[PATCH v2] dpkg: acquire a read (shared) locks while building packages

[Cedric Hombourger <Cedric_Hombourger@mentor.com>]

As debhelper scripts involved during the actual build process
(dpkg-buildpackage) may access the dpkg database, we need to
make sure it does not get modified in the background by other
build tasks for other packages (such as install_build_deps).
An example of a reader is dh_shlibdeps/dpkg-shlibdeps: it
needs to find packages providing the shared libraries the
package depends on to substitute shlibs variables found in its
control file with package dependencies. Acquire the isar-apt
lock as a shared lock to block writers.

Special care may be needed in other classes/users of the
dpkg_runbuild API. Due to some limitations with bitbake (to be
discussed with the maintainers), we could not grab the lock
from dpkg-base. A statement such as "flock <my-lock.lock>
dpkg_run_build" would fail as flock can't exec() dpkg_run_build
(since it is a shell function, not an executable). The popular
shell construct "( flock -n 9; dpkg_run_build ) 9> <my-lock.lock>"
cannot be parsed by bitbake. And finally the lockfiles task
attribute (implemented in bitbake) only handles exclusive locks.

This commit is only addressing dpkg.bbclass. dpkg-raw.bbclass
is left intact as the generated control file does require
dh_shlibdeps. linux-custom.inc was also left out as the Debian
packaging scripts only use low-level packaging tools and do not
use debhelper scripts during packaging (at this time). In
agreement with the Isar project maintainers, we will not grab
the Isar lock from linux-custom.inc to let kernel builds run
in parallel with do_install_deps() and do_deploy_deb() that
may be executed by other bitbake threads.

Signed-off-by: Cedric Hombourger <Cedric_Hombourger@mentor.com>
---
 meta/classes/dpkg.bbclass | 1 +
 1 file changed, 1 insertion(+)

diff --git a/meta/classes/dpkg.bbclass b/meta/classes/dpkg.bbclass
index 24b9fe3..f4e3d7a 100644
--- a/meta/classes/dpkg.bbclass
+++ b/meta/classes/dpkg.bbclass
@@ -19,5 +19,6 @@ do_install_builddeps[stamp-extra-info] = "${DISTRO}-${DISTRO_ARCH}"
 # 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}
 }
-- 
2.11.0