composer/doc/articles/resolving-merge-conflicts.md

Resolving merge conflicts

When working as a team on the same Composer project, you will eventually run into a scenario where multiple people added, updated or removed something in the composer.json and composer.lock files in multiple branches. When those branches are eventually merged together, you will get merge conflicts. Resolving these merge conflicts is not as straight forward as on other files, especially not regarding the composer.lock file.

Note: It might not immediately be obvious why text based merging is not possible for lock files, so let's imagine the following example where we want to merge two branches;

• Branch 1 has added package A which requires package B. Package B is locked at version 1.0.0.
• Branch 2 has added package C which conflicts with all versions below 1.2.0 of package B.

A text based merge would result in package A version 1.0.0, package B version 1.0.0 and package C version 1.0.0. This is an invalid result, as the conflict of package C was not considered and would require an upgrade of package B.

1. Reapplying changes

The safest method to merge Composer files is to accept the version from one branch and apply the changes from the other branch.

An example where we have two branches:

1. Package 'A' has been added
2. Package 'B' has been removed and package 'C' is added.

To resolve the conflict when we merge these two branches:

• We choose the branch that has the most changes, and accept the composer.json and composer.lock files from that branch. In this case, we choose the Composer files from branch 2.
• We reapply the changes from the other branch (branch 1). In this case we have to run composer require package/A again.

2. Validating your merged files

Before committing, make sure the resulting composer.json and composer.lock files are valid. To do this, run the following commands:

php composer.phar validate
php composer.phar install [--dry-run]

Automating merge conflict resolving with git

Some improvement could be made to git's conflict resolving by using a custom git merge driver.

An example of this can be found at balbuf's composer git merge driver.

Important considerations

Keep in mind that whenever merge conflicts occur on the lock file, the information, about the exact version new packages were locked on for one of the branches, is lost. When package A in branch 1 is constrained as ^1.2.0 and locked as 1.2.0, it might get updated when branch 2 is used as baseline and a new composer require package/A:^1.2.0 is executed, as that will use the most recent version that the constraint allows when possible. There might be a version 1.3.0 for that package available by now, which will now be used instead.

Choosing the correct version constraints and making sure the packages adhere to semantic versioning when using next significant release operators should make sure that merging branches does not break anything by accidentally updating a dependency.

Recovering from incorrectly resolved merge conflicts

If the above steps aren't followed and text based merges have been done anyway, your Composer project might be in a state where unexpected behaviour is observed because the composer.lock file is not (fully) in sync with the composer.json file.

There are two things that can happen here:

1. There are packages in the require or require-dev section of the composer.json file that are not in the lock file and as a result never installed

Note: Starting from Composer release 2.5, having packages that are required but not present in composer.lock results in an error when running install

2. There are packages in the composer.lock file that are not a direct or indirect dependency of any of the packages required. As a result, a package is installed, even though running composer why vendor/package says it is not required.

There are several ways to fix these issues;

A. Start from scratch

The easiest but most impactful option is run a composer update to resolve to a correct state from scratch.

A drawback to this is that previously locked package versions are now updated, as the information about previous package versions has been lost. If all your dependencies follow semantic versioning and your version constraints are using next significant release operators this should not be an issue, otherwise you might inadvertently break your application.

B. Reconstruct from the git history

An option that is probably not very feasible in a lot of situations but that deserves an honorable mention;

It might be possible to reconstruct the correct package state by going back into the git history and finding the most recent valid composer.lock file, and re-requiring the new dependencies from there.

C. Resolve issues manually

There is an option to recover from a discrepancy between the composer.json and composer.lock file without having to dig through the git history or starting from scratch. For that, we need to solve issue 1 and 2 separately.

1. Detecting and fixing missing required packages

To detect any package that is required but not installed, you can simply run:

php composer.phar validate

If there are packages that are required but not installed, you should get output similar to this:

./composer.json is valid but your composer.lock has some errors
# Lock file errors
- Required package "vendor/package-name" is not present in the lock file.
This usually happens when composer files are incorrectly merged or the composer.json file is manually edited.
Read more about correctly resolving merge conflicts https://getcomposer.org/doc/articles/resolving-merge-conflicts.md
and prefer using the "require" command over editing the composer.json file directly https://getcomposer.org/doc/03-cli.md#require

To recover from this, simply run composer update vendor/package-name for each package listed here. After doing this for each package listed here, running composer validate again should result in no lock file errors:

./composer.json is valid

2. Detecting and fixing superfluous packages

To detect and fix packages that are locked but not a direct/indirect dependency, you can run the following command:

php composer.phar remove --unused

If there are no packages locked that are not a dependency, the command will have the following output:

No unused packages to remove

If there are packages to be cleaned up, the output will be as follows:

vendor/package-name is not required in your composer.json and has not been removed
./composer.json has been updated
Running composer update vendor/package-name
Loading composer repositories with package information
Updating dependencies
Lock file operations: 0 installs, 0 updates, 1 removal
  - Removing vendor/package-name (1.0)
Writing lock file
Installing dependencies from lock file (including require-dev)
Nothing to install, update or remove