Re: [Qemu-devel] [PATCH v2 1/5] qemu-io: Don't die on second open

[Eric Blake]

On 05/31/2017 09:18 AM, Max Reitz wrote:
> On 2017-05-24 22:28, Eric Blake wrote:
>> Most callback commands in qemu-io return 0 to keep the interpreter
>> loop running, or 1 to quit immediately.  However, open_f() just
>> passed through the return value of openfile(), which has different
>> semantics of returning 0 if a file was opened, or 1 on any failure.
>>
>> As a result of mixing the return semantics, we are forcing the
>> qemu-io interpreter to exit early on any failures, which is rather
>> annoying when some of the failures are obviously trying to give
>> the user a hint of how to proceed (if we didn't then kill qemu-io
>> out from under the user's feet):
>>
>> $ qemu-io
>> qemu-io> open foo
>> qemu-io> open foo
>> file open already, try 'help close'
>> $ echo $?
>> 0
>>
>> Meanwhile, we WANT openfile() to report failures, as it is the
>> way that 'qemu-io -c "$something" no_such_file' knows to exit
>> early rather than attempting $something.  So the solution is to
>> fix open_f() to always return 0 (when we are in interactive mode,
>> even failure to open should not end the session), and save the
>> return value of openfile() for command line use in main().
>>
>> This has been awkward since at least as far back as commit
>> e3aff4f, in 2009.
>>
>> Signed-off-by: Eric Blake <address@hidden>
> 
> This still makes qemu-io -c "$something" no_such_file fail, right. But
> now qemu-io -c "open no_such_file" -c "$something" will execute
> $something; and I'm not sure we can convert all -c open users to not use
> that command (maybe we can, now that we have --image-opts).

Oh, so we do have some uses of -c "open..." -c "$something" (iotest 103
for example).

What happens during "$something" if there was no file currently open? It
may be a command-by-command behavior.  But as long as we get graceful
secondary failures rather than crashes for those subsequent $somethings,
we may be okay.

> 
> I don't think it's absolutely necessary for -c open to exit on error,
> but you seem to do, if I understand your penultimate paragraph
> correctly. :-)

I don't think it's absolutely necessary either.  You're right that this
patch is just worried about 'qemu-img name', not 'qemu-img -c "open
name"'.  So maybe all I need to do is update the commit message to
document that the change in semantics to -c "open..." is a side-effect,
that may or may not be changed in a followup patch?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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