core/reproducible: do not override SOURCE_DATE_EPOCH

SOURCE_DATE_EPOCH is currently forcibly set (to either the git commit
date, or the last release date).

However, the spec mandates that it should not be modified if already
set: https://reproducible-builds.org/specs/source-date-epoch/

    Build systems MUST NOT overwrite this variable for child
    processes to consume if it is already present.

Abide by the rule, and only set it if not already set.

This will allow users to pass it from an upper-layer buildsystem (e.g. a
jenkins or gitlab-ci job, for example), when they have a reson to do so.

Reported-by: Peter Korsgaard <peter@korsgaard.com>
Reported-by: Einar Jón Gunnarsson <tolvupostur@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Einar Jón Gunnarsson <tolvupostur@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

diff --git a/Makefile b/Makefile
index 79db7fe48a7cf54023fca1d3d7ae91ac077ff0d9..7a727075ea98b2fc17332f24b066e784a017378e 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -254,7 +254,7 @@ export LANG = C
 export LC_ALL = C
 export GZIP = -n
 BR2_VERSION_GIT_EPOCH = $(shell GIT_DIR=$(TOPDIR)/.git $(GIT) log -1 --format=%at)
-export SOURCE_DATE_EPOCH = $(if $(wildcard $(TOPDIR)/.git),$(BR2_VERSION_GIT_EPOCH),$(BR2_VERSION_EPOCH))
+export SOURCE_DATE_EPOCH ?= $(if $(wildcard $(TOPDIR)/.git),$(BR2_VERSION_GIT_EPOCH),$(BR2_VERSION_EPOCH))
 DEPENDENCIES_HOST_PREREQ += host-fakedate
 endif