]BACKEND[0]: fd:12 filter:-1 flags:0x00000001 EVFILT_READ(204) [1962238720]BACKEND[0]: POLLRDNORM|POLLIN on 12 [1962238720]BACKEND[0]: hooking in box for fd 12 [1962238720]BACKEND[0]: fd:16 filter:-1 flags:0x00000001 EVFILT_READ(212) [1962238720]BACKEND[0]: POLLRDNORM|POLLIN on 16 [1962238720]BACKEND[0]: hooking in box for fd 16 [1962238720]BACKEND[0]: fd:15 filter:-1 flags:0x00000001 EVFILT_READ(102) [1962238720]BACKEND[0]: POLLRDNORM|POLLIN on 15 [1962238720]BACKEND[0]: hooking in box for fd 15 [1962238720]BACKEND[0]: fd:14 filter:-1 flags:0x00000001 EVFILT_READ(102) [1962238720]BACKEND[0]: POLLRDNORM|POLLIN on 14 [1962238720]BACKEND[0]: hooking in box for fd 14 [1962238720]BACKEND[0]: fd:13 filter:-1 flags:0x00000001 EVFILT_READ(205) [1962238720]BACKEND[0]: POLLRDNORM|POLLIN on 13 [1962238720]BACKEND[0]: hooking in box for fd 13 [1962238720]BACKEND[0]: Examining box for fd 13 revents:0x0001 [1962238720]BACKEND[0]: read_callback(13, 0x1008d9770) ({ /* 2 elements */ "212.247.28.6", "yp2.roxen.com" }) [1962238720]BACKEND[0]: Examining box for fd 14 revents:0x0001 [1962238720]BACKEND[0]: read_callback(14, 0x1008d8ae0) ({ /* 2 elements */ "212.247.28.7", 0 }) [1962238720]BACKEND[0]: Examining box for fd 15 revents:0x0001 [1962238720]BACKEND[0]: read_callback(15, 0x1008d91a0) ({ /* 2 elements */ "212.247.28.8", 0 }) [1962238720]BACKEND[0]: Examining box for fd 16 revents:0x0001 [1962238720]BACKEND[0]: read_callback(16, 0x1008d92c0) ({ /* 2 elements */ "212.247.28.9", "ldapmaster.roxen.com" }) [1962238720]BACKEND[0]: Examining box for fd 12 revents:0x0001 [1962238720]BACKEND[0]: read_callback(12, 0x1008d8ed0) [New Thread 0x190b of process 55510] ^C Program received signal SIGINT, Interrupt. 0x00007fff8a8e133a in ?? () (gdb) bt #0 0x00007fff8a8e133a in ?? () #1 0x000000010018a8ae in udp_read (args=<optimized out>) at /Users/marty/projects/Pike/8.0/src/modules/_Stdio/udp.c:812 #2 0x0000000100014704 in lower_mega_apply (args=0, o=0x1008d8ed0, fun=51) at /Users/marty/projects/Pike/8.0/src/interpret.c:2172 #3 0x0000000100014ba2 in jump_opcode_F_MARK_CALL_LFUN (arg1=<optimized out>) at /Users/marty/projects/Pike/8.0/src/interpret_functions.h:2406 #4 0x0000000101be98cd in ?? () #5 0x4b00365126ec1321 in ?? () #6 0x000000000000a028 in ?? () #7 0x000000000000000c in ?? () #8 0x0000000100479e60 in ?? () #9 0x00000001001dc2b8 in ?? () #10 0x0000000000000150 in ?? () #11 0x00007fff5fbfea50 in ?? () #12 0x0000000100017215 in eval_instruction (pc=<optimized out>) at /Users/marty/projects/Pike/8.0/src/interpret.c:1711 #13 mega_apply (type=<optimized out>, args=1606346880, arg1=0x10000, arg2=<optimized out>) at /Users/marty/projects/Pike/8.0/src/interpret.c:2694 Backtrace stopped: frame did not save the PC (gdb)