buildsystem: use an update lock when updating image and sysroot

cp (and potentially mkdir -p) are not atomic, and we have seen situations where two packages
concurrently copying the same file (eg. the udev rule for xf86-video-nvidia and
xf86-video-nvidia-legacy) will succeed for one package but the other package fails with
a "file exists" error (as the file didn't exist when it checked, but does exist when it
actually copies the file). Not even cp -f will avoid this issue.

There are several workarounds, but the most practical (and general) solution is to ensure
sequential updates of the image and shared sysroot directories.

config/functions

@@ -1371,6 +1371,13 @@ enable_service() {
 
 
 ### MULTI-THREADED FUNCTION HELPERS ###
+# flocks: 94 (pkg_lock_status)
+#         95 (scripts/pkgbuild)
+#         96 (acquire_exclusive_lock)
+#         97 (acquire_update_lock)
+#         98 (pkg_lock)
+#         99 (scripts/get)
+
 # Test build type so that these functions are a no-op during non-multithreaded builds.
 
 # Prevent concurrent modifications to a package during certain activities.
@@ -1540,6 +1547,20 @@ exec_thread_safe() {
   return ${result}
 }
 
+# A lightweight target specific lock (eg. image, sysroot)
+acquire_update_lock() {
+  is_sequential_build && return 0
+
+  exec 97>"${THREAD_CONTROL}/locks/.update.${1}"
+  flock --exclusive 97
+}
+
+release_update_lock() {
+  is_sequential_build && return 0
+
+  flock --unlock 97 2>/dev/null
+}
+
 # Use distribution functions if any
 if [ -f "distributions/$DISTRO/config/functions" ]; then
   . distributions/$DISTRO/config/functions

scripts/build

@@ -444,8 +444,13 @@ for i in $(find "${SYSROOT_PREFIX}" -type l 2>/dev/null); do
 done
 
 # Transfer the new sysroot content to the shared sysroot
+acquire_update_lock sysroot
+
 mkdir -p "${PKG_ORIG_SYSROOT_PREFIX}"
 cp -PRf "${SYSROOT_PREFIX}"/* "${PKG_ORIG_SYSROOT_PREFIX}"
+
+release_update_lock
+
 rm -rf "${SYSROOT_PREFIX}"
 
 export SYSROOT_PREFIX="${PKG_ORIG_SYSROOT_PREFIX}"

scripts/install

@@ -64,6 +64,8 @@ pkg_lock_status "ACTIVE" "${PKG_NAME}:${TARGET}" "install"
 
 build_msg "CLR_INSTALL" "INSTALL" "${PKG_NAME} $(print_color CLR_TARGET "(${TARGET})")" "indent"
 
+acquire_update_lock image
+
 mkdir -p ${INSTALL}
 
 if [ "${TARGET}" = "target" ] ; then
@@ -169,6 +171,8 @@ if [ "${TARGET}" = "target" ] ; then
   pkg_call_exists post_install && pkg_call post_install
 fi
 
+release_update_lock
+
 touch ${STAMP}
 
 pkg_lock_status "UNLOCK" "${PKG_NAME}:${TARGET}" "install" "installed"