Disable dir_index ext2/3/4 option

[bmwiedemann]

This reduces indeterminism from filesystem readdir order,
e.g. this snippet suddenly delivers the same output every time:

mkdir -p in; for i in $(seq 10) ; do touch in/$i ; done
find in/ -type f > unreproducible

Since we usually do not have large directories, it should not slow
down the build processes.

Quick testing with

osc co openSUSE:Factory/python-service_identity && cd $_
time osc build --no-service --clean --vm-type=kvm \
    --offline --build-opt=--vm-disk-filesystem=ext4

showed that the difference is 0 +-0.2%

Alternative approach is to use a constant hash_seed via
tytso/e2fsprogs@e1f7100
but that is not yet in Leap 15.0

Helps to limit the effect of problems like
http://bugzilla.opensuse.org/show_bug.cgi?id=1110339

[mlschroe]

Hmm, but is that a good thing? We want to find and fix cases where the build depends on the filesystem ordering, don't we?

[bmwiedemann]

We can still find and fix those outside of OBS by having a local revert of this.
I thought, in OBS we want to have reduced rebuilds and re-publishs from indeterminism.

btw: does OBS use ext4 to build?
Otherwise, I might have to add this flag to ext3 as well.

[bmwiedemann]

found it: /usr/lib/build/queryconfig reads .osc/_buildconfig-standard-x86_64 to see

# Use ext4 as FS in builder workers (instead of default ext3)
BuildFlags: vmfstype:ext4

[bmwiedemann]

Maybe I should make it an option, so I can enable/disable it easily for testing if it makes a difference...

[bmwiedemann]

dir_index is now an option and default-off here.

[mlschroe]

Why is the default off?

[bmwiedemann]

default off, to make builds more reproducible by default.
The only reason to switch it on would be performance, if there were cases with 10000+ files in a single dir happening during build.

[mlschroe]

I'm not in favor of changing the current default. Lets see what the others have to say.

[adrianschroeter]

default change is indeed not a good idea. factory project might decide for it and change it there

[adrianschroeter]

@bmwiedemann shall I adapt the code to make it an opt-in?

[bmwiedemann]

dir_index is already an opt-in option. Or you want it unreproducible by default with a --vm-disk-no-dirindex option?

[adrianschroeter]

I have submitted a reworked version now:

#634

it is really making this an opt-in without changing the default. It can also be configured via prjconf for OBS setups now.
So closing this one.