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Re: [Qemu-devel] [PATCH 5/5] iotests: Let 233 run concurrently

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH 5/5] iotests: Let 233 run concurrently
Date: Tue, 7 May 2019 15:38:59 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1

On 5/7/19 1:36 PM, Max Reitz wrote:
> common.nbd's nbd_server_set_tcp_port() tries to find a free port, and
> then uses it for the whole test run.  However, this is racey because


> even if the port was free at the beginning, there is no guarantee it
> will continue to be available.  Therefore, 233 currently cannot reliably
> be run concurrently with other NBD TCP tests.
> This patch addresses the problem by dropping nbd_server_set_tcp_port(),
> and instead finding a new port every time nbd_server_start_tcp_socket()
> is invoked.  For this, we run qemu-nbd with --fork and on error evaluate
> the output to see whether it contains "Address already in use".  If so,
> we try the next port.
> On success, we still want to continually redirect the output from
> qemu-nbd to stderr.  To achieve both, we redirect qemu-nbd's stderr to a
> FIFO that we then open in bash.  If the parent process exits with status
> 0 (which means that the server has started successfully), we launch a
> background cat process that copies the FIFO to stderr.  On failure, we
> read the whole content into a variable and then evaluate it.
> While at it, use --fork in nbd_server_start_unix_socket(), too.  Doing
> so allows us to drop nbd_server_wait_for_*_socket().
> Note that the reason common.nbd did not use --fork before is that
> qemu-nbd did not have --pid-file.
> Signed-off-by: Max Reitz <address@hidden>
> ---
>  tests/qemu-iotests/233        |  1 -
>  tests/qemu-iotests/common.nbd | 93 ++++++++++++++++-------------------
>  2 files changed, 42 insertions(+), 52 deletions(-)

> @@ -34,76 +39,62 @@ nbd_server_stop()
>          fi
>      fi
>      rm -f "$nbd_unix_socket"
> -}
> -
> -nbd_server_wait_for_unix_socket()
> -{
> -    echo "Failed in check of unix socket created by qemu-nbd"
> -    exit 1
> +    rm -f "$nbd_stderr_fifo"

You could use a single 'rm -f "$nbd_unix_socket" "$nbd_stderr_fifo"'.
That's cosmetic, though.

Are we sure that even on failure, our fifo will not fill up and cause
deadlock? If the failing qemu-nbd has so much output as to be non-atomic
so that it blocks waiting for a reader, but we don't read anything until
after qemu-nbd exits after forking the daemon, then we have deadlock.
But in the common case, I don't think qemu-nbd ever spits out that much
in errors, even when it fails to start whether due to a socket in use or
for other reasons.  And even if it does hang, it is our testsuite (and
our CI tools will probably notice it), rather than our main code.

Otherwise, it's a lot of shell code with quite a few bash-isms, but we
already require bash, and I didn't spot anything blatantly wrong.

Reviewed-by: Eric Blake <address@hidden>

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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