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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 the 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.

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.
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.
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