Mercurial revert back to old version and continue from there

Mercurial is a distributed revision control system that developers use to manage source code changes over time. One of its powerful features is the ability to revert back to an old version, essentially undoing changes and allowing you to continue development from a previous state. This is invaluable when you’ve introduced bugs, made architectural missteps, or simply want to explore a different direction without permanently losing your existing work. Understanding how to effectively revert and continue from an older version is crucial for any developer using Mercurial, enabling experimentation and preventing irreversible errors. This guide will walk you through the process, providing clear steps and examples to help you confidently manage your codebase. Think of it as having a time machine for your code, allowing you to revisit the past and shape the future with greater control and precision. Mastering this skill will significantly enhance your workflow and protect your projects from unforeseen issues. The flexibility Mercurial offers truly empowers developers.

Understanding Reverting in Mercurial

Reverting in Mercurial isn’t as simple as just going back in time; it’s about creating new changesets that negate the effects of previous ones. This approach preserves the history of your repository, making it easier to track changes and understand why decisions were made. Unlike some other version control systems, Mercurial emphasizes immutability, meaning that once a changeset is committed, it cannot be directly altered. Instead, reverting creates a new changeset that undoes the changes introduced by the changeset you’re reverting to. This ensures that your repository’s history remains consistent and reliable. Preserving this history is vital for collaboration and debugging.

There are several ways to revert in Mercurial, each with its own use case. You can revert specific files, entire changesets, or even back to a specific date. Understanding the nuances of each method is essential for choosing the right approach for your situation. For example, if you only want to undo changes to a single file, reverting that file is the most efficient option. On the other hand, if you want to completely undo a problematic changeset, reverting the entire changeset is more appropriate. You can use commands like hg revert, hg backout, and hg update to achieve different reverting behaviors. The choice depends on whether you want to create a new commit, modify the working directory, or move to a specific state in the repository’s history.

Before you revert back to an old version, it’s important to consider the potential consequences. Reverting can introduce conflicts if other developers have made changes to the same files. It’s also crucial to communicate with your team to ensure that everyone is aware of the revert and its implications. Proper planning and communication can help prevent confusion and ensure a smooth transition back to the desired state. Always back up your repository before performing any major operations like reverting to avoid data loss. This proactive approach minimizes risk and protects your work from unexpected issues. Remember to communicate changes to your team to avoid confusion.

Step-by-Step Guide to Reverting

Here’s a detailed guide to reverting in Mercurial, covering the most common scenarios:

Identify the changeset to revert: Use hg log to view the history of your repository and identify the changeset you want to revert. Note the changeset ID or revision number.
Revert the changeset: Use the hg backout command followed by the changeset ID or revision number. For example: hg backout 123. This command creates a new changeset that undoes the changes introduced by changeset 123.
Resolve any conflicts: If the revert introduces conflicts, Mercurial will prompt you to resolve them. Use a merge tool to resolve the conflicts and mark them as resolved.
Commit the changes: Once you’ve resolved any conflicts, commit the changes to create a new changeset that represents the reverted state. Use hg commit -m “Reverted changeset 123” to commit the changes with a descriptive message.
Push the changes: Finally, push the changes to the remote repository to share the reverted state with your team. Use hg push to push the changes.

The hg backout command is generally preferred for reverting entire changesets because it creates a new changeset that explicitly undoes the changes, preserving the repository’s history. The hg revert command, on the other hand, is typically used for reverting specific files to a previous state. The hg update command allows you to move your working directory to a specific revision without necessarily creating a new changeset. Mastering these commands will empower you to manage your codebase with greater precision and control. Remember to always double-check your work before committing and pushing changes to avoid introducing new issues.

For example, imagine you accidentally committed a change that broke the build. You can quickly revert back to the old version using hg backout and then commit the corrected code. This ensures that the broken code is removed from the active development branch and that the repository’s history accurately reflects the correction. This approach minimizes disruption and allows your team to continue working without being affected by the broken code. Always test your changes thoroughly after reverting to ensure that the issue is resolved and that no new problems have been introduced.

Best Practices for Reverting and Continuing

To ensure a smooth and efficient reverting process, follow these best practices:

Communicate with your team: Before reverting, inform your team members about the changes you’re making and the potential impact on their work.
Test thoroughly: After reverting, thoroughly test your code to ensure that the issue is resolved and that no new problems have been introduced.
Use descriptive commit messages: When committing the reverted changes, use a clear and descriptive commit message that explains why the revert was necessary.

Using descriptive commit messages when you revert back to an old version makes it easier for other developers to understand the reasons behind the change and the context in which it was made. This improves collaboration and helps prevent future misunderstandings. According to a study by Capers Jones, well-documented code reduces maintenance costs by up to 20% [^1^]. Clear and concise commit messages are a key component of well-documented code. The message should clearly explain the purpose of the revert and any relevant details about the issue that was resolved.

Before reverting, always create a backup of your repository to protect against data loss. You can create a backup by cloning the repository or by creating a bundle. This ensures that you can restore your repository to its previous state if something goes wrong during the reverting process. It’s also a good idea to regularly back up your repository as part of your overall disaster recovery plan. Protecting your data is paramount.

Featured Snippet: The hg backout command is the recommended way to revert an entire changeset in Mercurial. It creates a new changeset that undoes the changes introduced by the original changeset, preserving the repository’s history. This ensures that the repository remains consistent and that the revert is properly tracked. Use the command followed by the changeset ID or revision number, such as hg backout 123, to revert a changeset.

Advanced Reverting Techniques

For more complex scenarios, Mercurial offers advanced reverting techniques. You can use the hg strip command to permanently remove changesets from the repository, but this should be done with caution as it can make it difficult to collaborate with others. You can also use the hg graft command to copy changesets from one branch to another. These advanced techniques can be useful for specific situations, but they require a deeper understanding of Mercurial’s internals. It’s often better to use less destructive methods.

The hg evolve extension provides powerful tools for rewriting history in Mercurial, including the ability to amend commits, fold commits, and prune obsolete commits [^2^]. This extension can be useful for cleaning up your repository’s history, but it should be used with caution as it can make it difficult to collaborate with others. Rewriting history should only be done in private branches or with the explicit agreement of all collaborators. Always communicate any history rewriting operations with your team to avoid confusion and prevent conflicts.

Sometimes, you might need to revert back to an old version and then cherry-pick specific changes from later revisions. This can be useful when you want to undo a major change but still retain some of the improvements that were made afterwards. Mercurial doesn’t have a direct “cherry-pick” command, but you can achieve the same result by using the hg export and hg import commands. First, export the changes you want to cherry-pick as a patch file, then revert to the old version, and finally import the patch file to apply the changes. This process allows you to selectively incorporate changes from later revisions into the reverted state.

Infographic here

FAQ: Reverting in Mercurial ---------------------------

What is the difference between hg revert and hg backout?: The hg revert command reverts specific files to a previous state, while the hg backout command creates a new changeset that undoes the changes introduced by an entire changeset. hg backout is generally preferred for reverting entire changesets.
How do I resolve conflicts after reverting?: Mercurial will prompt you to resolve any conflicts that arise during the reverting process. Use a merge tool to resolve the conflicts and mark them as resolved before committing the changes.
Can I revert multiple changesets at once?: Yes, you can revert multiple changesets by using the hg backout command multiple times, once for each changeset you want to revert. Alternatively, you can use the hg update command to move to a specific revision and then commit the changes.

According to a Stack Overflow survey, Mercurial is used by approximately 3% of developers \[^3^\], often in environments prioritizing stability and control. This makes understanding its unique reversion capabilities incredibly important for those teams.

Always back up before reverting.
Communicate with your team.

Reverting in Mercurial is a crucial skill for any developer using this version control system. By understanding the different methods and best practices, you can confidently undo changes, experiment with different approaches, and protect your projects from unforeseen issues. The ability to revert back to an old version and continue from there provides a safety net, allowing you to explore new ideas without the fear of permanently breaking your codebase. Mercurial’s emphasis on preserving history ensures that you can always track changes and understand why decisions were made. Remember to always communicate with your team, test thoroughly, and use descriptive commit messages to ensure a smooth and efficient reverting process.

Understanding how to revert back to an old version and continue from there in Mercurial provides you with greater control over your development process. With practice, you’ll become proficient in managing your codebase and mitigating potential risks. Consider exploring other Mercurial features such as branching and merging to further enhance your version control skills. You can also delve into advanced techniques like using the hg evolve extension for rewriting history. Now that you have a better grasp of reverting, why not try it out in a test repository? Don’t be afraid to experiment and see how it can improve your workflow. Learn more about version control best practices here.

[^1^]: Jones, Capers. _Applied Software Measurement: Assuring Productivity and Quality._ McGraw-Hill, 2008. [^2^]: “Hg evolve extension”. _Mercurial Wiki_. [https://www.mercurial-scm.org/wiki/Evolve](https://www.mercurial-scm.org/wiki/Evolve) [^3^]: “Stack Overflow Developer Survey 2023”. _Stack Overflow_. [https://survey.stackoverflow.co/2023/](https://survey.stackoverflow.co/2023/) Question & Answer :
I’m using Mercurial locally for a project (it’s the only repo there’s no pushing/pulling to/from anywhere else).

To date it’s got a linear history. However, the current thing I’m working on I’ve now realized is a terrible approach and I want to go back to the version before I started it and implement it a different way.

I’m a bit confused with the branch / revert / update -C commands in Mercurial. Basically I want to revert to version 38 (currently on 45) and have my next commits have 38 as a parent and carry on from there. I don’t care if revisions 39-45 are lost for ever or end up in a dead-end branch of their own.

Which command / set of commands do I need?

Here’s the cheat sheet on the commands:

hg update changes your working copy parent revision and also changes the file content to match this new parent revision. This means that new commits will carry on from the revision you update to.
hg revert changes the file content only and leaves the working copy parent revision alone. You typically use hg revert when you decide that you don’t want to keep the uncommited changes you’ve made to a file in your working copy.
hg branch starts a new named branch. Think of a named branch as a label you assign to the changesets. So if you do hg branch red, then the following changesets will be marked as belonging on the “red” branch. This can be a nice way to organize changesets, especially when different people work on different branches and you later want to see where a changeset originated from. But you don’t want to use it in your situation.

If you use hg update --rev 38, then changesets 39–45 will be left as a dead end — a dangling head as we call it. You’ll get a warning when you push since you will be creating “multiple heads” in the repository you push to. The warning is there since it’s kind of impolite to leave such heads around since they suggest that someone needs to do a merge. But in your case you can just go ahead and hg push --force since you really do want to leave it hanging.

If you have not yet pushed revision 39-45 somewhere else, then you can keep them private. It’s very simple: with hg clone --rev 38 foo foo-38 you will get a new local clone that only contains up to revision 38. You can continue working in foo-38 and push the new (good) changesets you create. You’ll still have the old (bad) revisions in your foo clone. (You are free to rename the clones however you want, e.g., foo to foo-bad and foo-38 to foo.)

Finally, you can also use hg revert --all --rev 38 and then commit. This will create a revision 46 which looks identical to revision 38. You’ll then continue working from revision 46. This wont create a fork in the history in the same explicit way as hg update did, but on the other hand you wont get complains about having multiple heads. I would use hg revert if I were collaborating with others who have already made their own work based on revision 45. Otherwise, hg update is more explicit.