Welcome to the sixth edition of Git Rev News, a digest of all things Git. For our goals, the archives, the way we work, and how to contribute or to subscribe, see the Git Rev News page on git.github.io.
This edition covers what happened during the month of July 2015.
Git 2.5 is out! The project maintainer, Junio C. Hamano, has shared his thoughts on the release at his blog. Git 2.5 is packed full of new features, and includes contributions from 21 new contributors. In his post, Junio describes how git help has changed:
One interesting change is to git help. We now list commands, grouped by the situation in which you would want to use them. This came from discussion on usability, inspired by one of the talks at GitMerge conference we had in spring.
He goes on to talk about some of his favourite new features included in the release, such as a new short hand branch@{push} that “denotes the remote-tracking branch that tracks the branch at the remote the branch would be pushed to”, and a new option --ws-error-highlight that can be used with git diff and friends to show whitespace breakages in deleted and context lines.
Be sure to see the post for more on the new features, or checkout the release notes in the source for all the nitty gritty details.
Did you know that the Linux kernel you clone normally contains only a part of its entire history? If you need access to its uninterrupted evolution since the first commit you have to “graft” a few separate repositories together chronologically.
Git has a local - per-repository - mechanism to change and specify explicitly
the parent of existing commits: These are called Grafts. In a repository they
are defined in a file called “.git/info/grafts” (check the Git repository
layout manpage for details).
From the Git Wiki:
When Linus started using Git for maintaining his kernel tree there didn’t exist any tools to convert the old kernel history. Later, when the old kernel history was imported into Git from the bkcvs gateway, grafts was created as a method for making it possible to tie the two different repositories together.
To re-assemble the complete kernel history these three repositories are needed:
The syntax of the Grafts file in “.git/info/grafts” is simple: each line
lists a commit and it’s fake parent using the SHA-1 identifiers. So to
re-assemble the full history of the Linux kernel add the following grafts to
.git/info/grafts:
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 e7e173af42dbf37b1d946f9ee00219cb3b2bea6a
7a2deb32924142696b8174cdf9b38cd72a11fc96 379a6be1eedb84ae0d476afbc4b4070383681178
With these grafts, you can get a complete and continuous history of the kernel
since 0.01. Read the rest of the article on grafts and git replace in
Nicola’s just published piece.
Working on the Linux kernel, Olaf Hering wondered why the following command does not show all the commits that modified a function in the given file:
git log -p -M --stat -- drivers/hv/channel_mgmt.c
and asked for a command that could show them all.
John Keeping replied that the change not shown by the above command
was added in an evil merge and suggested the following command, which
adds --cc, instead:
git log -p -M --stat --cc -- drivers/hv/channel_mgmt.c
Olaf thanked John, but said that the output from the above command is rather useless. Andreas Schwab asked why and explained how to read the output:
- static void init_vp_index(struct vmbus_channel *channel, uuid_le *type_guid) -static u32 get_vp_index(const uuid_le *type_guid) ++static void init_vp_index(struct vmbus_channel *channel, const uuid_le *type_guid)One branch renamed get_vp_index to init_vp_index, the other branch added the const attribute. This hunk combines both changes.
Then Linus Torvalds explained the situation this way:
That’s not an evil merge. That’s just a regular merge. One side had changed the argument to “const”:
- 1b9d48f2a579 (“hyper-v: make uuid_le const”)
while the other side had renamed the function and added an argument, and changed the return type:
- d3ba720dd58c (“Drivers: hv: Eliminate the channel spinlock in the callback path”)
an “evil merge” is something that makes changes that came from neither side and aren’t actually resolving a conflict.
Linus then started a discussion about whether the -p option in git
log should imply --cc:
That said, I do wonder if we should just make “-p” imply “–cc”. Right now we have the kind of odd situation that “git log -p” doesn’t show merge patches, but “git show
" does show them. And you kind of have to know a lot about git to know the "--cc" option.
Linus, Junio and Jeff King tried to find a sensible way to improve on this, but in the end it looks like nothing will be done as the new behavior might have some drawbacks. Junio concluded the thread with:
It just is that ‘-p’, that clearly stands for ‘patch’ but does more than ‘patch’ to produce something that cannot be fed to ‘apply’ by defaulting to ‘–cc’, makes me hesitate. By making it a lot more convenient for experienced people who understand these issues, I have this suspicion that it would make the options less orthgonal and harder to explain to new people.
Gaurav Chhabra reported that in his company they have a script launched by a pre-receive hook in which they use the following Git command to check if a given object is a tag instead of a commit:
git describe --exact-match $sha1 2>&1 | grep -o fatal | wc -w
If the output of the command is 0, the script considers that $sha1 is a tag.
Unfortunately it sometimes doesn’t work and Gaurav, though acknowledging that it’s a bad implementation, wondered why.
Junio Hamano replied that commits that are pointed to by annotated tags would be considered as tags, and wondered if that was on purpose and asked why that would be wanted.
Junio also found something strange in an earlier part of the script and suggested an alternative implementation.
Jacob Keller, aka Jake, agreed with Junio that it was not clear what
was really needed, and suggested that the problem might come from the
fact that, when the pre-receive hook that launches the script is run,
refs that are received have not yet been accepted, so git describe
cannot see them yet.
Agreeing with Jake, Junio replied to Gaurav with the following description of what happens on the server when it receives a push:
When you push a branch ‘master’ and a tag ‘v1.0’, these things happen in order:
(1) all objects that are necessary to ensure that the receiving repository has everything reachable from ‘master’ and ‘v1.0’ are sent to it and stored. If the receiving end fails to store this correctly, everything below is skipped and the operation fails.
(2) pre-receive is run. Notice that at this point, no refs have been updated yet, so “describe” will not know v1.0 tag (if it is a new one) exists. But this step can assume that all new objects are already accessible using their object name, so
case "$old" in 0000000000000000000000000000000000000000) range=$new ;; *) range=$old..$new ;; esac && git rev-list $range | while read commit do check $commit object, e.g. git cat-file commit $commit | grep FooBarId done is expected to work.(3) for each ref being updated (in this case, refs/heads/master and refs/tags/v1.0), the following happens:
(3-1) built-in sanity checks may reject the push to the ref, e.g. refusing to update a checked out branch, refusing to accept a non-fast-forward push that is not forced, etc. (3-2) update-hook is run, which may reject the push to this ref. (3-3) the ref is updated (unless the push is atomic).(4) if the push is atomic, the refs are updated.
(5) post-receive hook is run. This is for logging and cannot affect the outcome of the push.
(6) for each ref that was updated (in this case, refs/heads/master and refs/tags/v1.0), post-update hook is run. This is for logging and cannot affect the outcome of the push.
and this opinion about the problematic command:
As your requirement seems to be to validate any and all new commits, I think it is totally unnecessary to check if any of them happens to have a tag pointing at it in the first place.
Gaurav eventually said that the purpose of the problematic command was only to check if a ref that is update is a tag. Jake then detailed how to properly check in the hook that a ref being updated is a tag:
To check whether the ref being updated is a tag, you need to check the 3rd parameter. pre-receive receives in the format
so you need to check each line's 3rd value which is the ref-name being updated. If it's in refs/tags then it's a tag update. If it's not, you can check it as a branch update.
Gaurav thanked Junio and Jake for their support.
Before we move on to the link section of this edition, we want to make a “call-to-arms” for people reading the Git mailing list. We didn’t get enough material in this edition on what was really discussed on the list the last month. In order to to be the window into the Git community that we set out to be, we need your help.
So please, the next time you read through an interesting discussion on the mailing list, note down a few lines about it and share them with us.
Light reading
git worktree add command that arrived with Git 2.5.git bisect, so here is Kruno Knego giving beginners a fresh introduction.Git tools and sites
This edition of Git Rev News was curated by Christian Couder <christian.couder@gmail.com>, Thomas Ferris Nicolaisen <tfnico@gmail.com> and Nicola Paolucci <npaolucci@atlassian.com>, with help from Andrew Ardill.