Add shallow repository detection and handling to JGit timestamp provider

[Copilot]

Problem

When using the JGit build timestamp provider with repositories cloned using --depth=1 (shallow clones), timestamp determination fails or produces incorrect results because the full git history is not available locally. This is particularly problematic in CI/CD environments where shallow clones are commonly used to speed up checkout times.

As noted in the issue, for large repositories like SWT binaries (750 MB vs 9 MB for HEAD only), the difference can be significant.

Solution

This PR adds shallow repository detection and configurable handling to the JGit build timestamp provider.

Changes

1. Shallow Repository Detection

The implementation detects shallow repositories by checking for the presence of the .git/shallow file, which is Git's standard mechanism for marking repositories cloned with limited depth.

2. Configuration Option: jgit.shallow

A new configuration parameter with three modes:

• error (default) - Throws an exception with a clear message explaining the issue and resolution options
• warning - Logs a warning and falls back to the default timestamp provider (uses current build time)
• ignore - Uses timestamps from the shallow history with a warning about potential inaccuracy

3. Clear Error Messages

When a shallow repository is detected in ERROR mode, users receive helpful guidance:

Git repository is shallow (cloned with --depth). This prevents accurate timestamp determination.
To resolve this issue, you can:
  1. Clone the full repository history (without --depth)
  2. Configure jgit.shallow=warning in tycho-packaging-plugin to fall back to the default timestamp provider
  3. Configure jgit.shallow=ignore to use timestamps from the shallow history (may be incorrect)

Usage Example

For CI/CD builds using shallow clones, configure the provider to fall back gracefully:

<plugin>
  <groupId>org.eclipse.tycho</groupId>
  <artifactId>tycho-packaging-plugin</artifactId>
  <version>${tycho-version}</version>
  <dependencies>
    <dependency>
      <groupId>org.eclipse.tycho</groupId>
      <artifactId>tycho-buildtimestamp-jgit</artifactId>
      <version>${tycho-version}</version>
    </dependency>
  </dependencies>
  <configuration>
    <timestampProvider>jgit</timestampProvider>
    <jgit.shallow>warning</jgit.shallow>
  </configuration>
</plugin>

Alternatively, set via system property:

mvn clean verify -Djgit.shallow=warning

Design Decisions

Why not query remote repositories?

The issue comments discussed querying remote repository history using git log commands. However, this approach was not implemented because:

1. Complexity - Remote queries require handling network operations, authentication, various Git protocols, and error cases
2. Reliability - Network operations can fail, making builds unreliable and non-deterministic
3. Performance - Remote queries could be slower than the shallow clone they're meant to optimize
4. Practicality - The detection + configuration approach solves the real problem: helping users understand the limitation and choose appropriate behavior

The current implementation provides a solid foundation that can be extended with remote querying in the future if needed, while immediately solving the problem for users encountering shallow clone issues.

Benefits

• Fail-fast by default - Prevents incorrect timestamps in builds without user awareness
• Flexible configuration - Supports different use cases (CI/CD vs reproducible builds)
• Clear guidance - Error messages explain the problem and provide solutions
• Minimal changes - ~80 lines of code, no new dependencies
• Consistent design - Follows existing pattern used for jgit.dirtyWorkingTree

Fixes issue regarding JGit timestamp provider not working with --depth=1 shallow clones.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>jgit timestamp provider do not work with --depth=1 </issue_title> <issue_description>If one only checkout the head with --depth=1 the jgit timestamps are wrong. We should investigate if it is possible to use the remote history in that case as it is vast-full in most cases (verification builds) to check out the whole history.</issue_description>

<agent_instructions>We should support a new "shallow" option in the JGitTimestamp provider that the uses querying the upstream log to figure out timestamps.

If possible we should optimize it to only fetch the latest timestamp if that makes a difference</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@HannesWell Maybe git log can be used together with the remote: https://git-scm.com/docs/git-log

Using the argument -n 1 should only return the latest commit that touches a given file/folder.

But in general I don't expect the checkout depth to have a significant influence on the overall runtime of the build. At least for the p2 build the runtime of the actions/checkout was only increased from 5 to 8sec. Of course for larger repos the difference would be greater, but larger repos probably also build longer. So I expect the fraction of the overall runtime will not change significantly.</comment_new> <comment_new>@laeubi

At least for the p2 build the runtime of the actions/checkout was only increased from 5 to 8sec.

I think github is really good at caching its own infra. But for example the swt binaries is 750 MB where the actual HEAD is only 9 MB ... Also its more that its hard to catch that issue, I would also be okay with failing the build as a first step if such a state is detected.</comment_new> <comment_new>@HannesWell

At least for the p2 build the runtime of the actions/checkout was only increased from 5 to 8sec.

I think github is really good at caching its own infra. But for example the swt binaries is 750 MB where the actual HEAD is only 9 MB ...

In such extreme cases it could be useful.

Also its more that its hard to catch that issue, I would also be okay with failing the build as a first step if such a state is detected.

That would indeed be very useful in general! I wonder how this could be detected. Does git log or however the timestamp is obtain then return a special value if the file/folder is not found in the available history?</comment_new> <comment_new>@laeubi

I have not checked this, but if I checkout a single commit then i get a "you are in a detached head state" message from git, don't know if depth=1 is such a detached state, I'm more a git user than understanding all these special features ;-)</comment_new> <comment_new>@mickaelistria "detached HEAD" is a valid case, and almost all CI builds are in detached head. Various workflows make that even on the development workstation one can be in detached head. And Tycho can still build. Tycho is confused when history is missing. With "--depth=1", there is only 1 commit in history. I don't know whether there are some flags or other stuff that Tycho can use to detect history is missing; but for sure "detached head" isn't an indicator here.</comment_new> <comment_new>@laeubi

I don't know whether there are some flags or other stuff that Tycho can use to detect history is missing; but for sure "detached head" isn't an indicator here.

I don't wanted to indicate that it is exactly that, I just wanted to say that git seems to know about the current (special) state so maybe there is a similar indicator for the history depth at least git has an "unshallow" command, so there must be some kind of way to detect this:

https://stackoverflow.com/questions/58704510/how-to-get-whole-git-history-afterwards-git-clone-depth-1

here it i indicated that there is a git rev-parse --is-shallow-repository but I don't knwo if Jgit support it

https://stackoverflow.com/questions/37531605/how-to-test-if-git-repository-is-shallow</comment_new> <comment_new>@laeubi

@tomaswolf thanks for the insigh, tycho itself do not clone the repo but "Github Action" do it, so if the repository itself is shallow I assume it does not make a difference if I use --depth=1 or "full" depth?

3. f a repository has shallow commits, there is a .git/shallow

So effectivly one could check if that file exits and emmit a warning? Or could the file be present even if I do git fetch --unshallow and we need to parse its contents? Doe JGit maybe already support getting this "shallow" info form a Repository?

@laeubi > I don't know what the correct way to deal with a shallow repo would be ...

Fixes eclipse-tycho/tycho#947

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Test Results

1 017 files  1 017 suites   5h 36m 45s ⏱️ 1 303 tests 1 280 ✅ 20 💤 0 ❌ 3 🔥 3 909 runs  3 837 ✅ 63 💤 0 ❌ 9 🔥

For more details on these errors, see this check.

Results for commit b22969f5.

[laeubi]

We must somehow prevent for these test tha they detect the git repo in the root ... but it also implies we where testing against a shallow clone of Tycho here in the past... an alternative would be to set the modes to ignore the problem.