[yocto] externalsrc + sstate why is not compatible?

[Marcelo E. Magallon]

On Wed, Dec 06, 2017 at 10:59:37AM -0600, Koehler, Yannick wrote:
> Ok, will try that.  If that works, I may see if I can alter the file
> fetcher to use symlinks, not sure if sstate subsystem will like that
> or not.

That was my idea, but I've never tried it. I'm sure the devil is in the
details. If you can elide the copy and simply replace it with a symlink,
you'd get faster builds.

> If we do so, and someone change the file in /src/somedir will yocto
> redo the fetch/unpack pattern to recopy over the original content by
> itself?

Yes, it does. You change a file, and the fetcher runs again. This means
S and B are deleted (if split), do_fetch runs again and S is populated
fresh. do_configure and do_compile populate B again.

Once you get that working, people will complain as to why the system is
rebuilding everything instead of just the file that changed :-) It might
work "as expected" if S = B, and your build system in the module is
smart enough to produce the correct dependencies (e.g. cmake or bazel).

Marcelo