log -p hides changes in merge commit

git log -p never shows a diff for merge commits. It is nice that it does not show a giant diff that is the sum of all of the changes being merged, but any manual changes made on top of the merge are also lost from view, and this is not good. Here is a reproduction recipe:

git init echo foo > a git add a git commit -m "added a" git branch other git checkout other echo bar > b git add b git commit -m "added b" git checkout master git merge other git log -p

At this point there is no diff shown in the log output. This is fine since there were no changes needed to complete the merge, and the addition of b is already documented in the merged commit. The problem is that if you add --no-merge to the git merge, and then:

echo bart > a git add a git commit

Now in addition to merging b, you have modified a, but git log -p still shows no diff, effectively hiding the fact that you snuck in a modification to a during the merge.

It seems that adding -c or --cc to the log correctly shows the change to a, but why is this not shown by default?

Phillip Susi <psusi@cfl.rr.com> writes:

Depends on the definition of "correctly", but perhaps you have a definition different from ours ;-) The "patches" shown with -c/--cc are designed to be different from normal diff so that people do not accidentally try to apply them with "patch" or "git apply".

"log -p" omits merge commits by default because diffs of merges are mostly not useful for ordinary purposes. If you are trying to use "log -p" to reproduce a (part of) history, perhaps you would want to also study -m option.

On 1/6/2011 2:43 PM, Junio C Hamano wrote:

What I would like to do is be able to review a merge to sign off on it.
 While the full diff against the left parent would be a large and
unhelpful amalgamation of the changes in the merged branch, any
additional changes made during the commit should not be hidden.  This
allows someone performing the merge to effectively sneak in unintended
changes.  I would expect any such changes to be shown by log -p, but
this only seems to happen if you add -c.

Phillip Susi wrote:

To be more precise, here is what -c and --cc do. Consider the following history (time flowing left to right):

    -- [topic]
   /
 B --- [master]

from B to topic and B to master touch entirely different sections of code (though perhaps within the same files), so one could just apply the two diffs in succession to make a merge automatically. (2) Almost as good is the case when they touch code a couple of lines apart --- "git merge" still figures it out automatically. (3) Less nice is the case when they touch the same line, say --- but even here the correct merge can be obvious. (4) Worst of all is when the changes semantically conflict but syntactically do not:

	$ git merge topic
	$ make test;	# fails!
	$ ... hack hack hack ...
	$ git commit --amend

   -- o [topic] ------
  /                   \
 B -- o --------------- [master]

In case (1), -c will show a "combined diff" for files where master does not match either the old master or topic. --cc, on the other hand, will correctly suppress these uninteresting diffs.

In case (2), -c will show a noisy "combined diff" as before. --cc will show a combined diff when the changes from both parents touch nearby code, even if it merged trivially.

In case (3), -c and --cc will show the semantically boring but syntactically interesting merge.

Case (4) is underspecified. So let's give a more precise example: the old master and topic tried to fix the same bug in two incompatible ways. When merging, I decide I like the topic's way better, so I resolve conflicts in favor of the topic. Hopefully all unrelated changes on master were preserved!

In this case, -c and --cc will very likely show nothing at all. Each file matches one of the two parents (old master or topic) so there is no easy way to distinguish the case from (0) or (1).

By now it should be clear how to get the diff you are looking for. One makes a test merge, perhaps using the iffy "resolve in favor of one side or the other" feature to save time on conflicts:

	git checkout oldmaster^0
	git merge topic
	git reset --merge ORIG_HEAD; # meh, too many conflicts
	git merge -Xours topic

and then makes a diff.

	git diff master

Hope that helps, Jonathan

On 1/6/2011 4:04 PM, Jonathan Nieder wrote:

That does help me understand the difference between -c and -cc, but I still don't see why one or the other is not the default behavior of log -p, instead of opting to never show anything at all for merges?

Phillip Susi <psusi@cfl.rr.com> writes:

Assuming this is about the default, the answer is very simple. The features to support -c and --cc came _much_ later than -p, and many people have got used to the default behaviour after using "log -p" for a long time. It simply is rude to change the default on them.

As -c or --cc won't do anything special on a non-merge commit, you can always say "log -p --cc" if that output is what you want to see.