[Libguestfs] question on nbdkit shutdown behavior

I'm observing a difference in timing on nbdkit shutdown that is dependent on client behavior, and I wonder if that's a bug, but I can't figure out where to patch it. When there is no connection present, sending SIGINT to nbdkit terminates immediately. If, on the other hand, there is a single client currently connected, the termination behavior on SIGINT depends on what the client has done: if the client is currently silent with regards to issuing transmission-phase transactions, nbdkit hangs for 5 seconds, then forcibly tears down the connection: term1$ ./nbdkit -f -v -n file file=TODO ... nbdkit: debug: bound to IP address <any>:10809 (2 socket(s)) term2$ qemu-io -f raw nbd://localhost:10809/ -r qemu-io> term1$ ^C nbdkit: debug: waiting for running threads to complete nbdkit: debug: waited 5s for running threads to complete nbdkit: debug: /home/eblake/nbdkit/plugins/file/.libs/nbdkit-file-plugin.so: unload term2$ qemu-io> r 0 1 read failed: Input/output error qemu-io> q But, if the client makes ANY transaction during that window, then nbdkit lets that transaction succeed, then tears down the connection without any further wait: term1$ ./nbdkit -f -v -n file file=TODO ... nbdkit: debug: bound to IP address <any>:10809 (2 socket(s)) term2$ qemu-io -f raw nbd://localhost:10809/ -r qemu-io> r 0 1 # wait to hit Enter until right after ^C in term1 term1$ ^C nbdkit: debug: waiting for running threads to complete term2$ # hit enter term1$ nbdkit: file[1]: debug: acquire per-connection request lock nbdkit: file[1]: debug: pread count=1 offset=0 nbdkit: file[1]: debug: release per-connection request lock nbdkit: file[1]: debug: close nbdkit: debug: waited 2s for running threads to complete nbdkit: debug: /home/eblake/nbdkit/plugins/file/.libs/nbdkit-file-plugin.so: unload term2$ qemu-io> r 0 1 read 1/1 bytes at offset 0 1 bytes, 1 ops; 0.0002 sec (4.138 KiB/sec and 4237.2881 ops/sec) Why does current traffic from the client cause the plugin to be torn down faster? Does it matter? -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org