This also changes the structure of the object store, though the
basic idea is the same.
The object store contains a directory of objects, which are content
addressable filesystem trees. Currently we only ever use their
content-hash internally, but the idea for this is basically Lars
Karlitski and Kay Sievers' `treesum()`. We may exopse this in the
future.
Moreover, it contains a directory of refs, which are symlinks named
by the stage id they correspond to (as before), pointing to an object
generated from that stage-id.
The ObjectStore exposes three method:
`has_tree()`: This checks if the content store contains the given tree.
If so, we can rely on the tree remaining there.
`get_tree()`: This is meant to be used with a `with` block and yields
the path to a read-only instance of the tree with the given id. If the
tree_id is passed in as None, an empty directory is given instead.
`new_tree()`: This is meant to be used with a `with` block and yields
the path to a directory in which the tree by the given id should be
created. If a base_id is passed in, the tree is initialized with the
tree with the given id. Only when the block is exited successfully
is the tree written to the content store, referenced by the id in
question.
Use this in Pipeline.run() to avoid regenerating trees unneccessarily.
In order to trigger a regeneration, the content store must currently
be manually flushed.
Update the travis test to run the noop pipeline twice, verifying that
the stage is only run the first time.
Signed-off-by: Tom Gundersen <teg@jklm.no>
