store: add logic to group revlog file together For now each file get its own entry, this will help stopping this, soon™. We use such gathering in the `basicstore` code.

store: change `_walk` return to `(filename, (type, size))` If we are to group file per revlog, having the filename as the "main key" will be useful. This change will make the following changes clearer.

store: lazily get file size on demand for the fncache case We don't have the information in the first place, so we can avoid querying the file system inconditionnaly for use case we don't needs it. This change requires the StoreFile class to be passed a vfs to retrieve the file_size if needed. In the non-fncache case, we already have the information from file system walking, so we keep it and use it.

store: only access is_volatile information through the file object This make sure other code only access this information through the proper API, and it prepare for store entries to be able to agregate multiple files.

store: only access file_size information through the file object This make sure other code only access this information through the proper API, and it prepare for store entries to be able to agregate multiple files.

store: have custom init for entries class This will get useful to add special processing later in this series.

store: use specialized class for store entries We introduce two different classes for revlog and other entries. For now, we still have multiple entry for the same revlog, but we will work toward grouping the different file in a single entry in this series. Having the distinction is a step toward this goal.

store: introduce a EntryFile object to actually access file info For now a StoreEntry match a single file, but the goal is to eventually combine multiple file in a higher level Entry, so we need to introduce this distinction and use it first.

store: use a StoreEntry object instead of tuple for store files We want to make the store return more semantic information instead of a stream of file path. To achieve this, we start with adding a simple object that hold the same information as the tuple it replace, and do a simple update to the user code to fetch and use the same information. From there, we will be able to iteratively upgrade the codebase toward better objects.

store: no longer explicitly mark the splitted index as ignored Since the fncache is going to automatically recognised it as non "normal" revlog file, we don't need to explicitly make it ignored.

store: cleanup what is recognized as a revlog file The temporary file from `censor` are not relevant to the store as they are just temporary. So let `fncache` and `datafiles` forget about them.

store: only inclure revlog related file in `datafiles` This is a "reading" equivalent of the previous patches (see that patch for details).

store: do not record file that are not part of a revlog in fncache The fncache exist to list file to copy/stream when cloning a repository, it should only contains file that are relevant for a revlog in such case. For example, temporary file are not relevant. So we now skip the addiction of non-relevant file to the fn-cache in the first place.

revlog-split: expand the some test to cover the success case too It seems useful to double check the case of a transaction successfully committed before trying the one with transaction abort.

transaction: use a ".bck" extension for all backup file There are too much code in Mercurial that assume file function from extension. Keeping the original extension when doing backup is just creating confusion (for exemple, backup of revlog's confused with new version revlogs). So we change the backup name from now on to be explicilty backup file and prevent this kind of problem. (note that repository using fncache is less affected (but still affected) by this kind of problem as the backup are not listed in the fncache).

repair-issue6528: remove a now useless continue The condition for that continue have been excplicly filtered out of the `files` list a handful of line earlier.

branching: merge stable into default

stabletailgraph: clarify omission of linear parts in test sorts Those are not interesting and are collapsed using globs. Only the tagged nodes are interesting.

stabletailgraph: omit uninteresting linear parts in test log The test uses tags for the nodes of interest. There is no need to print the linear parts which exist only to force the rank.

stabletailgraph: fix typo in test

stabletailgraph: fix terminology in doc

perf: introduce a `perf::stream-locked-section` command This command benchmark the initial part of a stream clone, where the repository is locked.

transaction: tr._names are actually bytes, use byte string to join them Looks like this is never checked in the tests? But trying e.g. `print(tr)` by hand before this change would give: TypeError: sequence item 0: expected str instance, bytes found Looks like tr._names are always bytes (although this isn't actually enforced, but maybe at some point it will at least be type checked e.g. by pytype).

nix: add nix-defined package and devel env (flake) This adds a Nix Flake which defines: - package recipies to build and run Mercurial (C, C+Rust variants), - a meta package to run the test suite in an isolated sandbox, - a development environment with some tools (notably Python and the Black formatter, pinned to match the CI). Python is pinned to the recommended version. The Rust toolchain is a fairly recent one provided by the Nixpkgs channel. It is not yet pinned to the same version as the "reference" Debian package, but this does not seem to cause any issue. Example usage of local commands are provided in the `flake.nix` file. Once merged, it should also be possible to pull and run directly from the source repository directly with something like: `nix run hg+https://foss.heptapod.net/mercurial/mercurial-devel?ref=TOPIC&dir=contrib/nix' -- version`