author | Olle Lundberg <geek@nerd.sh> |
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.. Copyright 2011 Pierre-Yves David <pierre-yves.david@ens-lyon.org> |
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.. Logilab SA <contact@logilab.fr> |
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----------------------------------- |
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The instability Principle |
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----------------------------------- |
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An intrinsic contradiction |
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----------------------------------- |
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XXX starts by talking about getting ride of changeset. |
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DVCSes bring two new major concepts to the version control scene: |
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* History is organized as a robust DAG, |
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* History can be rewritten. |
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However, the two concepts are in contradiction: |
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To achieve a robust history, three key elements are gathered in *changesets*: |
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* Full snapshot of the versioned content, |
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* Reference to the previous full snapshot used to build the new one, |
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* A description of the change which leads from the old content to the new content. |
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All three elements are used to compute a *unique* hash that identifies the changeset |
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(with various other metadata). This identification is a key part of DVCS design. |
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This is a very useful property because changing B's parent means |
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changing B's content too. This requires the creation of **another** |
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changeset, which is semantically good. |
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.. figure:: ./figures/edit-is-rewrite-step2.svg |
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To avoid duplication, the older changeset is usually discarded from accessible |
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history. I'm calling them *obsolete* changesets. |
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But rewriting a changeset with children does not change the |
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children's parents! And because children of the rewritten changeset |
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still **depend** on the older "dead" version of the changeset, we |
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cannot get rid of this dead version. |
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:: |
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Schema base, A and A' and B. |
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I'm calling these children **unstable** because they are based on a |
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dead changeset and prevent people from getting rid of it. |
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This instability is an **unavoidable consequence** of the strict dependency of |
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changesets. Rewriting history always needs to take it into account and |
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provide a way to rewrite the descendants of the new changeset to avoid |
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coexistence of the old and new versions of a rewritten changeset. |
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Everybody is working around the issue |
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------------------------------------------------ |
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I'm not claiming that rewriting history is impossible. People have been successfully |
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doing it for years. However they all need to work around *instability*. Several |
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workaround strategies exist. |
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Rewriting all at once |
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`````````````````````````` |
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The simplest way to avoid instability is to ensure rewriting |
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operations always end in a stable situation. This is achieved by |
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rewriting all affected changesets at the same time. |
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Rewriting all descendants at the same time when rewriting a changeset. |
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:: |
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Schema! |
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Several Mercurial commands apply it: rebase, collapse, histedit. |
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Mercurial also refuses to amend changesets with descendants. The git |
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branch design enforces this approach in git too. |
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However, DVCS are **distributed**. This means that you do not control what |
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happens outside your repository. Once a changeset has been exchanged *outside*, |
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there is no way to be sure it does not have descendants somewhere else. |
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Therefore **if you rewrite changeset that exists elsewhere, you can't eradicate |
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the risk of instability.** |
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Do not rewrite exchanged changesets |
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``````````````````````````````````` |
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To work around the issue above, Mercurial introduced phases, which |
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prevent you from rewriting shared changesets and ensure others can't |
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pull certain changesets from you. But this is a very frustrating |
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limitation that prevents you to efficiently sharing, reviewing and |
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collaborating on mutable changesets. |
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In the Git world, they use another approach to prevent instability. By |
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convention only a single developer works on a changeset contained in |
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a named branch. But once again this is a huge blocker for |
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collaborating. Moreover clueless people **will** mess up social |
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convention soon or later. |
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Lose the DAG robustness |
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```````````````````````````` |
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The other approach in Mercurial is to keep the mutable part of the |
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history outside the DVCS constraint. This is the MQ approach of |
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sticking a quilt queue over Mercurial. |
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This allow much more flexible workflow but two major feature are lost in the |
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process: |
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:graceful merge: MQ uses plain patch to store changeset contents, which has |
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problems in changing context. Applying your queue |
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becomes very painful when context changes. |
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:easy branching: A quilt queue is by definition a linear queue, increasing risk |
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of conflict. |
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It is possible to collaborate over versioned MQ! But you are going to |
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have a lot of trouble. |
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Ignore conflicts |
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``````````````````````````````````` |
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Another ignored issue is a conflicting rewrite of the same changeset. |
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If a changeset is rewritten two times we have two newer versions, |
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and duplicated history is complicated to merge. |
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Mercurial work around by |
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The "One set of mutable changset == One developer" mantra is also a way to work |
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around conflicting rewriting of changeset. If two different people are able to |
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The git branch model allows overwriting changeset version by another |
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one, but it does not care about divergent version. It is the equivalent |
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of "common ftp" source management for changesets. |
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Facing The Danger Once And For All |
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------------------------------------------------ |
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Above we saw that the more effort you put to avoid instability, the more options |
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you deny. And even the most restrictive workflow can't guarantee that instability |
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will never show up! |
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Obsolete marker can handle the job |
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``````````````````````````````````` |
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It is time to provide a full-featured solution to deal with |
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instability and to stop working around the issue! This is why I |
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am developing a new feature for Mercurial called "Obsolete markers". |
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Obsolete markers have two key properties: |
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* Any "old" changeset we want to get rid of is **explicitly** marked |
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as "obsolete" by history rewriting operations. |
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By explicitly marking the obsolete part of the history, we will be able to |
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easily detect instability situation. |
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* Relations between old and new version of changesets are tracked by obsolete |
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markers. |
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By storing a meta-history of changeset evolution we are able to easily resolve |
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instability and edit conflicts [#]_ . |
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.. [#] Edit conflicts is another major obstable to collaboration. See the |
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section dedicated to obsolete marker for details. |
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Improved robustness == improved simplicity |
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```````````````````````````````````````````````` |
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This proposal should **first** be seen as a safety measure. |
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It allows detecting instability as soon as possible. |
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:: |
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$ hg pull |
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added 3 changeset |
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+2 unstable changeset |
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(do you want "hg evolve" ?) |
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working directory parent is obsolete! |
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$ hg push |
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outgoing unstable changesets |
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(use "hg evolve" or force the push) |
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And it should not not encourage people to create instability. |
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:: |
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$ hg up 42 |
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$ hg commit --amend |
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changeset have descendant. |
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$ hg commit --amend -f |
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+5 unstable changeset |
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$ hg rebase -D --rev 40::44 |
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rebasing already obsolete changeset 42:AAA will conflict with newer version 48:BBB |
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While allowing powerful feature |
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```````````````````````````````````````````````` |
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* Help to automatically solve instability. |
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* "prune" changeset remotely. |
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* track resulting changeset when submitting patch//pull request. |
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* Focus on what you do: |
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I do not like the "all at once" model of history rewriting. I'm comfortable |
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with instability and obsolete marker offer all the tool to safely create and |
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handle instability locally. |
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