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Stacked Git

StGit Tutorial

StGit is a command-line application that provides functionality similar to Quilt or the Mercurial Queues extension, i.e. pushing and popping patches to/from a stack, but using Git instead of diff and patch. StGit patches are stored in a Git repository as Git commits, but can be manipulated by StGit commands in a variety of powerful ways beyond what can easily be done with regular Git commits.

This tutorial assumes familiarity with the basics of Git, including commits, branches, and merge conflicts. For more information on Git, see git(1) or the Git home page.

Getting Started

Online Help

For a full list of StGit commands:

$ stg help

For quick help on an individual stg subcommand:

$ stg help <cmd>

For more extensive help on a subcommand:

$ man stg-<cmd>

The documentation is also available as online man pages.

Setup a Repository

StGit operates in the context of a regular Git repository, providing additional capabilities above and beyond those provided by Git.

This tutorial uses StGit’s own Git repository for its examples, but any regular Git repository may be used to work through this tutorial.

Use git init to create or git clone to clone a Git repository.

To clone the StGit repository:

$ git clone https://github.com/stacked-git/stgit.git
$ cd stgit

Before creating StGit patches, the StGit stack must be initialized on the current Git branch using stg init:

$ stg init

This initializes the StGit stack metadata for the current branch. To have StGit patches on another branch, stg init must be run again on that branch.

NOTE As a shortcut, stg clone will perform a git clone followed by stg init.

Patches

Create a Patch

With the StGit stack initialized, patches may be created:

$ stg new my-first-patch

This will create a patch called my-first-patch, and open an editor to edit the patch’s commit message. This patch is empty, which can be seen by running stg show:

$ stg show

NOTE stg new may be called without a patch name, in which case the patch name will be generated by StGit based on the first line of the commit message.

But empty patches are not particularly interesting. So the next step is to make a modification to a file in the working tree using a regular text editor.

$ $EDITOR setup.py
$ stg status
M stgit/setup.py

To update a patch with changes from the working tree, stg refresh is used:

$ stg refresh

And voilĂ  – the patch is no longer empty:

$ stg show
commit 3de32068c600d40d8af2a9cf1f1c762570ae9610
Author: Audrey U. Thor <author@example.com>
Date:   Sat Oct 4 16:10:54 2008 +0200

    Tis but a patch

diff --git a/setup.py b/setup.py
index 808cd7fa9..9bdad1f86 100755
--- a/setup.py
+++ b/setup.py
@@ -1,5 +1,6 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
+"""My first patch!"""

 from distutils.core import setup

Since the patch is also a regular Git commit, it can be seen by regular Git tools such as gitk.

Another Topic, Another Patch

When making a change for a new topic, create a new patch. This time the working tree is modified before creating the new patch. It is a feature of StGit that a patch can be created independent of the working tree state.

$ echo 'Audrey U. Thor' > AUTHORS
$ stg new credit --message 'Give me some credit'
$ stg refresh

NOTE Use the --message (-m) option to stg new to give the patch a message without invoking an editor.

The stack now contains two patches:

$ stg series --description
+ my-first-patch # This is my first patch
> credit         # Give me some credit

stg series lists the patches from bottom to top; + means that a patch is ‘applied’, and > that it is the current, or topmost, patch.

Further changes to the topmost patch can be made by just editing files in the working tree running stg refresh to capture those changes in the topmost patch.

But how to change my-first-patch? The simplest way is to pop the credit patch. Doing so will make my-first-patch the topmost applied patch again:

$ stg pop credit
Checking for changes in the working directory ... done
Popping patch "credit" ... done
Now at patch "my-first-patch"
$ stg series --description
> my-first-patch # This is my first patch
- credit         # Give me some credit

stg series now indicates that my-first-patch is topmost again. And running stg refresh will update my-first-patch with any changes made to the working tree.

The minus sign (-) in front of credit in the stg series output indicates that the credit patch is ‘unapplied’, which means that the changes embodied in the credit patch are not currently applied to the work tree. Unapplied patches are also not seen in the regular Git history as seen by git log or gitk.

An unapplied patch is reapplied and made the topmost patch using stg push:

$ stg push credit
Checking for changes in the working directory ... done
Fast-forwarded patch "credit"
Now at patch "credit"

NOTE stg push and stg pop may be called without specifying a patch name. Doing so causes the next unapplied patch to be pushed, or the topmost patch to be popped, respectively.

Advanced Patch Refresh

By default stg refresh captures changes from the work tree into the topmost applied patch, however when working with multiple patches it is often the case that a change in the work tree should be captured by an already applied patch. The --patch option may be used with stg refresh to do just that.

$ stg series
+ my-first-patch
> credit
$ $EDITOR setup.py
$ stg refresh --patch my-first-patch
Popped refresh-temp -- credit
Pushing patch "refresh-temp" ... done
Pushing patch "credit" ... done
Now at patch "credit"
$ stg status

After the above refresh operation, the topmost patch remains credit, but the changes from the work tree are now part of my-first-patch and the work tree is clean as evidenced by running stg status.

NOTE Another way to update a non-topmost patch is to create a new patch, refresh the new patch with work tree changes, and then combine the new patch with the other already applied patch using stg squash.

About Commit Messages

An important part of StGit’s value is to help create useful Git history. And critical to Git commit history are good commit messages.

When creating a patch with stg new, an initial commit message is drafted, but as a patch evolves with refreshes and reorderings, the commit message typically needs to evolve as well.

StGit makes it easy to modify (and re-modify) a patch’s commit message using stg edit:

$ stg edit

In addition to using stg edit anytime, a patch’s message may also be modified when refreshing by using stg refresh --edit.

NOTE Using stg edit’s --diff option causes the patch’s diff text to be present inline when editing the commit message, which can be helpful aid for writing a thorough commit message.

NOTE The commit message of any patch in the stack may be modified at any time using stg edit <patchname>. StGit does not need to apply (push) a patch in order to modify it’s commit message, so editing patches’ commit messages can be done without risk of encountering a merge conflict.

Renaming Patches

If a patch changes considerably, it might even deserve a new name. Use stg rename to rename a patch.

Conflicts

Like with regular Git, there are various times in the normal use of StGit when conflicts can occur. With regular Git commands, a conflict may occur during merge, rebase, or pull. With StGit, conflicts may occur when applying or reordering a patch using one of the following commands:

Normally, when re-pushing a patch after popping it and making a change to another patch, StGit is able to re-push the patch without conflict. In the following example, two patches are created that each modify a different file. First a test repository is initialized:

$ git init test-repo
$ cd test-repo
$ touch a.txt b.txt
$ git add a.txt b.txt
$ git commit -m "Add files"
$ stg init

Then the two patches are created:

$ stg new first -m 'First patch'
$ echo 'a change' >> a.txt
$ stg refresh
$ stg new second -m 'Second patch'
$ echo 'b change' >> b.txt
$ stg refresh

Then both patches are popped:

$ stg pop --all

Next, the patches are reordered by pushing in the opposite order:

$ stg push second first
$ stg series
+ second
> first

StGit had no problems reordering these patches since they did not affect the same lines or even the same files. When using StGit, it is the typical case that no conflicts emerge when pushing or reordering patches; especially when each patch is limited to one coherent topic.

But it is inevidable that sometimes multiple patches necessarily affect the same lines of a file. This is when a conflict may arise.

$ stg pop
Checking for changes in the working directory ... done
Popping patch "first" ... done
Now at patch "second"
$ echo 'another change' >> a.txt
$ stg refresh

Now, both patches add a line to the end of a.txt. What happens when attempting to apply both patches at once?

$ stg push
Pushing patch "first" ... done (conflict)
Error: 1 merge conflict(s)
       CONFLICT (content): Merge conflict in a.txt
Now at patch "first"

StGit indicates that when it pushed first on top of second that since both modify the same lines of the same file (a.txt), there is a conflict. stg status can be used to see the status of files in the work tree, including those with conflicts:

$ stg status
UU a.txt

As indicated by stg push, the conflict is in the file a.txt. If the patch modified multiple files, all modified files would be listed in the status output, prefixed with UU if there were unresolvable conflicts, or M if StGit was able to resolve all diff hunks within the file.

When conflicts occur, there are two general options for how to respond:

  1. Undo the command caused the conflict(s).
  2. Resolve the conflicts.

Undo

The stg undo command can rewind the state of the StGit stack and work tree.

$ stg undo --hard
Now at patch "second"

NOTE The --hard flag for stg undo is required when there are modifications in the work tree or index, as is the case when there are unresolved conflicts in the work tree.

Undoing may be helpful when inter-patch dependencies are uncovered when attempting to reorder patches. In such cases, the best approach may be to undo the command that caused conflicts and not reorder the patches.

In other cases, however, it may be that the conflict must be resolved manually…

Resolve Conflicts

Resolving conflicts incurred while using StGit commands is the same process as when conflicts are incurred using Git commands directly:

  • Modify the affected files to decide on how the conflict should be resolved, removing conflict markers. This can either be done manually using a regular text editor, or with a tool such as git mergetool

  • Tell Git that the conflict is resolved using git add or its StGit alias, stg add.

NOTE It may be helpful to read the Git Book’s section on handling merge conflicts

Back to the example, opening a.txt in a text editor reveals the following:

<<<<<<< current
another change
=======
a change
>>>>>>> patched

The ‘conflict markers’ <<<<<<<, =======, and >>>>>>> indicate which lines were in the file before the patch was applied (current), and which conflicting lines were added by the patch (patched).

To resolve this conflict a.txt is modified to choose which lines of text to retain while also removing the conflict markers. In this case, both lines are retained:

a change
another change

And the next step is to tell Git that the conflict has been resolved:

$ stg add a.txt
$ stg status
M  a.txt

At this point, the status indicates that a.txt is modified, but no longer in conflict. The patch may now be refreshed:

$ stg refresh

The state of the resolved and refreshed patch:

$ stg show
commit 8e3ae5f6fa6e9a5f831353524da5e0b91727338e
Author: Audrey U. Thor <author@example.com>
Date:   Sun Oct 5 14:43:42 2008 +0200

    First patch

diff --git a/a.txt b/a.txt
index 0a131d8..b4c7416 100644
--- a/a.txt
+++ b/a.txt
@@ -1 +1,2 @@
+a change
 another change

Workflows

Development branch workflow

One common use of StGit is to “polish” a Git branch before publishing it to another public repository. The kinds of polish a that StGit can help with include:

  • Complete and correct commit messages.
  • Each patch limited to one coherent topic.
  • Each patch standing on its own: passing tests, etc.
  • Considerate patch (commit) order

Careful curation of Git commit history, as enabled by StGit, can be of high value to those reviewing pull requests or trying to understand why or how code came to be the way it is.

There are limits, however, to what history may be safely modified. As a general rule, any commits that have been made public (i.e. by pushing to a public repository) should be off-limits to history modification.

As a concrete example, consider a situation where several Git commits have been made in a repository with commit messages such as:

  • “Improve the snarfle cache”
  • “Remove debug printout”
  • “New snarfle cache test”
  • “Oops, spell function name correctly”
  • “Fix documentation error”
  • “More snarfle cache”

While the above may be the “true” histor of commits to the repository, it may not be the history that is most helpful to code reviewers or the developer who needs to understand what happended in this are of the code six months after the fact. Using StGit, this history can be revised to be higher quality and higher value.

The first step is to make StGit patches from the Git commits to be revised:

$ stg uncommit --number 6
Uncommitting 6 patches ...
  Now at patch "more-snarfle-cache"
done
$ stg series --description
+ improve-the-snarfle-cache      # Improve the snarfle cache
+ remove-debug-printout          # Remove debug printout
+ new-snarfle-cache-test         # New snarfle cache test
+ oops-spell-function-name-corre # Oops, spell function name correctly
+ fix-documentation-error        # Fix documentation error
> more-snarfle-cache             # More snarfle cache

The stg uncommit command adds StGit metadata to the last few Git commits, turning them into StGit patches, and thus readying them to be operated on by other StGit commands.

NOTE With the --number flag, stg uncommit uncommits that many commits and generates patch names based on their commit messages. Alternativly, patch names may be specified on the stg uncommit command line.

A number of possible history revisions are possible at this point:

  • Continue developing, and take advantage of, for example stg goto or stg refresh --patch to place modifications in the most appropriate patches.

  • Use stg float, stg sink, stg push, and stg pop to reorder patches.

  • Use stg squash to combine two or more patches into one. squash pushes and pops so that the patches to be squashed are consecutive, then makes one big patch out of the patches to be squashed, and finally pushes other patches back on the stack such that the topmost patch is the same as it was prior to running stg squash.

NOTE The above commands all cause patches to be pushed, either implicitly or explicitly. Thus these commands may trigger conflicts. If a push results in a conflict, the operation will be halted and the choice will have to be made to either undo or resolve the conflicts.

Once the history in the StGit stack satisfactorily revised, the patches can be converted back into regular Git commits:

$ stg commit --all

TIP: stg commit can commit specific patches, leaving other patches as-is. This can be used to retire patches as they mature, while keeping newer and more volatile changes as patches.

When completely done using StGit with a branch, stg branch can be used to cleanup (remove) all StGit metadata from the branch or completely delete the branch:

$ stg branch --cleanup branchname
$ stg branch --delete branchname

NOTE A branch must have an empty stack (no patches) before it is either cleaned-up or deleted.

Email-based workflow

In the ‘Development branch workflow’ described above, it was assumed that only single developer was working on her own branch without having to worry about parallel development by others. While common, this is not the only use model for Git.

An alternative use model is for many contributors to send their patches via email to a mailing list. This model is used, for example, by the Linux kernel community. In this use model, others read the patches posted to the mailing list, trying them out and providing feedback. Often, the patch author is asked to send updated versions of patches. When the project maintainer is satisfied with the patches, she will apply them and publish to a public repository.

StGit is ideally suited for the process of creating patches, emailing them out for review, revising them, mailing them off again, and eventually getting them accepted into an upstream repository.

Getting patches upstream

Two StGit commands useful for sharing patches in an email-based workflow are stg mail and stg export.

  • stg mail sends an email containing one or more patches from a StGit stack.
  • stg export exports patches from a StGit stack to a filesystem directory, one text file per patch. This may be useful if patches need to be transported by something other than email.

NOTE Git has its own capability for sending commits via email: git send-email. Since StGit patches are Git commits, git send-email may be a better choice for sending patches via email than stg mail.

NOTE For exporting a single patch stg show may be used instead of stg export.

Mailing a patch is as easy as this:

$ stg mail --to recipient@example.com <patches>

One or more patches may be listed on the command line. Each patch will be sent as a separate email, with the first line of the commit message used as the email’s subject.

NOTE stg mail uses the sendmail program to send the emails. If sendmail is not properly set up, the --smtp-server option may alternatively be provided to stg mail to use an SMTP server instead of invoking sendmail.

There are many command-line options to control exactly how patch emails are sent, as well as user-modifyable message templates. The man page has all the details, but two worth mentioning here are:

  • --edit-cover opens an editor for writing an introductory message. All patch emails are then sent as replies to this “cover message”. Using a cover message is advised whenever sending more than one patch in order to give reviewers a quick overview of the patches.

  • --edit-patches enables editing each patch before it is sent. Any part of the patch email may be modified, but it is not advised to edit the diff itself since that will only affect the outgoing email and not the underlying patch in the StGit stack. What --edit-patches is useful for, however, is to add notes for the patch recipients:

From: Audrey U. Thor <author@example.com>
Subject: [PATCH] First line of the commit message

The rest of the commit message

---

Everything after the line with the three dashes and
before the diff is just a comment, and not part of the
commit message. If there is anything you want the patch
recipients to see, but that should not be recorded in
the history if the patch is accepted, write it here.

 stgit/main.py |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)


diff --git a/stgit/main.py b/stgit/main.py
index e324179..6398958 100644
--- a/stgit/main.py
+++ b/stgit/main.py
@@ -171,6 +171,7 @@ def _main():
     sys.exit(ret or utils.STGIT_SUCCESS)

 def main():
+    print 'My first patch!'
     try:
         _main()
     finally:

Working with Remote Changes

In a project with multiple developers, while a local StGit stack is being developed, others will be doing the same. As a result, a stack will need to periodically incorporate changes from other developers. StGit has a few different tools to help with this.

Because multiple developers may be working on the same files, pulling or rebasing remote changes may result in conflicts. It is almost always less work to rebase often so that smaller sets of conflicts can be resolved versus waiting and having larger sets of conflicts to resolve. And in most workflows, patches must be rebased prior to emailing or creating a pull request.

Pulling remote changes

The most straightforward way to incorporate changes from a remote repository is to use stg pull.

$ stg pull

Under the hood, stg pull first pops any applied patches, fetches changes from a remote repository, and then reapplies any previously applied patches. The net effect is that the patch stack is rebased on top of the new remote head.

The outcome of stg pull is thus similar to the outcome of using git pull with its --rebase option.

NOTE Pulling changes from a remote repository may result in conflicts when patches are reapplied. When this occurs, the stg pull command will halt after pushing the first conflicting patch. After resolving conflicts and refreshing the conflicting patch, it becomes the user’s responsibility to push or goto the desired patch.

NOTE stg pull pulls changes from the default remote associated with the branch, but a remote may also be explicitly specified on the stg pull command line.

Fetch remote changes and rebase

Whereas stg pull fetches remote changes, updates the local branch, and rebases the branch’s stack with a single command, there are times when those steps need to be performed separately. stg rebase can be used as part of a two-step process to rebase a StGit stack with remote changes.

Step one is to fetch changes from a remote repository using either git fetch or git remote update:

$ git remote update

This updates remote branch pointers, but does not modify corresponding local branches. Step two of this process is to update a local branch with the remote changes and rebase the StGit stack on top of that. The following example rebases to the master branch of a remote repository “origin”:

$ stg rebase remotes/origin/master

Like stg pull, rebase will first pop any applied patches, update the local branch to point at the same commit as the specified remote branch, and then reapply (push) any previously applied patches.

The end result is that patches are now applied on top of the latest remotes/origin/master.

When patches are accepted upstream

When patches are accepted into an upstream repository, a good practice is to pull or rebase those commits into the local repository. The one difference in the process from above is the use of the --merged (-m) flag with stg pull or stg rebase:

$ stg pull --merged

or:

$ git remote update
$ stg rebase --merged remotes/origin/master

The --merged flag helps StGit detect that local patches have been merged upstream, at some cost in performance.

The merged patches will remain present in the StGit stack after the pull or rebase, but empty (no diff) since the change they added is now present in the stack base. stg series --empty will prefix any empty patches with a 0. And stg clean will delete all empty patches from the stack:

$ stg series --empty
0+ patch1
 + patch2
0> patch3
$ stg clean
$ stg series --empty
 > patch2

Importing patches

StGit supports importing patches from several non-Git sources using the stg import command.

  1. A patch (diff) file.

  2. Several patch files containing one patch each along with a series file listing the patch files in their correct order.

  3. An email containing a single patch.

  4. A mailbox file (in standard Unix mbox format) containing multiple emails with one patch in each.

Importing a plain patch

Importing a plain patch, such as produced by e.g. GNU diff, git diff, git show, stg diff, or stg show, is simply:

$ stg import patch-file

The imported patch will be at the top of the stack.

If a path is not provided on the command line, stg import will read the patch from its standard input. Thus a patch may be imported by piping the diff into stg import.

By default, the imported patch’s name will be derived from the file name. And if present, the patch’s commit message and author info will be taken from the beginning of the patch. However, command line options may be used to override these defaults.

Importing a patch series

Some programs—among them stg export—will create a directory of files with one patch per file, along with a ‘series’ file (often called series) listing the correct patch order. Passing --series with name of the series file to stg import will import the entire series in its correct order.

Importing a patch from an e-mail

Importing a patch from an email is simple too:

$ stg import --mail my-mail

The email should be in standard Git mail format (which is what stg mail produces)—that is, with the patch in-line in the mail, not attached. The authorship info is taken from the mail headers, and the commit message is read from the ‘Subject:’ header and the mail body.

If no filename is provided, stg import --mail will read from stdin. Thus a mail reader may be configured to pipe email contents into stg import --mail to import (and apply) a patch email.

Importing a mailbox full of patches

Finally, in case importing one patch at a time is too much work, stg import also accepts an entire Unix mbox-format mailbox, either on the command line or on its standard input; just use the --mbox flag. Each mail should contain one patch, and is imported just like with --mail.

Mailboxes full of patches are produced by e.g. stg mail with the --mbox flag, but most mail readers can produce them too, meaning that patch emails may be copied or moved to a separate mailbox and then imported.