2 files changed, 13 insertions, 2 deletions

diff --git a/vmdb/plugins/cache_rootfs.mdwn b/vmdb/plugins/cache_rootfs.mdwn
index c4270a7..2077de5 100644
--- a/vmdb/plugins/cache_rootfs.mdwn
+++ b/vmdb/plugins/cache_rootfs.mdwn
@@ -6,8 +6,19 @@ Create a tarball of the root filesystem in the image.
 Step keys:
 
 * `cache-rootfs` — REQUIRED; tag of root filesystem on image.
+* `force` — OPTIONAL; boolean that enables overwriting of an existing rootfs
+    tarball by `vmdb2`, allowing the tarball to be used as both a build input and a
+    build output. This can be useful in multi-stage build chains where a "common base
+    OS" rootfs filesystem is populated and packaged as a rootfs tarball by `vmdb2`, and
+    this tarball is consumed by multiple downstream `vmdb2` builds that extract the
+    tarball into disk images that have different partition layouts and/or filesystems.
 
 Example (in the .vmdb file):
 
+    # typical use
     - cache-rootfs: root
       unless: rootfs_unpacked
+
+    # create a rootfs tarball output at the end of a build process
+    - cache-rootfs: root
+      force: true
diff --git a/vmdb/plugins/cache_rootfs_plugin.py b/vmdb/plugins/cache_rootfs_plugin.py
index c7ebb6d..cdbd3e6 100644
--- a/vmdb/plugins/cache_rootfs_plugin.py
+++ b/vmdb/plugins/cache_rootfs_plugin.py
@@ -28,7 +28,7 @@ class CacheRootFSPlugin(vmdb.Plugin):
 
 class MakeCacheStepRunner(vmdb.StepRunnerInterface):
     def get_key_spec(self):
-        return {"cache-rootfs": str, "options": "--one-file-system"}
+        return {"cache-rootfs": str, "options": "--one-file-system", "force": False}
 
     def run(self, values, settings, state):
         fs_tag = values["cache-rootfs"]
@@ -49,7 +49,7 @@ class MakeCacheStepRunner(vmdb.StepRunnerInterface):
 
         vmdb.progress("caching rootdir {}".format(rootdir))
         vmdb.progress("caching relative {}".format(dirs))
-        if not os.path.exists(tar_path):
+        if values["force"] or not os.path.exists(tar_path):
             vmdb.runcmd(["tar"] + opts + ["-C", rootdir, "-caf", tar_path] + dirs)
 
     def _find_cacheable_mount_points(self, tags, rootdir):