It turned out we cannot currently use slapos.cookbook in software part
of SR - the reason is slapos.cookbook egg depends on lxml egg, which in
turn needs libxml to be also installed via slapos.

For this reason stack/slapos.cfg has

    [slapos-cookbook]   # NOTE _not_ slapos.cookbook
    recipe = zc.recipe.egg
    eggs =
      ${lxml-python:egg}
    ...

and lxml-python is lxml building recipe to build it together with
libxml:

    [lxml-python]
    recipe = zc.recipe.egg:custom
    egg = lxml

    rpath =
      ${libxml2:location}/lib/
      ${libxslt:location}/lib/
      ${zlib:location}/lib/
    environment = lxml-python-env
    ...

So underlying idea, as I understand it, is: every SR contains
slapos-cookbook in parts and this way lxml-python gets build. Then when
there is slapos.cookbook egg usage, it is already correctly built.

BUT

This works when such slapos.cookbook egg usage happens _only_ in instance
part of an SR: otherwise, if buildout sees slapos.cookbook egg usage in
some recipe, e.g. like it currently is in helloweb-ruby:

    [helloweb-ruby]
    recipe  = slapos.cookbook:wrapper
    ...

it _first_ tries to install slapos.cookbook egg directly - as needed for
recipes eggs are installed as a first step, _before_ further buildout
processing. What happens then is that slapos.cookbook (note not "-") egg
sources and dependencies are downloaded from pypi, including lxml egg,
all are tried to build, and in lxml egg it fails this way:

    ...
    Processing lxml-3.5.0
    Writing /tmp/tmpLGK4xWbuild/lxml-3.5.0/setup.cfg
    Running setup.py -q bdist_egg --dist-dir /tmp/tmpLGK4xWbuild/lxml-3.5.0/egg-dist-tmp-DJvofa
    Building lxml version 3.5.0.
    Building without Cython.
    ERROR: /bin/sh: 1: xslt-config: not found

    ** make sure the development packages of libxml2 and libxslt are installed **

    Using build configuration of libxslt
    In file included from src/lxml/lxml.etree.c:323:0:
    src/lxml/includes/etree_defs.h:14:31: fatal error: libxml/xmlversion.h: No such file or directory
     #include "libxml/xmlversion.h"
                                   ^
    compilation terminated.
    Compile failed: command 'gcc' failed with exit status 1
    /tmp/tmpLGK4xWbuild/lxml-3.5.0/temp/xmlXPathInitGlEAOF.c:1:26: fatal error: libxml/xpath.h: No such file or directory
     #include "libxml/xpath.h"
                              ^
    compilation terminated.
    *********************************************************************************
    Could not find function xmlCheckVersion in library libxml2. Is libxml2 installed?
    *********************************************************************************
    error: Setup script exited with error: command 'gcc' failed with exit status 1
    An error occurred when trying to install lxml 3.5.0. Look above this message for any errors that were output by easy_install.
    Could't load zc.buildout entry point wrapper
    from slapos.cookbook:
    Couldn't install: lxml 3.5.0.
    While:
      Installing.
      Getting section helloweb-ruby.
      Initializing section helloweb-ruby.
      Installing recipe slapos.cookbook.
    Error: Couldn't install: lxml 3.5.0

Previously it probably used to work because we had system libxml
installed, and this way lxml compilation succeeded (but was incorrect
from slapos point of view).

( The problem turned out to be already known somehow - see e.g. c7d00913
  "Initial neoppod commit." and look for "Note on LXML/END LXML" there )

Solution could be: either fix slapos.cookbook installation via e.g.
teaching buildout to take into account pre-dependencies for eggs (for
lxml) or just to avoid using slapos.cookbook:wrapper for executable
generation.

While @kazuhiko is working on the first more-generic solution, here goes
a simpler one to just make helloweb component alive again: like it is
done in a lot of places (e.g. in software/kvm/) let's use
collective.recipe.template to generate a short shell script.

NOTE

previously the command line was

    ${bundler:bundle} exec sh -c 'helloweb.rb "$@"' ${:_buildout_section_name_}

but now it is the same with "$@" appended:

    exec ${bundler:bundle} exec sh -c 'helloweb.rb "$@"' ${:_buildout_section_name_} "$@"

The reason is slapos.cookbook:wrapper uses slapos.recipe.librecipe.execute.generic_exec()
in generated script, which appends sys.argv[1:] to the command-line implicitly:

    def generic_exec(args):
      ...
      os.execve(exec_list[0], exec_list + sys.argv[1:], exec_env)
                                        ^^^^^^^^^^^^^^

    https://lab.nexedi.com/nexedi/slapos/blob/54bbe0a9/slapos/recipe/librecipe/execute.py#L84

that's why last "$@" was not present in original version.

P.S.

Otherwise currently slapos.cookbook is used only in instance parts of
recipes in whole slapos.git

/reviewed-by TrustMe
/debugged-with @kazuhiko
/cc @vpelletier