[poky] summary about recent do_populate_sdk failures

[Tian, Kevin]

Lianhao and I are looking into recent master instability, more specifically starting
from reported do_populate_sdk failures. There have been 3 problems revealed:

1) unexpected do_package/do_package_write* rebuild
2) change PACKAGE_CLASSES causes more do_package rebuilds
3) do_populate_sdk finally exits due to eglibc not matching expected version

We've root caused 1), and the patch has been sent out:
http://git.pokylinux.org/cgit/cgit.cgi/poky-contrib/commit/?h=tk/master1&id=68ad8ead1a83161afb8c2a65a28dfc205181d80e

We're not sure whether 3) is caused by too many unexpected do_package rebuilds,
and is now testing whether 3) will disappear with the fix for 1). We'll also look into
original failure to see whether real cause of 3) may be hidden with the fix.

Now the remaining open is about 2). We got the reason, but would like to welcome
Comments on an elegant solution.

The phenomenon is that do_package checksum changes with switching the order
In PACKAGE_CLASSES, e.g:

From
PACKAGE_CLASSES = "package_rpm package_ipk"
To
PACKAGE_CLASSES = "package_ipk package_rpm"

This is undesired since the order only matters for final rootfs generation. The actual
cause is related to how do_package is generated, which depends on when 
package.bbclass is first brought in.

Take package_rpm.bbclass for example, which inherit package.bbclass. Bitbake
generates below wrappers implicitly:

Do_package () {
	bb.build.exec_func('package_rpm_do_package', d)
}

Package_rpm_do_package() {
	bb.build.exec_func('package_do_package', d)
}

Above is implicitly created when bitbake parses class files based on inheritance tree,
and once a class file is parsed it's then saved to the cache. When later package_ipk is
parsed, bitbake will simply read from the cache for package.bbclass and then
do_package remains tying to package_rpm_do_package.

If package_ipk is the 1st in PACKAGE_CLASSES, then do_package becomes:

Do_package () {
	bb.build.exec_func('package_ipk_do_package', d)
}

This sure generates different checksum to break sstate installation, but in reality it's
just name difference, no actual functional change. We considered to use vardeps and 
varexcldeps which don't help though since it's the do_package itself changed.

One possible option is to sort the INHERIT list, which ensures we always parse
Inherited class files in same order if no content change. We'll test whether this works,
and also hear your comments.

Thanks
Kevin