view rust/rhg/README.md @ 48409:005ae1a343f8

rhg: add support for narrow clones and sparse checkouts

This adds a minimal support that can be implemented without parsing the narrowspec.
We can parse the narrowspec and add support for more operations later.

The reason we need so few code changes is as follows:

Most operations need no special treatment of sparse because
some of them only read dirstate (`rhg files` without `-r`),
which bakes in the filtering,
some of them only read store (`rhg files -r`, `rhg cat`),
and some of them read no data at all (`rhg root`, `rhg debugrequirements`).

`status` is the command that might care about sparse, so we just disable
rhg on it.

For narrow clones, `rhg files` clearly needs the narrowspec to work
correctly, so we fall back.
`rhg cat` seems to work consistently with `hg cat` if the file exists.
If the file is hidden by narrow spec, the error message is different and confusing, so
that's something that we should improve in follow-up patches.

Differential Revision: https://phab.mercurial-scm.org/D11764

# `rhg`

The `rhg` executable implements a subset of the functionnality of `hg`
using only Rust, to avoid the startup cost of a Python interpreter.
This subset is initially small but grows over time as `rhg` is improved.
When fallback to the Python implementation is configured (see below),
`rhg` aims to be a drop-in replacement for `hg` that should behave the same,
except that some commands run faster.


## Building

To compile `rhg`, either run `cargo build --release` from this `rust/rhg/`
directory, or run `make build-rhg` from the repository root.
The executable can then be found at `rust/target/release/rhg`.


## Mercurial configuration

`rhg` reads Mercurial configuration from the usual sources:
the user’s `~/.hgrc`, a repository’s `.hg/hgrc`, command line `--config`, etc.
It has some specific configuration in the `[rhg]` section:

* `on-unsupported` governs the behavior of `rhg` when it encounters something
  that it does not support but “full” `hg` possibly does.
  This can be in configuration, on the command line, or in a repository.

  - `abort`, the default value, makes `rhg` print a message to stderr
    to explain what is not supported, then terminate with a 252 exit code.
  - `abort-silent` makes it terminate with the same exit code,
    but without printing anything.
  - `fallback` makes it silently call a (presumably Python-based) `hg`
    subprocess with the same command-line parameters.
    The `rhg.fallback-executable` configuration must be set.

* `fallback-executable`: path to the executable to run in a sub-process
  when falling back to a Python implementation of Mercurial.

* `allowed-extensions`: a list of extension names that `rhg` can ignore.

  Mercurial extensions can modify the behavior of existing `hg` sub-commands,
  including those that `rhg` otherwise supports.
  Because it cannot load Python extensions, finding them
  enabled in configuration is considered “unsupported” (see above).
  A few exceptions are made for extensions that `rhg` does know about,
  with the Rust implementation duplicating their behavior.

  This configuration makes additional exceptions: `rhg` will proceed even if
  those extensions are enabled.


## Installation and configuration example

For example, to install `rhg` as `hg` for the current user with fallback to
the system-wide install of Mercurial, and allow it to run even though the
`rebase` and `absorb` extensions are enabled, on a Unix-like platform:

* Build `rhg` (see above)
* Make sure the `~/.local/bin` exists and is in `$PATH`
* From the repository root, make a symbolic link with
  `ln -s rust/target/release/rhg ~/.local/bin/hg`
* Configure `~/.hgrc` with:

```
[rhg]
on-unsupported = fallback
fallback-executable = /usr/bin/hg
allowed-extensions = rebase, absorb
```

* Check that the output of running
  `hg notarealsubcommand`
  starts with `hg: unknown command`, which indicates fallback.

* Check that the output of running
  `hg notarealsubcommand --config rhg.on-unsupported=abort`
  starts with `unsupported feature:`.