Fixing Git Incompatibility In `rust:slim-bookworm`

Let's dive into a tricky issue encountered while using the rust:slim-bookworm Docker image. Specifically, we're seeing test failures because of an older Git version. This article breaks down the problem, explains why it happens, and offers several solutions to get your tests passing again. This issue falls under the discussion category of acunniffe and git-ai.

Environment Details

• Docker Image: rust:slim-bookworm
• OS: Debian 12 (Bookworm)
• Git Version: 2.39.5
• Rust Version: 1.90.0

Issue Description

We've pinpointed that running cargo test within the rust:slim-bookworm Docker image results in four failing tests. The culprit? A Git version incompatibility. These tests are trying to use the --merge-base option with git merge-tree --write-tree, a feature not supported in Git 2.39.5.

Failed Tests

The following tests are failing:

1. authorship::rebase_authorship::tests::test_prepare_working_log_simple_squash
2. authorship::rebase_authorship::tests::test_prepare_working_log_squash_multiple_files
3. authorship::rebase_authorship::tests::test_prepare_working_log_squash_multiple_sessions
4. authorship::rebase_authorship::tests::test_prepare_working_log_squash_with_main_changes

Error Message

When these tests fail, you'll likely see an error message similar to this:

error: unknown option `merge-base=<sha>`
usage: git merge-tree [--write-tree] [<options>] <branch1> <branch2>
   or: git merge-tree [--trivial-merge] <base-tree> <branch1> <branch2>

Command Being Executed

The specific command causing the trouble is:

git merge-tree --write-tree --merge-base=<commit-sha> -X ours <branch1> <branch2>

Root Cause Analysis

The core issue lies in the version of Git included in Debian 12 (Bookworm). The --merge-base option for git merge-tree --write-tree wasn't introduced until Git 2.46.0 (or possibly a version between 2.39.5 and 2.47.0). Since Debian 12 ships with Git 2.39.5, it lacks this functionality, leading to the test failures. Understanding the root cause is essential for implementing the correct solution. The Git version plays a crucial role in this scenario.

Test Results: A Comparative Look

To illustrate the problem, here's a table comparing test results across different environments:

Docker Image	Debian Version	Git Version	Test Result
rust:latest	Trixie (13)	2.47.3	✅ All tests pass
rust:slim-bookworm	Bookworm (12)	2.39.5	❌ 4 tests fail

Reproduction Steps

Want to reproduce the issue yourself? Here's how:

docker run --rm -v $(pwd):/workspace -w /workspace rust:slim-bookworm bash -c "
  apt-get update && apt-get install -y libssl-dev pkg-config git &&
  cargo test -- --test-threads=1
"

This command will spin up a rust:slim-bookworm container, update the package list, install necessary dependencies (including Git), and then run the cargo test command. You should observe the same four test failures.

Possible Solutions: Getting Your Tests to Pass

Now, let's explore some potential solutions to address this Git version incompatibility:

1. Update Minimum Git Version Requirement

The simplest approach is to update the minimum Git version requirement in your project's documentation to 2.46.0 or higher. This clearly communicates the dependency to users and helps prevent issues. This ensures that users are aware of the required Git version for the project to function correctly.

2. Add Fallback Logic for Older Git Versions

A more robust solution involves adding fallback logic to your code. You can detect the Git version at runtime and use alternative commands if the --merge-base option is not supported. Here's a conceptual example:

use std::process::Command;

fn git_merge_tree(branch1: &str, branch2: &str, commit_sha: &str) -> Result<String, String> {
    let git_version = Command::new("git")
        .arg("--version")
        .output()
        .map_err(|e| format!("Failed to execute git --version: {:?}", e))?;

    let version_string = String::from_utf8_lossy(&git_version.stdout);
    let version = version_string
        .split_whitespace()
        .nth(2)
        .and_then(|s| s.split('.').nth(0).and_then(|major| major.parse::<i32>().ok()))
        .unwrap_or(0);

    if version >= 2 {
        let output = Command::new("git")
            .args(&["merge-tree", "--write-tree", format!("--merge-base={}", commit_sha).as_str(), "-X", "ours", branch1, branch2])
            .output()
            .map_err(|e| format!("Failed to execute git merge-tree with --merge-base: {:?}", e))?;

        if output.status.success() {
            Ok(String::from_utf8_lossy(&output.stdout).to_string())
        } else {
            Err(String::from_utf8_lossy(&output.stderr).to_string())
        }
    } else {
        // Implement alternative logic for older Git versions (e.g., using `git merge`)
        Err("Git version too old.  Implement fallback logic here.".to_string())
    }
}

This is a simplified example. In a real-world scenario, you'd need to implement the alternative logic for older Git versions, potentially using git merge and handling conflicts appropriately. This method makes the project more adaptable and compatible with older systems.

3. Use Alternative Git Commands

Instead of relying on git merge-tree --write-tree --merge-base, explore alternative Git commands that achieve the same result but are compatible with Git 2.39.5. This might involve a combination of git merge, git checkout, and other commands. Thorough testing is crucial to ensure the alternative commands produce the desired outcome. Consider the compatibility of Git commands across different versions.

4. Update CI/CD Environment

For CI/CD pipelines, the most straightforward solution is often to update the Docker image to one with a newer Git version (2.46.0+). This avoids the need for code changes and ensures consistent behavior across environments. Using rust:latest or a similar image that includes a recent Git version can resolve the issue. CI/CD pipeline updates can provide a quick and effective solution.

Related Code

The failures are specifically occurring in src/authorship/rebase_authorship.rs, where git merge-tree is invoked with the --merge-base option to handle squash merge scenarios. Reviewing this code section is essential when implementing any of the proposed solutions. Focus on code compatibility within the rebase_authorship.rs file.

Impact Assessment

• Users running on Debian 12 (Bookworm) stable will experience test failures, which can be frustrating and time-consuming.
• CI/CD pipelines using rust:slim-bookworm will fail, potentially blocking deployments and hindering development workflows.
• Production environments using Debian 12 might be affected if the failing code paths are executed, leading to unexpected behavior or errors. Understanding the potential impact on users and systems is crucial for prioritizing the fix.

Conclusion

By understanding the root cause of the Git version incompatibility in the rust:slim-bookworm Docker image, we can implement effective solutions to ensure test pass and smooth development workflows. Whether it's updating the minimum Git version, adding fallback logic, using alternative Git commands, or updating CI/CD environments, addressing this issue promptly is essential for maintaining code quality and reliability.

For more information on Git merge-tree, visit the Git documentation.