author | Pierre-Yves David <pierre-yves.david@logilab.fr> |
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Why Do We Need a New Concept |
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Current DVCSes are great tools for forging a series of flawless |
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changesets on your own. But they perform poorly when it comes to |
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**sharing** some work in progress and **collaborating** on such work |
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in progress. |
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When people forge a new version of a changeset they actually create a |
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new changeset and get rid of the original changeset. Difficulties to |
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collaborate mostly came from the way old content is *removed* from |
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a repository. |
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Mercurial Approach: Strip |
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With the current version of mercurial, every changeset that exists in |
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your repository is *visible* and *meaningful*. To delete old |
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(rewritten) changesets, mercurial removes them from the repository |
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storage with an operation called *strip*. After the *stripping*, the |
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repository looks as if the changeset never existed. |
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This approach is simple and effective except for one big |
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drawback: you can remove changesets from **your repository only**. If |
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a stripped changeset exists in another repository it touches, it will |
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show up again. This is because a shared changeset becomes |
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part of a shared global history. Stripping a changeset from all |
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repositories is at best impractical and in most case impossible. |
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As consequence, **you can not rewrite something once you exchange it with |
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others**. The old version will still exist along side the new one [#]_. |
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Moreover stripping changesets creates backup bundles. This allows |
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restoration of the deleted changesets, but the process is painful. |
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Finally, as the repository format is not optimized for deletion. stripping a |
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changeset may be slow in some situations. |
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To sum up, the strip approach is very simple but does not handle |
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interaction with the outer world, which is very unfortunate for a |
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*Distributed* VCS. |
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.. [#] various work around exists but they require their own workflows |
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which are distinct from the very elegant basic workflow of |
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Mercurial. |
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Git Approach: Overwrite Reference |
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The Git approach to repository structure is a bit more complex: there |
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can be any amount of unrelated changesets in a repository, and **only |
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changesets referenced by a git branch** are *visible* and |
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*meaningful*. |
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.. warning:: add a schema:: |
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C |
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| B---<foo> |
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A |
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Only B and A are visible. |
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This simplifies the process of getting rid of old changesets. You can |
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just leave them in place and move the reference on the new one. You |
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can then propagate this change by moving the git-branch on remote host |
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with the newer version of the marker overwriting the older one. |
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This approach goes a bit further but still has a major drawback: |
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Because you **overwrite** the git-branch, you have no conflict |
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resolution. The last to act wins. This makes collaboration on multiple |
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changesets difficult because you can't merge concurrent updates on a |
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changeset. |
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Every overwrite is a forced operation where the operator says, "yes I |
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want this to replace that". In highly distributed environments, a user |
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may end up with conflicting references and no proper way to choose. |
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Because of this way to visualize a repository, git-branches are a core |
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part of git, which makes the user interface more complicated and |
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constrains moving through history. |
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Finally, even if all older changesets still exist in the repository, |
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accesing them is still painful. |
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The Obsolete Marker Concept |
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As none of the concepts was powerful enough to fulfill the need of |
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safely rewriting history, including easy sharing and collaboration on |
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mutable history, we needed another one. |
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Basic concept |
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Every history rewriting operation stores the information that old rewritten |
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changeset is replaced by newer version in a given set of changesets. |
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All basic history rewriting operation can create an appropriate obsolete marker. |
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.. figure:: ./figures/example-1-update.* |
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*Updating* a changeset |
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Create one obsolete marker: ``([A'] obsolete A)`` |
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.. figure:: ./figures/example-2-split.* |
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*Splitting* a changeset in multiple one |
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Create one obsolete marker ``([B1, B2] obsolete B)]`` |
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.. figure:: ./figures/simple-3-merge.* |
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.. figure:: ./figures/simple-4-reorder.* |
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*Moving* changeset around |
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To conclude, a single obsolete marker express a relation from **0..n** new |
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Second, because obsolete changeset content is still **available**. You can |
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you can |
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Finally, the obsolete marker can be **exchanged between |
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Conflicting history rewriting operation can be detected and |
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Detecting and solving tricky situations |
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----------------------------------------------------- |
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History rewriting can lead to complex situations. The obsolete marker |
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cases can be resolved automatically using the available information. |
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```````````````````` |
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Rebasing `B` and `C` on `A` (as `B'`, `C'`) |
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``````````````````` |
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While exploring the possibilities of the obsolete marker a bit |
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*non-obsolete* children. There is two common ways to achieve this: |
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* Pull a changeset based of an old version of a changeset [#]_. |
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children. |
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*Non-obsolete* changeset based on *obsolete* one are called **unstable** |
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Amend `A` into `A'` leaving `B` behind. |
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have all the necessary data to detect `B` as an *unstable* branch |
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we have enough data to automatically resolve this instability: we |
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again. |
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Proper warnings should be issued when part of the history becomes |
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for them. |
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XXX details on automatic resolution for |
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* movement |
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* handling deletion |
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* handling split on multiple head |
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.. [#] For this to happen one needs to explicitly enable exchange of draft |
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`````````````````````````````````````` |
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This kind of conflict is easy to detect with an obsolete marker |
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`````````````````````` |
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public. |
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* Public changesets are excluded from the obsolete set (public |
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Solving such an error is easy. Because we know what changeset a |
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.. warning:: add a schema |
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---------------- |
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The obsolete marker is a powerful concept that allows mercurial to safely handle |
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history rewriting operations. It is a new type of relation between Mercurial |
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This concept is simple to define and provides a very solid base for: |
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.. list-table:: Comparison on solution [#]_ |
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:header-rows: 1 |
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* - Strip |
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* - Reference |
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* - Obsolete |
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.. [#] To preserve good tradition in comparison table, an overwhelming advantage |
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goes to the defended solution. |