[Buildroot] [PATCH 1/1] core/pkg-infra: Fix package files statistics for parallel build

[Andreas Naumann <anaumann@ultratronik.de>]

When activating top level parallel build, multiple processes may
try to modify the various *files-list* files in the common build/
directory at the same time. This can cause racy build failures.

The fix here is to use flock to ensure exclusive execution of the
statistics gathering code. For this to work it is assumed that the
target/staging/host directories are isolated, which is true for
per-package builds.
For standard sequential builds, the locking is of course unnecessary
but no conditional handling is implemented  since the runtime cost is
expected to be next to nothing.

For flock to work, the statistics gathering code must be run in a
single subshell. Otherwise the lockfile descriptor would be closed
after the subshell (= the Makefile line where it was opened) ends
and thus the lock would be released before even entering the critical
code section.

Signed-off-by: Andreas Naumann <anaumann@ultratronik.de>
---

Notes:
This patch has been developed on top of Thomas ppsh-v7 branch, rebased
on master.

Of course this solution adds a dependency on flock. However flock is
part of util-linux which probably is a dependency of other buildroot
prerequisites anyway.

Also, even though this solution has a somewhat hacky feel to me, it does
work for both per-package and standard sequential builds. When converting
to per-package for good, the statistics gathering code could probably
be simplified because, well, all the files are installed isolated folders
which contents should be easier to compare before/after.

 package/pkg-generic.mk | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/package/pkg-generic.mk b/package/pkg-generic.mk
index 6d4442b824..0f875736d6 100644
--- a/package/pkg-generic.mk
+++ b/package/pkg-generic.mk
@@ -64,20 +64,22 @@ GLOBAL_INSTRUMENTATION_HOOKS += step_time
 # $(3): suffix of file  (optional)
 define step_pkg_size_inner
 	@touch $(BUILD_DIR)/.files-list$(3).stat
-	@touch $(BUILD_DIR)/packages-file-list$(3).txt
-	$(SED) '/^$(1),/d' $(BUILD_DIR)/packages-file-list$(3).txt
+	exec 3>$(BUILD_DIR)/packages-file-list$(3).txt; \
+	flock -x 3; \
+	$(SED) '/^$(1),/d' $(BUILD_DIR)/packages-file-list$(3).txt; \
 	cd $(2); \
 	LC_ALL=C find . \( -type f -o -type l \) -printf '%T@:%i:%#m:%y:%s,%p\n' \
-		| LC_ALL=C sort > $(BUILD_DIR)/.files-list$(3).new
+		| LC_ALL=C sort > $(BUILD_DIR)/.files-list$(3).new; \
 	LC_ALL=C comm -13 \
 		$(BUILD_DIR)/.files-list$(3).stat \
 		$(BUILD_DIR)/.files-list$(3).new \
-		> $($(PKG)_BUILDDIR)/.files-list$(3).txt
+		> $($(PKG)_BUILDDIR)/.files-list$(3).txt; \
 	sed -r -e 's/^[^,]+/$(1)/' \
 		$($(PKG)_BUILDDIR)/.files-list$(3).txt \
-		>> $(BUILD_DIR)/packages-file-list$(3).txt
+		>> $(BUILD_DIR)/packages-file-list$(3).txt; \
 	mv $(BUILD_DIR)/.files-list$(3).new \
-		$(BUILD_DIR)/.files-list$(3).stat
+		$(BUILD_DIR)/.files-list$(3).stat; \
+	exec 3>&-
 endef
 
 define step_pkg_size
-- 
2.21.0