Enabling generaldelta in Mercurial 2.0 Repositories: Compatibility Concerns

Should generaldelta be the default for new Mercurial 5.x repositories?

Enable generaldelta only if you can guarantee that every client accessing the repository runs Mercurial 1.9 or newer. If any collaborator might use an older client (≤1.8), keep the legacy delta format as the default to avoid the "unknown revision log version" error.

Confirmed facts

• Generaldelta requires revlog version 1, which is understood by Mercurial 1.9 and later.
• Mercurial ≤1.8 aborts with "unknown revision log version" when trying to read a generaldelta repository.
• Changing a repository’s storage format after creation requires a full store rewrite (e.g., via hg convert).

Likely explanation

The Mercurial project leaves the default undecided because distributions differ in their target audiences. Enabling generaldelta saves space but introduces a hard compatibility break for older clients.

Migration strategy for an existing legacy repository

1. Ensure you have a recent Mercurial (≥2.0) installed on the machine performing the conversion.
2. Run a conversion that sets the generaldelta flag:

   hg convert --config format.generaldelta=True /path/to/legacy-repo /path/to/new-repo

3. Verify the new repository:

   cd /path/to/new-repo
    hg debugrevlog --verbose | grep 'version'

4. Replace the old repository with the new one (or update mirrors) after confirming all collaborators can access it.

Guaranteeing collaborator compatibility

1. Determine the minimum Mercurial version used by your team (ask: "What is the oldest Mercurial version in use among collaborators?").
2. If the minimum is ≥1.9, you may safely enable generaldelta; otherwise keep the legacy format.
3. Optionally enforce a version check via a server‑side hook that rejects pushes from clients reporting hg version < 1.9.
4. Communicate the requirement clearly in your contribution documentation.