[Qemu-block] qemu-nbd vs 'simple' trace backend vs iotest 147

[Cornelia Huck]

Hi,

I recently noticed that iotest 147 was hanging on my laptop, but worked
fine on my s390x LPAR. Turned out that the architecture was a red
herring; on both platforms, things fail with the 'simple' trace backend
and work with e.g. the 'log' trace backend. Some details on the
failures with the 'simple' backend:

- The first run of 147 passes. However, there are two processes hanging
  around, one using a unix socket and one using an inet socket:

cohuck   22912  0.0  0.0 156580  3836 ?        Ss   14:32   0:00 
/home/cohuck/git/qemu/build/tests/qemu-iotests/../../qemu-nbd --fork -f qcow2 
/home/cohuck/git/qemu/build/tests/qemu-iotests/scratch/test.img -p 10811
cohuck   22925  0.0  0.0 156580  3840 ?        Ss   14:32   0:00 
/home/cohuck/git/qemu/build/tests/qemu-iotests/../../qemu-nbd --fork -f qcow2 
/home/cohuck/git/qemu/build/tests/qemu-iotests/scratch/test.img -k 
/home/cohuck/git/qemu/build/tests/qemu-iotests/scratch/nbd.socket

Attaching a gdb shows that we seem to be waiting on flushing:

(gdb) bt
#0  0x00007f461c078b99 in syscall () from /lib64/libc.so.6
#1  0x00007f461d13650f in g_cond_wait () from /lib64/libglib-2.0.so.0
#2  0x0000560cf3a1caf2 in flush_trace_file (wait=255)
    at /home/cohuck/git/qemu/trace/simple.c:139
#3  st_flush_trace_buffer () at /home/cohuck/git/qemu/trace/simple.c:374
#4  0x00007f461bfc01d8 in __run_exit_handlers () from /lib64/libc.so.6
#5  0x00007f461bfc022a in exit () from /lib64/libc.so.6
#6  0x0000560cf392eb7e in main (argc=<optimized out>, argv=<optimized out>)
    at /home/cohuck/git/qemu/qemu-nbd.c:1076

(for both processes)

- When I'm running 147 again, the test using the inet socket is unhappy
  as the port is already in use:

test_inet (__main__.QemuNBD) ... DEBUG:QMP:>>> {'execute': 'qmp_capabilities'}
DEBUG:QMP:<<< {u'return': {}}
Failed to find an available port: Address already in use

...but then it sits there, with ps showing a zombie qemu-nbd process,
until I Ctrl-C out of it. There's now a new qemu-nbd process (command
line identical to the first process above), and it's hanging with the
same backtrace (so same root problem, presumably). I would have
expected 147 to either fail or continue, though, instead of hanging
around.

Running under Fedora 27, current QEMU master.