[PATCH v2 4/4] reproducible: try to ensure reproducible xz archives

["André Draszik" <git@andred.net>]

xz suffers from a reproducibility problem when not using multi-
threaded mode:
a) archives are created differently in single- vs multi-threaded
   modes
b) xz will scale down the compression level so as to be able to
   work within any memory limit given to it when being launched
  in single-threaded mode.

Thus, for reproducible xz archives we need to launch xz with
at least two threads.

Add a little sanity test, and error out otherwise, so as to
guarantee no difference due this fact.

Assumptions:
* The thread limit should be set via XZ_THREADS, not via
  modifying XZ_DEFAULTS or XZ_OPTS, or any other way
* The thread limit should not be set to xz's magic value
  zero (0), as that will lead to single-threaded mode on
  single-core systems

This patch here doesn't prevent people from shooting themselves
into the foot by changing XZ_DEFAULTS to change the number
of threads directly, but it's can serve as a hint at least.

Signed-off-by: André Draszik <git@andred.net>
---
 meta/classes/reproducible_build.bbclass | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/meta/classes/reproducible_build.bbclass b/meta/classes/reproducible_build.bbclass
index 750eb950f2..e07bef87d8 100644
--- a/meta/classes/reproducible_build.bbclass
+++ b/meta/classes/reproducible_build.bbclass
@@ -35,6 +35,7 @@
 # SOURCE_DATE_EPOCH is set for all tasks that might use it (do_configure, do_compile, do_package, ...)
 
 BUILD_REPRODUCIBLE_BINARIES ??= '1'
+BUILD_REPRODUCIBLE_XZ_ARCHIVES ??= '1'
 inherit ${@oe.utils.ifelse(d.getVar('BUILD_REPRODUCIBLE_BINARIES') == '1', 'reproducible_build_simple', '')}
 
 SDE_DIR ="${WORKDIR}/source-date-epoch"
@@ -198,4 +199,8 @@ BB_HASHBASE_WHITELIST += "SOURCE_DATE_EPOCH"
 python () {
     if d.getVar('BUILD_REPRODUCIBLE_BINARIES') == '1':
         d.appendVarFlag("do_unpack", "postfuncs", " do_create_source_date_epoch_stamp")
+
+        if d.getVar('BUILD_REPRODUCIBLE_XZ_ARCHIVES') == '1':
+            if int(d.getVar('XZ_THREADS')) < 2:
+                bb.fatal("Can not build reproducible XZ archives without threading")
 }
-- 
2.23.0.rc1