🧨 `dotgit` mode fails on new repos with `reftable` format — `unsupported extension name extensions.refstorage`

[vegerot]

tl;dr: I enabled feature.experimental in my global git config, and now whenever I clone a new repo with git clone, sapling's dotgit mode fails.

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AI Slop

Muse Spark 1.1's Issue Summary

Sapling dotgit mode does not work in new repos cloned with git clone when Git uses reftable ref storage. Old repos still work. 🚨

🌍 Environment

• Git: 2.55.0.141.g55526a1826.dirty
• Sapling: SAPLING_VERSION 0.2
• Config: ~/.gitconfig has feature.experimental = true
• OS: darwin

💥 Error Message

abort: When constructing alloc::boxed::Box<dyn storemodel::StoreOutput> from dyn storemodel::StoreInfo, after being ignored by ["eager"], "git" reported error: opening git store: unsupported extension name extensions.refstorage; class=Repository (6)

Error comes from eden/scm/lib/gitstore/src/gitstore.rs:98 (GitStore::open -> git2::Repository::open).

🔁 Reproduction Steps

1. Enable experimental features in global git config: 📝

   [feature]
     experimental = true
   

2. Clone any repo with git clone. Example: 📥

   rm -rf /tmp/reftable-test
   git clone https://github.com/git/git.git /tmp/reftable-test
   cat /tmp/reftable-test/.git/config
   

   Config shows:

   [extensions]
     refstorage = reftable
   

   And .git/reftable/ exists. .git/refs/heads does not.

3. Run sl in repo: 🕹️

   cd /tmp/reftable-test && sl status
   

   This fails with error above.

4. Also reproduce in reporter path: 🔍

   cd ~/code/github.com/git/git.git/ && sl status
   

🔬 Root Cause

• Git 2.55 enables init.defaultRefFormat=reftable when feature.experimental=true. See Documentation/config/feature.adoc:16. 📚

  When this flag is set, git clone and git init create repos with extensions.refstorage = reftable and store refs in .git/reftable/ tables.

• Sapling uses libgit2 via git2 = "0.20.4" in eden/scm/lib/gitstore/Cargo.toml:24. This version does not support extensions.refstorage. git2::Repository::open in eden/scm/lib/gitstore/src/gitstore.rs:98 fails. 🧩

• Low level ref reader in eden/scm/lib/gitcompat/src/refs.rs:141 only reads loose files under refs/ and packed-refs. It does not read reftable format. 📂

📎 Related Code

• eden/scm/lib/gitstore/src/gitstore.rs:98 - opens repo with libgit2
• eden/scm/lib/gitstore/Cargo.toml:24 - git2 = "0.20.4"
• eden/scm/lib/gitcompat/src/refs.rs:141 - populate_packed_references
• eden/scm/lib/gitcompat/src/refs.rs:210 - populate_loose_directory_references

📈 Impact

• Any user with feature.experimental=true (or explicit init.defaultRefFormat=reftable) gets new clones that do not work with sl.
• feature.experimental is popular. Git will likely make reftable default in future. Impact will grow. ⚠️

🛠️ Suggested Fix

1. Update libgit2 to version that supports reftable. Check if libgit2 main branch already supports reftable. ⬆️

2. Update gitcompat/src/refs.rs to support reftable backend:

   • Option A: shell out to git refs / git for-each-ref / git show-ref for ref list. Then Sapling uses Git binary which supports reftable. 🔧
   • Option B: add reftable parser in Rust, or use gitoxide which supports reftable. 🦀

3. Alternatively, detect reftable and show clear error with migration hint. 💡

✅ Workaround For Users

Until fix is available:

• Force files format for new repos. Add to ~/.gitconfig: 🔧

  [init]
    defaultRefFormat = files
  

• Migrate existing broken repos: 🚑

  git refs migrate --ref-format=files
  

  This converts .git/reftable/ back to .git/refs/ and sets repositoryformatversion = 0.