All about handling missing repository requirements.
1. What is a missing requirement?
A repository stored on disk has some requirements. The requirements are listed in a file - that file is the first and most crucial file Mercurial reads before operating on the repository. Mercurial will fail with "abort: unknown repository format: requires features 'X'" if it doesn't know and provide all the listed features. This mechanism was designed for on disk formats but is also used for extensions providing alternative high level semantics of the stored data.
2. Repository formats
The support of repository formats is inherently tied to Mercurial versions. When an old version of Mercurial attempts to read an on-disk repository using features it doesn't support, it immediately aborts with a "requirement error" to avoid compatibility problems.
This only happens when a newer version of Mercurial creates a repository via init or clone and an older version is used to access that repository on the same disk or network filesystem. All versions of Mercurial are compatible over HTTP or SSH connections.
The following table shows which Mercurial versions support each requirement:
RevlogNG is used
The directory .hg/store contains the subdirectories data (see CaseFoldingPlan)
store files are named to work around Windows limitations described in fncacheRepoFormat.
shared store support
Leading '.' (period) or ' ' (space) in store filenames are encoded (34d8247a4595)
1.7 - 1.8.4
Use parentdelta for new revlogs (experimental, replaced by generaldelta)
Use generaldelta for improved delta chaining
For improved delta re-use inside revlog
Use zstd instead of zlib (default) compression
An "unknown repository format" abort can be resolved in different ways:
2.1. Use a newer Mercurial
See the above table to find a version that supports the missing requirement. Upgrading is generally painless, see UpgradingMercurial for more information.
2.2. Downgrade your repository over the network
Because all versions of Mercurial can interoperate over the network, an old client can simply pull from a repository served by a new client:
# run this on the new client $ hg serve listening at http://hostname:8000/ (bound to *:8000) # run this on the old client $ hg clone http://hostname:8000/ downgraded-repo
2.3. Downgrade your repository with a new client
The format configuration option may be used to instruct Mercurial to create older repository formats. For example, to convert a 'dotencode' repository into the previous format, the command
hg --config format.dotencode=0 clone --pull repoA repoB
can be used, which of course requires a Mercurial version that supports the 'dotencode' capability.
Some extensions implement alternative interpretation of the repository content. They register a "requirement" to ensure that all users of the repository will be aware of the special semantics.
For finding, installing and enabling extensions see UsingExtensions.
The largefiles extension is a part of Mercurial. It puts special semantics into files in the .hglf directory and assumes and ensure they are hashes of actual files. The actual files are fetched and pushed on demand using special protocol commands.