During the mail thread about "Pull is mostly evil" a user asked how the first parent could become reversed.
This howto explains how the first parent can get reversed when viewed by the project and then explains a method to keep the history correct. Signed-off-by: Stephen P. Smith <isch...@cox.net> --- Documentation/Makefile | 1 + .../howto/keep-canonical-history-correct.txt | 213 +++++++++++++++++++++ 2 files changed, 214 insertions(+) create mode 100644 Documentation/howto/keep-canonical-history-correct.txt diff --git a/Documentation/Makefile b/Documentation/Makefile index fc6b2cf..cea0e7a 100644 --- a/Documentation/Makefile +++ b/Documentation/Makefile @@ -59,6 +59,7 @@ SP_ARTICLES += howto/recover-corrupted-blob-object SP_ARTICLES += howto/recover-corrupted-object-harder SP_ARTICLES += howto/rebuild-from-update-hook SP_ARTICLES += howto/rebase-from-internal-branch +SP_ARTICLES += howto/keep-canonical-history-correct SP_ARTICLES += howto/maintain-git API_DOCS = $(patsubst %.txt,%,$(filter-out technical/api-index-skel.txt technical/api-index.txt, $(wildcard technical/api-*.txt))) SP_ARTICLES += $(API_DOCS) diff --git a/Documentation/howto/keep-canonical-history-correct.txt b/Documentation/howto/keep-canonical-history-correct.txt new file mode 100644 index 0000000..927154c --- /dev/null +++ b/Documentation/howto/keep-canonical-history-correct.txt @@ -0,0 +1,213 @@ +From: Junio C Hamano <gits...@pobox.com> +Date: Wed, 07 May 2014 13:15:39 -0700 +Subject: Beginner question on "Pull is mostly evil" +Abstract: This how-to explains a method for keeping a + project's history correct when using git pull. +Content-type: text/asciidoc + +Keep authoritative canonical history correct with git pull +========================================================== + +Sometimes a new project integrator will end up with project history +that appears to be "backwards" from what other project developers +expect. This howto presents a suggested integration workflow for +maintaining a central repository. + +Suppose that that central repository has this history: + +------------ + ---o---o---A +------------ + +which ends at commit `A` (time flows from left to right and each node +in the graph is a commit, lines between them indicating parent-child +relationship). + +Then you clone it and work on your own commits, which leads you to +have this history in *your* repository: + +------------ + ---o---o---A---B---C +------------ + +Imagine your coworker did the same and built on top of `A` in *his* +repository in the meantime, and then pushed it to the +central repository: + +------------ + ---o---o---A---X---Y---Z +------------ + +Now, if you "git push" at this point, beause your history that leads +to `C` lacks `X`, `Y` and `Z`, it will fail. You need to somehow make +the tip of your history a descendant of `Z`. + +One suggested way to solve the problem is "fetch and then merge", aka +"git pull". When you fetch, your repository will have a history like +this: + +------------ + ---o---o---A---B---C + \ + X---Y---Z +------------ + +Once you run merge after that, while still on *your* branch, i.e. `C`, +you will create a merge `M` and make the history look like this: + +------------ + ---o---o---A---B---C---M + \ / + X---Y---Z +------------ + +`M` is a descendant of `Z`, so you can push to update the central +repository. Such a merge `M` does not lose any commit in both +histories, so in that sense it may not be wrong, but when people want +to talk about "the authoritative canonical history that is shared +among the project participants", i.e. "the trunk", the way they often +use is to do: + +------------ + $ git log --first-parent +------------ + +For all other people who observed the central repository after your +coworker pushed `Z` but before you pushed `M`, the commit on the trunk +used to be `o-o-A-X-Y-Z`. But because you made `M` while you were on +`C`, `M`'s first parent is `C`, so by pushing `M` to advance the +central repository, you made `X-Y-Z` a side branch, not on the trunk. + +You would rather want to have a history of this shape: + +------------ + ---o---o---A---X---Y---Z---M' + \ / + B-----------C +------------ + +so that in the first-parent chain, it is clear that the project first +did `X` and then `Y` and then `Z` and merged a change that consists of +two commits `B` and `C` that achieves a single goal. You may have +worked on fixing the bug #12345 with these two patches, and the merge +`M'` with swapped parents can say in its log message "Merge +'fix-bug-12345'". Having a way to tell "git pull" to create a merge +but record the parents in reverse order may be a way to do so. + +Note that I said "achieves a single goal" above, because this is +important. "swapping the merge order" only covers a special case +where the project does not care too much about having unrelated +things done on a single merge but cares a lot about first-parent +chain. + +There are multiple schools of thought about the "trunk" management. + + 1. Some projects want to keep a completely linear history without any + merges. Obviously, swapping the merge order would not match their + taste. You would need to flatten your history on top of the + updated upstream to result in a history of this shape instead: ++ +------------ + ---o---o---A---X---Y---Z---B---C +------------ ++ + with `git pull --rebase` or something. + + 2. Some projects tolerate merges in their history, but do not worry + too much about the first-parent order, and allow fast-forward + merges. To them, swapping the merge order does not hurt, but + it is unnecessary. + + 3. Some projects want each commit on the "trunk" to do one single + thing. The output of `git log --first-parent` in such a project + would show either a merge of a side branch that completes a single + theme, or a single commit that completes a single theme by itself. + If your two commits `B` and `C` (or they may even be two groups of + commits) were solving two independent issues, then the merge `M'` + we made in the earlier example by swapping the merge order is + still not up to the project standard. It merges two unrelated + efforts `B` and `C` at the same time. + +For projects in the last category (Git itself is one of them), +individual developers would want to prepare a history more like +this: + +------------ + C0--C1--C2 topic-c + / + ---o---o---A master + \ + B0--B1--B2 topic-b +------------ + +That is, keeping separate topics on separate branches, perhaps like +so: + +------------ + $ git clone $URL work && cd work + $ git checkout -b topic-b master + $ ... work to create B0, B1 and B2 to complete one theme + $ git checkout -b topic-c master + $ ... same for the theme of topic-c +------------ + +And then + +------------ + $ git checkout master + $ git pull --ff-only +------------ + +would grab `X`, `Y` and `Z` from the upstream and advance your master +branch: + +------------ + C0--C1--C2 topic-c + / + ---o---o---A---X---Y---Z master + \ + B0--B1--B2 topic-b +------------ + +And then you would merge these two branches separately: + +------------ + $ git merge topic-b + $ git merge topic-c +------------ + +to result in + +------------ + C0--C1---------C2 + / \ + ---o---o---A---X---Y---Z---M---N + \ / + B0--B1-----B2 +------------ + +and push it back to the central repository. + +It is very much possible that while you are merging topic-b and +topic-c, somebody again advanced the history in the central repository +to put `W` on top of `Z`, and make your "git push" fail. + +In such a case, you would rewind to discard `M` and `N`, update the +tip of your 'master' again and redo the two merges: + +------------ + $ git reset --hard origin/master + $ git pull --ff-only + $ git merge topic-b + $ git merge topic-c +------------ + +------------ + C0--C1--------------C2 + / \ + ---o---o---A---X---Y---Z---W---M'--N' + \ / + B0--B1---------B2 +------------ + +See http://git-blame.blogspot.com/2012/03/fun-with-first-parent.html -- 2.0.0.rc1 -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
