Question

Is Mercurial always using the external merge tools when two branches that are merging have changes to the same file?

Or does it first see if it can merge the file itself, and only punting to the external tool if it can’t?

The reason I’m asking is that I am (once again) re-reading the tutorial written by Joel Spolsky on Mercurial and one thing he says, when comparing how Subversion and Mercurial is merging is that:

By contrast, while we were working separately in Mercurial, Mercurial was busy keeping a series of changesets. And so, when we want to merge our code together, Mercurial actually has a whole lot more information: it knows what each of us changed and can reapply those changes, rather than just looking at the final product and trying to guess how to put it together.

Only, my experience tells me that it seems to involve the external merge tool when two branches have changes to the same files. And thus, doesn’t that render the above argument incorrect?

Or should I interpret this as follows:

• Subversion only merges the final state of the two branches, and has more work to do in a single unit
• Mercurial merges each changeset individually, which allows it to work with smaller units of changes, with higher chance of merge success

Can someone shed some light on this?

---

Edit: Let me give an example:

@echo off

setlocal

if exist repo rd /s /q repo

md repo
cd repo
hg init .

rem --- version 0 ---
echo 1 >test.txt
echo 2 >>test.txt
echo 3 >>test.txt
echo 4 >>test.txt
echo 5 >>test.txt
hg add test.txt
hg commit -m "v0"

rem --- version 1 ---
echo 1 >test.txt
echo 2 v1 >>test.txt
echo 3 >>test.txt
echo 4 >>test.txt
echo 5 >>test.txt
hg commit -m "v1"

rem --- version 2 ---
hg update 0
echo 1 >test.txt
echo 2 >>test.txt
echo 3 >>test.txt
echo 4 v2 >>test.txt
echo 5 >>test.txt
hg commit -m "v2"

rem --- merge ---
hg update 1
hg merge 2

This first creates a file with the following content:

1
2
3
4
5

Then it changes it to:

1
2 v1
3
4
5

Then it goes back to the initial version (changeset), and changes it to:

1
2
3
4 v2
5

Then it attempts to merge the two.

Now, according to the (currently) single answer, this should not pose a problem, since the changes are not in conflict.

However, at this point, Beyond Compare (my external merge tool) is invoked.

Answer 1

The mergetool is invoked only in case there’s a conflict that needs to be resolved. Changes to the same file in different branches constitutes such a conflict.

Other than that, the actual merge algorithm isn’t changeset based, it’s file based to allow for best merge results. See the Mercurial Wiki for more details.

Mercurial leaves your merge uncomitted, so you have a chance to check your code before comitting the merge changeset.