How to keep a project's canonical history correct.

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 <ischis2@cox.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 217 insertions, 0 deletions

diff --git a/Documentation/Makefile b/Documentation/Makefile
index fc6b2cf9ec..cea0e7ae3d 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 0000000000..35d48ef714
--- /dev/null
+++ b/Documentation/howto/keep-canonical-history-correct.txt
@@ -0,0 +1,216 @@
+From: Junio C Hamano <gitster@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, because 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", they often view
+it as "commits you see by following the first-parent chain", and use
+this command to view it:
+
+------------
+    $ 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
+------------
+
+The procedure will result in a history that looks like this:
+
+------------
+		 C0--C1--------------C2
+		/                     \
+    ---o---o---A---X---Y---Z---W---M'--N'
+		\                 /
+		 B0--B1---------B2
+------------
+
+See also http://git-blame.blogspot.com/2013/09/fun-with-first-parent-history.html