[PATCH 05/26] dpkg: Fix shared-locking of build task

[Jan Kiszka <jan.kiszka@siemens.com>]

From: Jan Kiszka <jan.kiszka@siemens.com>

Bitbake file locks are not just exclusive flock calls. For whatever
reason, bitbake prefers to delete the lock file before unlocking them.
That obviously creates a nasty race with other locker requests: Those
already waiting will get access to the removed lock, those coming later
will create a new one. To resolve that race, bitbakes lockfile() tests
if the inode of the acquired lock file is identical to the one that is
currently visible via the lock name. If not, it retries the lock. What a
pointless mess...

Our open-coded "flock -s" does not do that, thus will often acquire only
a stale lock. Fix that by using bitbake's services for locking. That
requires to move the lock acquisition at the top level of the task, ie.
do_build. Thus, also dpkg-raw and kernel builds are now under the shared
(read) lock. The better fix for this issue would actually be private
buildchroots for each build, but that will come later.

CC: Cedric Hombourger <Cedric_Hombourger@mentor.com>
Fixes: 568072765b08 ("dpkg: acquire a read (shared) locks while building packages")
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---
 meta/classes/dpkg-base.bbclass | 11 +++++++----
 meta/classes/dpkg.bbclass      |  5 ++---
 2 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/meta/classes/dpkg-base.bbclass b/meta/classes/dpkg-base.bbclass
index 2d26418..8146328 100644
--- a/meta/classes/dpkg-base.bbclass
+++ b/meta/classes/dpkg-base.bbclass
@@ -87,10 +87,13 @@ dpkg_runbuild() {
     die "This should never be called, overwrite it in your derived class"
 }
 
-do_build() {
-    dpkg_do_mounts
-    dpkg_runbuild
-    dpkg_undo_mounts
+python do_build() {
+    lock = bb.utils.lockfile(d.getVar("REPO_ISAR_DIR") + "/isar.lock",
+                             shared=True)
+    bb.build.exec_func("dpkg_do_mounts", d)
+    bb.build.exec_func("dpkg_runbuild", d)
+    bb.build.exec_func("dpkg_undo_mounts", d)
+    bb.utils.unlockfile(lock)
 }
 
 CLEANFUNCS += "repo_clean"
diff --git a/meta/classes/dpkg.bbclass b/meta/classes/dpkg.bbclass
index dd123c3..df47ced 100644
--- a/meta/classes/dpkg.bbclass
+++ b/meta/classes/dpkg.bbclass
@@ -18,7 +18,6 @@ do_install_builddeps[lockfiles] += "${REPO_ISAR_DIR}/isar.lock"
 # Build package from sources using build script
 dpkg_runbuild() {
     E="${@ bb.utils.export_proxies(d)}"
-    flock -s "${REPO_ISAR_DIR}/isar.lock" \
-        sudo -E chroot --userspec=$( id -u ):$( id -g ) ${BUILDCHROOT_DIR} \
-             /isar/build.sh ${PP}/${PPS} ${DISTRO_ARCH}
+    sudo -E chroot --userspec=$( id -u ):$( id -g ) ${BUILDCHROOT_DIR} \
+         /isar/build.sh ${PP}/${PPS} ${DISTRO_ARCH}
 }
-- 
2.16.4