[Libguestfs] Shebang sh plugins (was: Re: [PATCH nbdkit v2 03/10] python: Implement nbdkit API version 2.)

[Richard W.M. Jones]

On Fri, Nov 22, 2019 at 02:55:31PM -0600, Eric Blake wrote:
> Unrelated side topic: in your recent addition of eval.sh, you
> wondered if we should promote it to a full-blown plugin rather than
> just an example script.  But reading 'man nbdkit-sh-plugin', there
> is no mention of turning an executable script into a full-blown
> plugin via a shebang, the way that python documents it.  [I guess
> 'man nbdkit' sort of mentions it under Shebang scripts]

I believe it's not possible to do it for sh plugins.

For (eg) python plugins it works like this:

  #!/usr/sbin/nbdkit python
  -> runs nbdkit python <script name>
  -> <script name> is interpreted as magic script= parameter
  -> the python plugin works by loading the script using
     PyRun_SimpleFileEx which interprets the contents of the
     script as python code

However for shell it doesn't work:

  #!/usr/sbin/nbdkit sh
  -> runs nbdkit sh <script name>
  -> <script name> is interpreted as magic script= parameter
  -> the sh plugin works by actually executing the script
  -> executing the script repeats the steps over from the top,
     causing an infinite loop

If you can think of a way to make this work it would be a useful
feature IMO.

Rich.

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