On 11/23/19 9:46 AM, Richard W.M. Jones wrote:
I think an aim of nbdkit is to make it as widely usable as possible.
That's for example why we have apparent esoterica like plugins written
in shell script.
We want to in the future ship the (or "an") imageio plugin separately
from virt-v2v. A possibility is it might be shipped as a single
Python file which starts with ‘#!/usr/bin/env nbdkit python’ which
That is not portable; not all kernels permit more than one argument in
#!. You can work around it with modern env (both BSD and Coreutils have
recently added -S), as:
#!/usr/bin/env -S nbdkit python
which will then invoke the desired 'nbdkit' 'python' 'file' regardless
of whether the OS invoked 'env' '-S nbdkit python' (Linux) or 'env' '-S'
'nbdkit' 'python' (others). But that in turn assumes env -S is portable
(it's fairly new, so not yet). Otherwise, interpreted plugin shebangs
really do have to hard-code the expected path of nbdkit rather than
relying on env, unless we find a way to make:
nbdkit file
determine that when 'nbdkit-plugin-file.so' does not exist but 'file'
does, then inspect file to determine if 'nbdkit-plugin-interpreter.so'
exists based on the first line of file.
Except that would mean the first line changes to:
#!/usr/bin/env nbdkit
which lacks the desired interpreter, so we'd still have to add something
like documenting that if the first line of the #! contains 'nbdkit',
then the second line is some form of magic comment that includes the
substring 'PLUGIN=...' (we have to allow arbitrary slop as prefix or
suffix after a plugin name, to allow the second line to be a comment or
otherwise innocuous interpreted line in whatever desired target
language), so we end up with:
#!/usr/bin/env nbdkit
# PLUGIN=python
where the initial invocation of '/usr/bin/env' 'nbdkit' 'file'
then passes on to '/path/to/nbdkit' 'file'
which sees that 'file' is not a plugin, but does have magic, so it then
retries as if invoked as '/path/to/nbdkit' 'python' 'file'
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
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