feat(build_image): Record directories installed in stateful_partition

During builds var_overlay is always mounted over /var. We want to do the
same at run time but we also want to ensure everything expected to be
there always does. After emerge completes gen_tmpfiles.py will scan /var
for any .keep files that were installed and records their parent
directories' permissions and ownership to /usr/lib/tmpfiles.d. On each
boot systemd will automatically recreate anything that goes missing.

This also means that going forward any ebuild that needs a directory in
/var (or anywhere else the stateful partition is bound) can simply rely
on the 'keepdir' ebuild function instead of adding things to
coreos_startup.

build_library/base_image_util.sh

@@ -203,6 +203,11 @@ create_base_image() {
   # trim the image size as much as possible.
   emerge_to_image --root="${root_fs_dir}" ${BASE_PACKAGE}
 
+  # Record directories installed to the stateful partition.
+  sudo "${SCRIPTS_DIR}/gen_tmpfiles.py" --root="${root_fs_dir}" \
+      --output="${root_fs_dir}/usr/lib/tmpfiles.d/base_image.conf" \
+      "${root_fs_dir}/var"
+
   # Set /etc/lsb-release on the image.
   "${OVERLAY_CHROMEOS_DIR}/scripts/cros_set_lsb_release" \
   --root="${root_fs_dir}" \

gen_tmpfiles.py

@@ -0,0 +1,83 @@
+#!/usr/bin/python
+'''Scan an existing directory tree and record installed directories.
+
+During build a number of directories under /var are created in the stateful
+partition. We want to make sure that those are always there so create a record
+of them using systemd's tempfiles config format so they are recreated during
+boot if they go missing for any reason.
+'''
+
+import optparse
+import os
+import stat
+import sys
+import pwd
+import grp
+
+def main():
+    keep = set()
+    parser = optparse.OptionParser(description=__doc__)
+    parser.add_option('--root', help='Remove root prefix from output')
+    parser.add_option('--output', help='Write output to the given file')
+    opts, args = parser.parse_args()
+
+    if opts.root:
+        opts.root = os.path.abspath(opts.root)
+
+    for path in args:
+        path = os.path.abspath(path)
+        if opts.root:
+            assert path.startswith(opts.root)
+
+        for dirpath, dirnames, filenames in os.walk(path):
+            if any(f.startswith('.keep') for f in filenames):
+                keep.add(dirpath)
+
+    # Add all parent directories too
+    for path in frozenset(keep):
+        split = []
+        for pathbit in path.split('/'):
+            split.append(pathbit)
+            joined = '/'.join(split)
+            if not joined:
+                continue
+            if opts.root and not joined.startswith(opts.root):
+                continue
+            if opts.root == joined:
+                continue
+            keep.add(joined)
+
+    config = []
+    for path in sorted(keep):
+        if opts.root:
+            assert path.startswith(opts.root)
+            stripped = path[len(opts.root):]
+            assert len(stripped) > 1
+        else:
+            stripped = path
+
+        info = os.stat(path)
+        assert stat.S_ISDIR(info.st_mode)
+        mode = stat.S_IMODE(info.st_mode)
+
+        try:
+            owner = pwd.getpwuid(info.st_uid).pw_name
+        except KeyError:
+            owner = str(info.st_uid)
+        try:
+            group = grp.getgrgid(info.st_gid).gr_name
+        except KeyError:
+            group = str(info.st_gid)
+
+        config.append('d %-22s %04o %-10s %-10s - -'
+                % (stripped, mode, owner, group))
+
+    if opts.output:
+        fd = open(opts.output, 'w')
+        fd.write('\n'.join(config))
+        fd.close()
+    else:
+        print '\n'.join(config)
+
+if __name__ == '__main__':
+    main()