do not copy src directory if -S option is also given when src method is used

[avg-I]

This is useful for a scenario where I want to create a jail from a pre-built source tree, but for the builds that will use the jail I want to provide a different source tree.

[bdrewery]

The patch looks fine, but why would you want a different source tree in the jail than what it was installed from?

[avg-I]

As I understand, /usr/src is needed mostly for kernel modules and tools that talk to kernel. I use FreeBSD head, I update the kernel quite often, but the world not that often (because of how long it takes to build it).
So, I want my poudriere to pick the main (in sync with the running kernel) /usr/src via nullfs during builds. But when I rebuild the world I want to be able to use -m src=/usr/src to install or update the jail.

[bdrewery]

Are you using these jails for something other than port builds?

For port module builds the /usr/src needs to match the target kernel (and be stable for rebuilds where the jail was not updated). I suppose if you were using this jail for port builds to use packages locally you would always want the latest host /usr/src mounted in for kernel packages.