Description of problem:

This is very simple to reproduce using the following short
Python plugin:
-------------------------------
#!/usr/sbin/nbdkit python3
import time
def open(readonly):
    h = {}
    return h
def close(h):
    # take some time
    time.sleep(10)
def get_size(h):
    return 1024
def pread(h, count, offset):
    buf = bytearray(count)
    return buf
-------------------------------

$ ./test.py
$ qemu-img info nbd:localhost:10809
$ qemu-img info nbd:localhost:10809

nbdkit will crash with the stack trace at end.

What seems to be happening is that the lengthy close() call
which takes 10 seconds is still running when a second NBD
connection is made.  Python doesn't like reentrancy in the
interpreter.

Version-Release number of selected component (if applicable):

nbdkit-1.2.1-1.fc28.x86_64

How reproducible:

100%

Additional info:

Program terminated with signal SIGSEGV, Segmentation fault.
#0  PyUnicode_InternInPlace ()
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Objects/unicodeobject.c:15302
15302	    Py_ALLOW_RECURSION
[Current thread is 1 (Thread 0x7fe81bc34700 (LWP 5733))]
Missing separate debuginfos, use: dnf debuginfo-install glibc-2.27-6.fc28.x86_64 libgcc-8.0.1-0.19.fc28.x86_64 libidn2-2.0.4-3.fc28.x86_64 libselinux-2.7-11.fc28.x86_64 pcre2-10.31-3.fc28.x86_64
(gdb) t a a bt

Thread 3 (Thread 0x7fe82c0c8400 (LWP 5709)):
#0  0x00007fe82b044949 in poll () from /lib64/libc.so.6
#1  0x000000000040b04c in poll (__timeout=-1, __nfds=3, __fds=0x7ffe523c4520)
    at /usr/include/bits/poll2.h:46
#2  accept_incoming_connections (socks=socks@entry=0x1f283b0, nr_socks=2)
    at sockets.c:333
#3  0x0000000000404e41 in start_serving () at main.c:818
#4  main () at main.c:598
#5  0x00007fe82af781bb in __libc_start_main () from /lib64/libc.so.6
#6  0x000000000040529a in _start ()

Thread 2 (Thread 0x7fe81c435700 (LWP 5725)):
#0  0x00007fe82b046d27 in select () from /lib64/libc.so.6
#1  0x00007fe8293113ad in pysleep (secs=<optimized out>)
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Modules/timemodule.c:1417
#2  time_sleep ()
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Modules/timemodule.c:235
#3  0x00007fe8292a4f51 in _PyCFunction_FastCallDict ()
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Objects/methodobject.c:209
#4  0x00007fe8292b6f8a in _PyCFunction_FastCallKeywords ()
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Objects/methodobject.c:275
#5  0x00007fe82931a27d in call_function ()
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Python/ceval.c:4824
#6  0x00007fe82935f08a in _PyEval_EvalFrameDefault ()
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Python/ceval.c:3322
#7  0x00007fe829345ca8 in _PyEval_EvalCodeWithName (qualname=0x0, 
    name=<optimized out>, closure=0x0, kwdefs=0x0, defcount=0, defs=0x0, 
    kwstep=2, kwcount=<optimized out>, kwargs=<optimized out>, 
    kwnames=<optimized out>, argcount=<optimized out>, args=<optimized out>, 
    locals=0x0, globals=<optimized out>, _co=<optimized out>)
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Python/ceval.c:4174
#8  PyEval_EvalCodeEx ()
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Python/ceval.c:4174
#9  0x00007fe829346c33 in function_call ()
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Objects/funcobject.c:604
#10 0x00007fe8292a9aa3 in PyObject_Call ()
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Objects/abstract.c:2261
#11 0x00007fe8292d7e58 in PyEval_CallObjectWithKeywords ()
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Python/ceval.c:4744
#12 0x00007fe8292d7eb7 in PyObject_CallObject (o=<optimized out>, 
    a=a@entry=({},))
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Objects/abstract.c:2187
#13 0x00007fe829704b77 in py_close (handle=0x7fe82bffe750) at python.c:340
#14 0x000000000040a498 in plugin_close (b=<optimized out>, conn=0x7fe814000b80)
    at plugins.c:283
#15 0x0000000000406aa2 in free_connection (conn=0x7fe814000b80)
    at connections.c:378
#16 _handle_single_connection (sockout=<optimized out>, sockin=<optimized out>)
    at connections.c:316
#17 handle_single_connection (sockin=<optimized out>, sockout=<optimized out>)
    at connections.c:326
#18 0x000000000040a939 in start_thread (datav=0x1f8c610) at sockets.c:262
#19 0x00007fe82b31b564 in start_thread () from /lib64/libpthread.so.0
#20 0x00007fe82b04f33f in clone () from /lib64/libc.so.6

Thread 1 (Thread 0x7fe81bc34700 (LWP 5733)):
#0  PyUnicode_InternInPlace ()
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Objects/unicodeobject.c:15302
#1  0x00007fe8292ba1ee in PyUnicode_InternFromString ()
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Objects/unicodeobject.c:15336
#2  0x00007fe8292baa02 in PyObject_GetAttrString ()
    at /usr/src/debug/python3-3.6.5-1.fc28.x86_64/Objects/object.c:793
#3  0x00007fe82970358d in callback_defined (name=<optimized out>, 
    obj_rtn=0x7fe81bc33c70) at python.c:94
#4  0x00007fe829703fbb in py_open (readonly=0) at python.c:308
#5  0x000000000040a518 in plugin_open (b=0x1f4b9f0, conn=0x7fe80c000b80, 
    readonly=0) at plugins.c:249
#6  0x00000000004067d4 in _handle_single_connection (sockout=7, sockin=7)
    at connections.c:356
#7  handle_single_connection (sockin=7, sockout=7) at connections.c:326
#8  0x000000000040a939 in start_thread (datav=0x1f93fb0) at sockets.c:262
#9  0x00007fe82b31b564 in start_thread () from /lib64/libpthread.so.0
#10 0x00007fe82b04f33f in clone () from /lib64/libc.so.6