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Re: [Qemu-block] [Qemu-devel] [PATCH v10 09/16] block: Add QMP support f

From: Markus Armbruster
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v10 09/16] block: Add QMP support for streaming to an intermediate layer
Date: Tue, 11 Oct 2016 18:50:27 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Eric Blake <address@hidden> writes:

> On 10/11/2016 09:57 AM, Kevin Wolf wrote:
>> Should we introduce a new, clean blockdev-stream command that fixes this
>> and matches the common name pattern? Of course, block-stream vs.
>> blockdev-stream could be a bit confusing, too...
> A new command is easy to introspect (query-commands), lets us get rid of
> cruft, and makes it obvious that we support everything from the get-go.
> I'm favoring that option, even if it leads to slightly confusing names
> of the deprecated vs. the new command.

Let's take a step back and consider extending old commands vs. adding
new commands.

A new command is trivial to detect in introspection.

Command extensions are not as trivial to detect in introspection.  Many
extensions are exposed in query-qmp-schema, but not all.

Back when QMP was young, Anthony argued for always adding new commands,
never extend existing ones.  I opposed it, because it would lead to a
confusing mess of related commands, unreadable or incomplete
documentation, and abysmal test coverage.

However, the other extreme is also unwise: we shouldn't shoehorn new
functionality into existing commands just because we can.  We should ask
ourselves questions like these:

* Is the extended command still a sane interface?  If writing clear
  documentation for it is hard, it perhaps isn't.  Pay special attention
  to failure modes.  Overloaded arguments are prone to confusing errors.

* How will the command's users use the extension?  If it requires new
  code paths, a new command may be more convenient for them.

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