SDE calculations broken

I was worried about the failures I was seeing in reproducible builds. I
therefore put more instrumentation about where SDE is calculated in builds.
There are some situations we'd not expect to see a value of SDE so I tried to
write logic that describes it. It is harder than you'd think as the value isn't
valid in fetch, unpack, prepare_recipe_sysroot or *_setscene tasks. I tried to
make the warnings conditional upon BB_CURRENTTASK only to find that gets set
later that the SDE function is called. I did add some hacks to make it get set
earlier but we'll come back to how early it runs.

Once it has a value, the function writes it to the datastore, since re-reading
it all the time is high overhead. This means that the value is cached and once
set, doesn't change.

In testing on the autobuilder, I found this log extract:

NOTE: Running task 196 of 2214 (virtual:native:/home/pokybuild/yocto-worker/buildtools/build/meta/recipes-devtools/rsync/rsync_3.2.3.bb:do_patch)
[...]
WARNING: Cannot find /home/pokybuild/yocto-worker/buildtools/build/build/tmp/work/core2-64-poky-linux/rsync/3.2.3-r0/source-date-epoch/__source_date_epoch.txt. SOURCE_DATE_EPOCH will default to 1302044400
[...]
NOTE: recipe rsync-native-3.2.3-r0: task do_patch: Started

buried in:

https://autobuilder.yoctoproject.org/typhoon/#/builders/20/builds/4430/steps/11/logs/stdio

What worries me there is that rsync-native is looking in core2-64-poky-linux,
i.e. the target rsync. How could that happen?

My theory is that SDE is being called early in parsing before the BBCLASSEXTEND
has modified PN and friends.

At this point I'm very worried as the whole situation looks error and race prone
. It may explain some of the instabilities I've been seeing though.

Cheers,

Richard