20201001

FreeBSD Subversion to Git Migration: Pt 2 Primer for Users

FreeBSD git Primer for Users

Today's blog is actually a preview of a git primer I'm writing for the FreeBSD project. It covers what a typical user will need, including those relatively rare users that may have some changes to the base. Please let me know what you think, and ways it can be improved. I'm keen on especially clear and useful pointers for the topics as well.

Also note: The cgit-beta mirror mentioned below is currently for testing purposes only.

If you have a lot of suggests, you can make them directly on the original for this on hackmd.

Scope

If you want to download FreeBSD, compile it from sources and generally keep up to date that way, this primer is for you. If you are looking to do more with the tree, contribute back, or commit changes, then you will need to wait for a later blog where I cover that. It covers getting the sources, updating the sources, how to bisect and touches briefly on how to cope with a few local changes. It covers the basics, and tries to give good pointers to more in-depth treatment for when the readers finds the basics insufficient.

Keeping Current With FreeBSD src tree

First step: cloning a tree. This downloads the entire tree. There’s two ways to download. Most people will want to do a deep clone of the repo. However, there are times that you may wish to do a shallow clone.

Branch names

The branch names in the new git repo are similar to the old names. For the stable branches, they are stable/X where X is the major release (like 11 or 12). The main branch in the new repo is ‘main’. The main branch in the old github mirror is ‘master’. Both reflecting the defaults of git at the time they were created. The main/master branch is the default branch if you omit the ‘-b branch’ or ‘–branch branch’ options below.

Repositories

At the moment, there’s two repositories. The hashes are different between them. The old github repo is similar to the new cgit repo. However, there are a large number of mistakes in the github repo that required us to regenerate the export when we migrated to having a git repo be the source of truth for the project.

The github repo is at https://github.com/freebsd/freebsd.git
The new cgit beta repo is at https://cgit-beta.freebsd.org/src.git
These will be $URL in the commands below.

Note: The project doesn’t use submodules as they are a poor fit for our workflows and development model. How we track changes in third-party applications is discussed elsewhere and generally of little concern to the casual user.

Deep Clone

A deep clone pulls in the entire tree, as well as all the history and branches. It’s the easiest to do. It also allows you to use git’s worktree feature to have all your active branches checked out into separate directories but with only one copy of the repository.

git clone $URL -b branch [dir]

is how you make a deep clone. ‘branch’ should be one of the branches listed in the previous section. It is optional if it is the main/master branch. dir is an optional directory to place it in (the default will be the name of the repo you are clone (freebsd or src)).

You’ll want a deep clone if you are interested in the history, plan on making local changes, or plan on working on more than one branch. It’s the easiest to keep up to date as well. If you are interested in the history, but are working with only one branch and are short on space, you can also use --single-branch to only download the one branch (though some merge commits will not reference the merged-from branch which may be important for some users who are interested in detailed versions of history).

Shallow Clone

A shallow clone copies just the most current code, but none or little of the history. This can be useful when you need to build a specific revision of FreeBSD, or when you are just starting out and plan to track the tree more fully. You can also use it to limit history to only so many revisions.

git clone -b branch --depth 1 $URL [dir]

This clones the repository, but only has the most recent version in the respository. The rest of the history is not downloaded. Should you change your mind later, you can do ‘git fetch --unshallow’ to get the old history.

Building

Once you’ve downloaded, building is done as described in the handbook, eg:

% cd src
% make buildworld
% make buildkernel
% make installkernel
% make installworld

so that won’t be coverd in depth here.

Updating

To update both types of trees uses the same commands. This pulls in all the revisions since your last update.

git pull --ff-only

will update the tree. In git, a ‘fast forward’ merge is one that only needs to set a new branch pointer and doesn’t need to re-create the commits. By always doing a ‘fast forward’ merge/pull, you’ll ensure that you have an identical copy of the FreeBSD tree. This will be important if you want to maintain local patches.

See below for how to manage local changes. The simplest is to use --autostash on the ‘git pull’ command, but more sophisticated options are available.

Selecting a Specific Version

In git, the ‘git checkout’ command can not only checkout branches, but it can also checkout a specific version. Git’s versions are the long hashes rather than a sequential number. You saw them above in the conflict when it said it couldn’t apply “646e0f9cda11”.

When you checkout a specific version, just specify the hash you want on the command line (the git log command can help you decide which hash you might want):

git checkout 08b8197a74

and you have that checked out.

However, as with many things git, it’s not so simple. You’ll be greeted with a message similar to the following:

Note: checking out '08b8197a742a96964d2924391bf9fdfeb788865d'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b <new-branch-name>

HEAD is now at 08b8197a742a hook gpiokeys.4 to the build

where the last line is generated from the hash you are checking out and the first line of the commit message from that revision. Also, a word about hashes: they can be abbreviated. That’s why you’ll see them have different lengths in different commands or their outputs. These super long hashes are often unique after 6 or 10 characters, so git lets you abbreviate and is somewhat inconsistent about how it presents them to users.

Bisecting

Sometimes, things go wrong. The last version worked, but the one you just updated to does not. A developer may ask to bisect the problem to track down which commit caused the regression.

If you’ve read the last section, you may be thinking to yourself “How the heck do I bisect with crazy version numbers like that?” then this section is for you. It’s also for you if you didn’t think that, but also want to bisect.

Fortunately, one uses the ‘git bisect’ command. Here’s a brief outline in how to use it. For more information, I’d suggest https://www.metaltoad.com/blog/beginners-guide-git-bisect-process-elimination or https://git-scm.com/docs/git-bisect for more details. The man page is good at describing what can go wrong, what to do when versions won’t build, when you want to use terms other than ‘good’ and ‘bad’, etc, none of which will be covered here.

‘git bisect start’ will start the bisection process. Next, you need to tell a range to go through. ‘git bisect good XXXXXX’ will tell it the working version and ‘git bisect bad XXXXX’ will tell it the bad version. The bad version will almost always be HEAD (a special tag for what you have checked out). The good version will be the last one you checked out.

A quick aside: if you want to know the last version you checked out, you should use ‘git reflog’:

5ef0bd68b515 (HEAD -> master, origin/master, origin/HEAD) HEAD@{0}: pull --ff-only: Fast-forward
a8163e165c5b (upstream/master) HEAD@{1}: checkout: moving from b6fb97efb682994f59b21fe4efb3fcfc0e5b9eeb to master
...

shows me moving the working tree to the master branch (a816…) and then updating from upstream (to 5ef0…). In this case, bad would be HEAD (or 5rf0bd68) and good would be a8163e165. As you can see from the output, HEAD@{1} also often works, but isn’t foolproof if you’ve done other things to your git tree after updating, but before you discover the need to bisect.

Back to git bisect. Set the ‘good’ version first, then set the bad (though the order doesn’t matter). When you set the bad version, it will give you some statistics on the process:

% git bisect start
% git bisect good a8163e165c5b
% git bisect bad HEAD
Bisecting: 1722 revisions left to test after this (roughly 11 steps)
[c427b3158fd8225f6afc09e7e6f62326f9e4de7e] Fixup r361997 by balancing parens.  Duh.

You’d then build/install that version. If it’s good you’d type ‘git bisect good’ otherwise ‘git bisect bad’. You’ll get a similar message to the above each step. When you are done, report the bad version to the developer (or fix the bug yourself and send a patch). ‘git bisect reset’ will end the process and return you back to where you started (usually tip of main). Again, the git-bisect manual (linked above) is a good resource for when things go wrong or for unusual cases.

Ports Considerations

The ports tree operates the same way. The branch names are different and the repos are in different locations.

The github mirror is at https://github.com/freebsd/freebsd-ports.git
The cgit mirror is https://cgit-beta.freebsd.org/src.git

As with ports, the ‘current’ branches are ‘master’ and ‘main’ respectively. The quarterly branches are named the same as in FreeBSD’s svn repo.

Coping with Local Changes

Here’s a small collections of topics that are more advanced for the user tracking FreeBSD. If you have no local changes, you can stop reading now (it’s the last section and OK to skip).

One item that’s important for all of them: all changes are local until pushed. Unlike svn, git uses a distributed model. For users, for most things, there’s very little difference. However, if you have local changes, you can use the same tool to manage them as you use to pull in changes from FreeBSD. All changes that you’ve not pushed are local and can easily be modified (git rebase, discussed below does this).

Keeping local changes

The simplest way to keep local changes (especially trivial ones) is to use ‘git stash’. In its simples form, you use ‘git stash’ to record the changes (which pushes them onto the stash stack). Most people use this to save changes before updating the tree as described above. They then use ‘git stash apply’ to re-apply them to the tree. The stash is a stack of changes that can be examined with ‘git stash list’. The git-stash man page (https://git-scm.com/docs/git-stash) has all the details.

This method is suitable when you have tiny tweaks to the tree. When you have anything non trivial, you’ll likely be better off keeping a local branch and rebasing. It is also integreated with the ‘git pull’ command: just add ‘–autostash’ to the command line.

Keeping a local branch

It’s much easier to keep a local branch with git than subversion. In subversion you need to merge the commit, and resolve the conflicts. This is managable, but can lead to a convoluted history that’s hard to upstream should that ever be necessary, or hard to replicate if you need to do so. Git also allows one to merge, along with the same problems. That’s one way to mange the branch, but it’s the least flexible.

Git has a concept of ‘rebasing’ which you can use to avoids these issues. The ‘git rebase’ command will basically replay all the commits relative to the parent branch at a newer location on that parent branch. This section will briefly cover how to do this, but will not cover all scenarios.

Create a branch

Let’s say you want to make a hack to FreeBSD’s ls command to never, ever do color. There’s many reasons to do this, but this example will use that as a baseline. The FreeBSD ls command changes from time to time, and you’ll need to cope with those changes. Fortunately, with git rebase it usually is automatic.

% cd src
% git checkout main
% git checkout -b no-color-ls
% cd bin/ls
% vi ls.c     # hack the changes in
% git diff    # check the changes
diff --git a/bin/ls/ls.c b/bin/ls/ls.c
index 7378268867ef..cfc3f4342531 100644
--- a/bin/ls/ls.c
+++ b/bin/ls/ls.c
@@ -66,6 +66,7 @@ __FBSDID("$FreeBSD$");
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+#undef COLORLS
 #ifdef COLORLS
 #include <termcap.h>
 #include <signal.h>
% # these look good, make the commit...
% git commit ls.c

The commit will pop you into an editor to describe what you’ve done. Once you enter that, you have your own local branch in the git repo. Build and install it like you normally would, following the directions in the handbook. git differs from other version control systems in that you have to tell it explicitly which files to use. I’ve opted to do it on the commit command line, but you can also do it with ‘git add’ which many of the more in depth tutorials cover.

Time to update

When it’s time to bring in a new version, it’s almost the same as w/o the branches. You would update like you would above, but there’s one extra command before you update, and one after. The following assumes you are starting with an unmodified tree. It’s important to start rebasing operations with a clean tree (git usually requires this).

% git checkout main
% git pull --no-ff
% git rebase -i main no-color-ls

This will bring up an editor that lists all the commits in it. For this example, don’t change it at all. This is typically what you are doing while updating the baseline (though you also use the git rebase command to curate the commits you have in the branch).

Once you’re done with the above, you’ve move the commits to ls.c forward from the old version of FreeBSD to the newer one.

Sometimes there’s merge conflicts. That’s OK. Don’t panic. You’d handle them the same as you would any other merge conflicts. To keep it simple, I’ll just describe a common issue you might see. A pointer to a more complete treatment can be found at the end of this section.

Let’s say the includes changes upstream in a radical shift to terminfo as well as a name change for the option. When you updated, you might see something like this:

Auto-merging bin/ls/ls.c
CONFLICT (content): Merge conflict in bin/ls/ls.c
error: could not apply 646e0f9cda11... no color ls
Resolve all conflicts manually, mark them as resolved with
"git add/rm <conflicted_files>", then run "git rebase --continue".
You can instead skip this commit: run "git rebase --skip".
To abort and get back to the state before "git rebase", run "git rebase --abort".
Could not apply 646e0f9cda11... no color ls

which looks scary. If you bring up an editor, you’ll see it’s a typical 3-way merge conflict resolution that you may be familiar with from other source code systems (the rest of ls.c has been omitted):

<<<<<<< HEAD
#ifdef COLORLS_NEW
#include <terminfo.h>
=======
#undef COLORLS
#ifdef COLORLS
#include <termcap.h>
>>>>>>> 646e0f9cda11... no color ls

The new code is first, and your code is second. The right fix here is to just add a #undef COLORLS_NEW before #ifdef and then delete the old changes:

#undef COLORLS_NEW
#ifdef COLORLS_NEW
#include <terminfo.h>

save the file. The rebase was interrupted, so you have to complete it:

% git add ls.c
% git rebase --cont

which tells git that ls.c has changed and to continue the rebase operation. Since there was a conflict, you’ll get kicked into the editor to maybe update the commit message.

If you get stuck during the rebase, don’t panic. git rebase --abort will take you back to a clean slate. It’s important, though, to start with an unmodified tree.

For more on this topic, https://www.freecodecamp.org/news/the-ultimate-guide-to-git-merge-and-git-rebase/ provides a rather extensive treatment. It goes into a lot of cases I didn’t cover here for simplicity that are useful to know since they come up from time to time.

Updating to a New FreeBSD Branch

Let’s say you want to main the jump from FreeBSD stable/12 to FreeBSD current. That’s easy to do as well, if you have a deep clone.

% git checkout main
% # build and install here...

and you are done. If you have a local branch, though, there’s one or two caveats. First, rebase will rewrite history, so you’ll likely want to do something to save it. Second, jumping branches tends to encounter more conflicts. If we pretend the example above was relative to stable/12, then to move to main, I’d suggest the following:

% git checkout no-color-ls
% git checkout -b no-color-ls-stable-12   # create another name for this branch
% git rebase -i stable/12 no-color-ls --onto main

What the above does is checkout no-color-ls. Then create a new name for it (no-color-ls-stable-12) in case you need to get back to it. Then you rebase onto the main branch. This will find all the commits to the current no-color-ls branch (back to where it meets up with the stable/12 branch) and then it will replay them onto the main branch creating a new no-color-ls branch there (which is why I had you create a place holder name).

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