go.bbclass: change GOTMPDIR to improve reproducibility

go.bbclass: change GOTMPDIR to improve reproducibility

When cgo is enabled, the Go toolchain writes temporary source files
(*.c) under GOTMPDIR and compiles them there. when -trimpath is passed
to go, Go passes options such as
-ffile-prefix-map=$WORK/b387=/tmp/go-build internally to the GCC
instance it invokes. The variable WORK is a temporary directory created
under GOTMPDIR, refer the following log:

[snip of compile log]
WORK=/build/tmp/work/x86-64-v3-wrs-linux/buildah/1.41.5/build-tmp/go-build377321751
cd $WORK/b387
TERM='dumb' x86_64-wrs-linux-gcc -m64 -march=x86-64-v3 -fstack-protector-strong -O2 -D_FORTIFY_SOURCE=2 -Wformat -Wformat-security -Werror=format-security --sysroot=/tmp/work/x86-64-v3-wrs-linux/buildah/1.41.5/recipe-sysroot -I /tmp/work/x86-64-v3-wrs-linux/buildah/1.41.5/sources/buildah-1.41.5/src/github.com/containers/buildah/vendor/github.com/proglottis/gpgme -fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=$WORK/b387=/tmp/go-build -gno-record-gcc-switches -v -D_FILE_OFFSET_BITS=64 -I $WORK/b387/ -O2 -g -ffile-prefix-map=/tmp/work/x86-64-v3-wrs-linux/buildah/1.41.5/sources/buildah-1.41.5=/usr/src/debug/buildah/1.41.5 -ffile-prefix-map=/tmp/work/x86-64-v3-wrs-linux/buildah/1.41.5/build=/usr/src/debug/buildah/1.41.5 -ffile-prefix-map=/tmp/work/x86-64-v3-wrs-linux/buildah/1.41.5/recipe-sysroot= -ffile-prefix-map=/tmp/work/x86-64-v3-wrs-linux/buildah/1.41.5/recipe-sysroot-native= -pipe -v -ffile-prefix-map=/tmp/work/x86-64-v3-wrs-linux/buildah/1.41.5/sources/buildah-1.41.5/src/github.com/containers/buildah/vendor=/_/vendor -frandom-seed=TZkSPVSBUvDMjg4wKjWS -o $WORK/b387/_x004.o -c unset_agent_info.cgo2.c
[snip of compile log]

OE also passes its own DEBUG_PREFIX_MAP to GCC(finally by CGO_CFLAGS),
including -ffile-prefix-map=${B}=${TARGET_DBGSRC_DIR}, where B is
${WORKDIR}/build. Because GOTMPDIR defaults to ${WORKDIR}/build-tmp, the
Go temporary directory looks like ${WORKDIR}/build-tmp/go-buildXYZ. Its
prefix therefore begins with ${WORKDIR}/build, so GCC matches the
DEBUG_PREFIX_MAP entry for ${B} first.

As a result, a path such as ${WORKDIR}/build-tmp/go-buildXYZ is
rewritten to ${TARGET_DBGSRC_DIR}-tmp/go-buildXYZ. This breaks the
-ffile-prefix-map option that Go itself adds, because the original WORK
path no longer matches the value Go expects. Since Go creates
go-buildXYZ directories randomly and internally, this causes the build
non-reproducible.

This patch changes GOTMPDIR from ${WORKDIR}/build-tmp to
${WORKDIR}/tmp-go-build so that the path no longer matches ${B}. This
prevents unintended replacements by OE's DEBUG_PREFIX_MAP and
restores reproducibility.

Note that pure go program like go-helloworld under OE will not have this
issue since it doen't use cgo, it is reproducible without this fix

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>