# Mercurial 5.8 (2021-05-01)

This is an overview of the 5.8 release.
## New Features 
 
- `hg purge` is now a core command using `--confirm` by default.
 
- The `rev-branch-cache` is now updated incrementally whenever changesets are added.

- The new options `experimental.bundlecompthreads` and `experimental.bundlecompthreads.<engine>` can be used to instruct the compression engines for bundle operations to use multiple threads for compression. The default is single threaded operation. Currently only supported for zstd.

## Default Format Change 

These changes affects newly created repositories (or new clone) done with Mercurial 5.8.

 - The `ZSTD` compression will now be used by default for new repositories when available. This compression format was introduced in Mercurial 5.0, released in May 2019.
    See `hg help config.format.revlog-compression` for details.

 - Mercurial installation built with the Rust parts will now use the "persistent nodemap" feature by default. This feature was introduced in Mercurial 5.4 (May 2020). However Mercurial instalation built without the fast Rust implementation will refuse to interract with them by default. This restriction can be lifted through configuration.

    See `hg help config.format.use-persistent-nodemap` for details

## New Experimental Features 

- There's a new `diff.merge` config option to show the changes relative to an automerge for merge changesets. This makes it easier to detect and review manual changes performed in merge changesets. It is supported by `hg diff --change`, `hg log -p` `hg incoming -p`, and `hg outgoing -p` so far.


## Bug Fixes 

- gracefully recover from inconsistent persistent-nodemap data from disk.


## Backwards Compatibility Changes 

- In normal repositories, the first parent of a changeset is not null, unless both parents are null (like the first changeset). Some legacy repositories violate this condition. The revlog code will now silentely swap the parents if this condition is tested. This can change the output of `hg log` when explicitly asking for first or second parent. The changesets "nodeid" are not affected.


## Internal API Changes 

- `changelog.branchinfo` is deprecated and will be removed after 5.8. It is superseded by `changelogrevision.branchinfo`.

- Callbacks for `revlog.addgroup` and the `changelog._nodeduplicatecallback` hook now get a revision number as argument instead of a node.

- revlog.addrevision returns the revision number instead of the node.

- `nodes.nullid` and related constants are being phased out as part of the deprecation of SHA1. Repository instances and related classes provide access via `nodeconstants` and in some cases `nullid` attributes.
