TortoiseHG: cannot partially commit a merge

Question

I tried to merge two heads in Mercurial. After merging, I didn't commit and did some more changes. Then I tried to commit and got the following message: abort: cannot partially commit a merge (do not specify files or patterns)

I'm using TortoiseHG as visual shell, and Beyond Compare for comparing and merging. And I'm relatively new to all of them.

What should I do to finish commit successfully?

Answer 1

What should I do to finish commit successfully?

One of the benefits of Mercurial (or git or any other DVCS) is that you can perform a commit at any point in your development process and it will be both fast and private. It will be fast because you should be committing to a local copy of the repository residing on your hard drive, and it will be private because no-one will see your change set until you push them to the server (or other master repository).

Therefore, to partially answer your question, the appropriate thing to do would have been to commit the merge without your addition changes, and then apply and commit your next wave of changes. If you are using TortoiseHG to peform the merge it will actually prompt you to commit the merge before leaving the GUI because this is the intended HG workflow.

That being said, I made some changes on a named branch (ie: new head), merged it back into the default branch, exited the TortoiseHG GUI without committing, made some more changes, and then committed with no problem. I will ask some clarifying questions below your original inquiry.

Answer 2

Mercurial/TortoiseHg is correct in telling you that you should not partially commit a merge. Partial means that you do not commit all files at once.

The underlying reason for this message is that is gives you the wrong results. When you merge two changesets in Mercurial, you are creating a new changeset with two parent changesets. This merge changeset shows others how you want everybody else to combine the two changesets.

Let us imagine that you start with changesets A and B and want to merge them. This creates a graph like this:

... --- [A]
           \
            [M]
           /
... --- [B]

Pretend that we added the line A! to a.txt in the A changeset and that we added B! to b.txt in the B changeset. Just two independent changes that does not conflict. If Mercurial allowed you to do a partial commit, then you could do this:

hg merge
hg commit -m 'Added A!' a.txt  # creates M
hg commit -m 'Added B!' b.txt  # creates M'

and the result is this graph:

... --- [A]
           \
            [M] --- [M']
           /
... --- [B]

If you look at b.txt along the path B, M, M', then you will see that the line with B! was introduced in B, removed in M and reintroduced in M'!

This is not what you want from a merge changeset: a partial merge throws away changes from one branch just to introduce them again in a followup commit. Mercurial trusts you when you create M: it really believes that M contains the correct mix of A and B. In particular, if the B! line is removed in M, then it will remain gone when you merge M with other changesets.

So Mercurial is trying to protect you from creating a bad history by not allowing partial merges.