Deadlock in reading stdout from cmd

[visr]

I want to read the stdout from an external program and process the results in Julia. However it sometimes hangs indefinitely.

It was previously discussed on Discourse, but @vtjnash suggested to make an issue here, he could reproduce it by running this on all cores (I also see it on one core):

while true
    readstring(`dd if=/dev/zero bs=1048576 count=2`)
end

It just reads in two MiB of zeroes, in two blocks.
Successful iterations of the loop will print this to stderr:

2+0 records in
2+0 records out
2097152 bytes (2,1 MB, 2,0 MiB) copied, 0,00447523 s, 469 MB/s

Until it hangs. Killing it with Ctrl-C then gives the following:

1+0 records in
0+0 records out
0 bytes copied, 11,1429 s, 0,0 kB/s

signal (2): Interrupt
while loading /home/martijn/test/dd.jl, in expression starting on line 1
unknown function (ip: 0x7fb138855498)
uv__epoll_wait at /home/martijn/src/julia/deps/srccache/libuv-52d72a52cc7ccd570929990f010ed16e2ec604c8/src/unix/linux-syscalls.c:321
uv__io_poll at /home/martijn/src/julia/deps/srccache/libuv-52d72a52cc7ccd570929990f010ed16e2ec604c8/src/unix/linux-core.c:267
uv_run at /home/martijn/src/julia/deps/srccache/libuv-52d72a52cc7ccd570929990f010ed16e2ec604c8/src/unix/core.c:354
process_events at ./libuv.jl:82 [inlined]
wait at ./event.jl:216
wait at ./event.jl:27
jl_call_fptr_internal at /home/martijn/src/julia/src/julia_internal.h:369 [inlined]
jl_call_method_internal at /home/martijn/src/julia/src/julia_internal.h:388 [inlined]
jl_apply_generic at /home/martijn/src/julia/src/gf.c:1929
jl_apply_2va at /home/martijn/src/julia/src/rtutils.c:287
stream_wait at ./stream.jl:42
close at ./stream.jl:326
open at ./process.jl:588
read at ./process.jl:645
readstring at ./process.jl:651
macro expansion at /home/martijn/test/dd.jl:2 [inlined]
anonymous at ./<missing> (unknown line)
jl_call_fptr_internal at /home/martijn/src/julia/src/julia_internal.h:369 [inlined]
jl_call_method_internal at /home/martijn/src/julia/src/julia_internal.h:388 [inlined]
jl_toplevel_eval_flex at /home/martijn/src/julia/src/toplevel.c:594
jl_parse_eval_all at /home/martijn/src/julia/src/ast.c:909
jl_load at /home/martijn/src/julia/src/toplevel.c:620
include_from_node1 at ./loading.jl:556
jl_call_fptr_internal at /home/martijn/src/julia/src/julia_internal.h:369 [inlined]
jl_call_method_internal at /home/martijn/src/julia/src/julia_internal.h:388 [inlined]
jl_apply_generic at /home/martijn/src/julia/src/gf.c:1929
include at ./sysimg.jl:14
jl_call_fptr_internal at /home/martijn/src/julia/src/julia_internal.h:369 [inlined]
jl_call_method_internal at /home/martijn/src/julia/src/julia_internal.h:388 [inlined]
jl_apply_generic at /home/martijn/src/julia/src/gf.c:1929
process_options at ./client.jl:315
_start at ./client.jl:383
jl_call_fptr_internal at /home/martijn/src/julia/src/julia_internal.h:369 [inlined]
jl_call_method_internal at /home/martijn/src/julia/src/julia_internal.h:388 [inlined]
jl_apply_generic at /home/martijn/src/julia/src/gf.c:1929
unknown function (ip: 0x401e40)
unknown function (ip: 0x401602)
__libc_start_main at /build/glibc-bfm8X4/glibc-2.23/csu/../csu/libc-start.c:291
unknown function (ip: 0x4016b8)
unknown function (ip: 0xffffffffffffffff)
Allocations: 210788 (Pool: 210123; Big: 665); GC: 35

As noted on Discourse, when using smaller block sizes, it always seems to hang after 65536 (2^16) bytes. This is seen on julia 0.5/0.6/master and all OS, though on Unix it seems to happen more quickly than Windows.

@vtjnash said that it seems like something is dropping an event notification.

cc @evetion