[libvirt] Why using AC_PATH_PROG to detect the location of run-time external programs

[Eric Blake]

On 03/19/2014 03:15 PM, Howard Tsai wrote:
> Hello,
> 
> I am wondering why AC_PATH_PROG is used to detect the location of run-time
> external programs (such as dnsmasq, ovs-vsctl, etc.), and hardcoded the
> locations in binary. It seems wrong to me.

It is only hard-coding the default location of the tools.  You are free
to override that, by using

./configure OVSVSCTL=/path/to/the/destinat*ion/ovs-vsctl

to force the binary to be built with the hard-coded location you plan on
using in your installation, or use

./configure OVSVSCTL=ovs-vsctl

to force libvirtd to do a PATH lookup every time.

> 
> At compile-time, configure script should only determine whether the
> resulting binary should support the use of these external programs. The
> actual location of these programs should be determined at run-time, either
> from libvirtd.conf or by $PATH.

Deferring the decision to libvirtd.conf might be feasible, albeit more
complex.  Propose a patch, and we'll look at it.  But for now, the
default is good enough, and over-ridable at configure time, so it's not
something that I'm personally bothered by enough to write a patch myself.

By the way, we DO prefer hard-coded locations over PATH lookups, because
PATH lookups can result in odd behavior when two people run
qemu:///session with different PATH, and where the lookup may pick up a
different binary for the two users.  Configuring a hard-coded location
requires a bit more planning, but is more secure - and since MOST users
go with pre-built distributions, the hard-coded binaries can be
guaranteed* to be present (for example, the rpm files used by Fedora
guarantee that for any tool where we hard-code the location into the
libvirt binary as living in /usr/bin will also be a prereq of the
package, so that if you install libvirt, you also install the prereq
packages that libvirt will try to use).  [*Of course, if you ever hit a
case where this guarantee is not met, file a bug against the distro to
inform them of the missing dependency]

> 
> I haven't looked into how to fix it.
> 
> The problem I ran into earlier is this: on my dev box, I have Open vSwitch
> utility ovs-vsctl installed locally in /usr/local/bin. Therefore, at
> build-time, libvirt detected the location of ovs-vsctl and hardcoded
> '/usr/local/bin/ovs-vsctl' in libvirtd. When I package libvirt and install
> it on my test box, it couldn't find ovs-vsctl, as ovs-vsctl is installed at
> /usr/bin/ovs-vsctl in the test box, so it failed.

Again, that's a matter of setting up OVSVSCTL on your dev box to match
the location where the binary will be present on your test box.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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