update readme

Author: hicksjacobp

Class1.cs

@@ -1,5 +1 @@
-
-/// <summary>
-/// This is a class.
-/// </summary>
 public class Class1 {}

README.md

@@ -1,3 +1,50 @@
-# checkout-codeql-test
-This repo is minimal test for the default CodeQL setup
-boop to trigger codeql
+# checkout-test
+
+This repo is minimal test to show that `actions/[email protected]` introduces a new requirement on consumers to use either a newer version of `git` or a newer version of other dependencies that need to handle the `repositoryformatversion` value of `1` in the `.git` directory.
+
+The difference between `actions/[email protected]` and `actions/[email protected]` is that `sparse-checkout` is disabled explicitly, which depending on the version of `git` being used, results in a `repositoryformatversion` value of `1` being written to the `.git` directory for older versions and a value of `0` for newer versions. What exact `git` version boundary this change occurs at is not clear. The following table shows the combinations of software and their results:
+
+| `actions/checkout` | `git` | `repositoryformatversion` | Test |
+|--------------------|-------|---------------------------| ---- |
+| `v4.1.1`           | `2.34.1` | `0` | :heavy_check_mark: <https://github.com/hicksjacobp/checkout-codeql-test/actions/runs/8842167272/job/24280382043> |
+| `v4.1.4`           | `2.34.1` | `1` | :x: <https://github.com/hicksjacobp/checkout-codeql-test/actions/runs/8842167272/job/24280382240> |
+| `v4.1.4`           | `2.43.2` | `0` | :heavy_check_mark: <https://github.com/hicksjacobp/checkout-codeql-test/actions/runs/8842167272/job/24280382420> |
+
+The scenario of `actions/[email protected]` and `[email protected]` is what has caused us unforeseen pain.
+This scenario is exacerbated by the fact that `actions/[email protected]` comes with `[email protected]` and the default CodeQL workflow uses `actions/checkout@v4` (and therefore resolves to `actions/[email protected]` as of the time of writing).
+Performing an `apt-get install git` does NOT update `git` to a newer version because the base and ancestor images which `actions/runner` is based off of does not use the `apt` feed which has newer versions of git, and is rather left behind at `2.34.1`.
+
+## Resolutions
+
+### Update `git`
+
+In order to update `git` to newer versions, in Ubuntu at least, you have to perform at least these steps:
+
+```bash
+apt update
+apt install software-properties-common
+add-apt-repository -y ppa:git-core/ppa
+apt install git
+```
+
+See <https://git-scm.com/download/linux> for reference.
+
+> [!TIP]
+> Do this in `actions/runner`?
+
+### Update dependencies to handle `repositoryformatversion` of `1`
+
+Personally, I found this issue through referencing `[email protected]`. With .NET SDK 8, there is now a `[email protected]` package (along with the transitive dependency which can now handle the `repositoryformatversion` of `1`). See <https://github.com/dotnet/sourcelink/pull/772>.
+
+### Downgrade `actions/checkout` to `v4.1.1`
+
+> [!WARNING]
+> This may not be possible for your situation, for example using the default CodeQL setup and using a CI infrastructure where updating `git` has not yet been done and is out of your control.
+
+## References
+
+- `actions/[email protected]` is based off of `mcr.microsoft.com/dotnet/runtime-deps:6.0-jammy` and none of these images do any additional setup for `git`:
+  <https://github.com/actions/runner/blob/14cea13ab5e7a5f385d805bf8a9034947d25f1b6/images/Dockerfile>
+  <https://github.com/dotnet/dotnet-docker/blob/main/src/runtime-deps/6.0/jammy/amd64/Dockerfile>
+- Install instructions for `git`:
+  <https://git-scm.com/download/linux>