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Re: [Libguestfs] [PATCH v2 0/3] library: improve handling of external tools

On Thu, Feb 02, 2017 at 02:33:17PM +0100, Pino Toscano wrote:
> Hi,
> the libguestfs library uses a number of external tools; for some of
> them, we search for them at build time, enabling some feature only if
> found, and later on assuming at runtime they are installed. However,
> the situation is more complex than that:
> - hardcoding the full path means that there is an incoherency in the
>   way some of the tools are used, as some other tools (e.g. qemu-img)
>   are just run as assumed to be in $PATH; also hardcoding means
>   libguestfs cannot use different versions of those tools

There is reasoning here, see m4/guestfs_programs.m4:

  # Check for external programs required to either build or run
  # libguestfs.
  # Use AC_CHECK_PROG(S) for programs which are only used during build.
  # Use AC_PATH_PROG(S) for program names which are compiled into the
  # binary and used at run time.  The reason is so that we know which
  # programs the binary actually uses.

As it says in the message, the main reason I like to hard code the
full path is because I want to know that the user is running the same
program that we tested against, ie. they didn't install some
half-baked binary in /usr/local/bin.

I think it's a mistake if we call qemu-img without the full path BTW ...

> - some of the tools are actually optional, and their runtime lack can
>   be handled gracefully instead of throwing a "failed to execute" error

Now, I do think this is a problem.  But why don't we just test if the
(full pathnamed) program exists using a simple stat or access?


> The chosen approach is to build a process-wide cache in the libguestfs
> library of paths of tools searched, even in case they are not available:
> this way it is easier to just skip something at runtime if a program is
> not there.  The patch series applies it for the libdb tools, and for
> the tools used to extract icons from guests.
> Thanks,
> Pino Toscano (3):
>   lib: add internal cache for external application
>   lib: improve libdb tools handling at runtime (RHBZ#1409024)
>   inspect: make netpbm and icoutils really optional runtime tools
>  lib/Makefile.am            |   1 +
>  lib/external-apps.c        | 190 +++++++++++++++++++++++++++++++++++++++++++++
>  lib/guestfs-internal.h     |   3 +
>  lib/inspect-apps.c         |  14 +++-
>  lib/inspect-icon.c         |  54 +++++++------
>  m4/guestfs-find-db-tool.m4 |   2 +-
>  m4/guestfs_progs.m4        |  24 ------
>  7 files changed, 237 insertions(+), 51 deletions(-)
>  create mode 100644 lib/external-apps.c
> -- 
> 2.9.3
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Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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