comparison rust/rhg/src/main.rs @ 46667:93e9f448273c

rhg: Add support for automatic fallback to Python

`rhg` is a command-line application that can do a small subset of what
`hg` can. It is written entirely in Rust, which avoids the cost of starting
a Python interpreter and importing many Python modules.
In a script that runs many `hg` commands, this cost can add up.

However making users decide when to use `rhg` instead of `hg` is
not practical as we want the subset of supported functionality
to grow over time.

Instead we introduce "fallback" behavior where, when `rhg` encounters
something (a sub-command, a repository format, …) that is not implemented
in Rust-only, it does nothing but silently start a subprocess of
Python-based `hg` running the same command.
That way `rhg` becomes a drop-in replacement for `hg` that sometimes
goes faster. Whether Python is used should be an implementation detail
not apparent to users (other than through speed).

A new `fallback` value is added to the previously introduced
`rhg.on-unsupported` configuration key. When in this mode, the new
`rhg.fallback-executable` config is determine what command to use
to run a Python-based `hg`.

The previous `rhg.on-unsupported = abort-silent` configuration was designed
to let a wrapper script call `rhg` and then fall back to `hg` based on the
exit code. This is still available, but having fallback behavior built-in
in rhg might be easier for users instead of leaving that script "as an
exercise for the reader".

Using a subprocess like this is not idea, especially when `rhg` is to be
installed in `$PATH` as `hg`, since the other `hg.py` executable needs
to still be available… somewhere. Eventually this could be replaced
by using PyOxidizer to a have a single executable that embeds a Python
interpreter, but only starts it when needed.

Differential Revision: https://phab.mercurial-scm.org/D10093

46666:33f2d56acc73

use hg::repo::{Repo, RepoError};
use hg::utils::files::{get_bytes_from_os_str, get_path_from_bytes};
use hg::utils::SliceExt;
use std::ffi::OsString;
use std::path::PathBuf;

mod blackbox;
mod error;
mod exitcode;
mod ui;

    }
}

fn exit(
    ui: &Ui,
    on_unsupported: OnUnsupported,
    result: Result<(), CommandError>,
) -> ! {
    match &result {
        Ok(_) => {}
        Err(CommandError::Abort { message }) => {
            if !message.is_empty() {
                // Ignore errors when writing to stderr, we’re already exiting

                        b"unsupported feature: {}\n",
                        message
                    ));
                }
                OnUnsupported::AbortSilent => {}
            }
        }
    }
    std::process::exit(exit_code(&result))
}

    /// Print an error message describing what feature is not supported,
    /// and exit with code 252.
    Abort,
    /// Silently exit with code 252.
    AbortSilent,
}

impl OnUnsupported {
    fn from_config(config: &Config) -> Self {
        let default = OnUnsupported::Abort;
        match config.get(b"rhg", b"on-unsupported") {
            Some(b"abort") => OnUnsupported::Abort,
            Some(b"abort-silent") => OnUnsupported::AbortSilent,
            None => default,
            Some(_) => {
                // TODO: warn about unknown config value
                default
            }
        }
    }
}

46667:93e9f448273c

use hg::repo::{Repo, RepoError};
use hg::utils::files::{get_bytes_from_os_str, get_path_from_bytes};
use hg::utils::SliceExt;
use std::ffi::OsString;
use std::path::PathBuf;
use std::process::Command;

mod blackbox;
mod error;
mod exitcode;
mod ui;

    }
}

fn exit(
    ui: &Ui,
    mut on_unsupported: OnUnsupported,
    result: Result<(), CommandError>,
) -> ! {
    if let (
        OnUnsupported::Fallback { executable },
        Err(CommandError::UnsupportedFeature { .. }),
    ) = (&on_unsupported, &result)
    {
        let mut args = std::env::args_os();
        let executable_path = get_path_from_bytes(&executable);
        let this_executable = args.next().expect("exepcted argv[0] to exist");
        if executable_path == &PathBuf::from(this_executable) {
            // Avoid spawning infinitely many processes until resource
            // exhaustion.
            let _ = ui.write_stderr(&format_bytes!(
                b"Blocking recursive fallback. The 'rhg.fallback-executable = {}' config \
                points to `rhg` itself.\n",
                executable
            ));
            on_unsupported = OnUnsupported::Abort
        } else {
            // `args` is now `argv[1..]` since we’ve already consumed `argv[0]`
            let result = Command::new(executable_path).args(args).status();
            match result {
                Ok(status) => std::process::exit(
                    status.code().unwrap_or(exitcode::ABORT),
                ),
                Err(error) => {
                    let _ = ui.write_stderr(&format_bytes!(
                        b"tried to fall back to a '{}' sub-process but got error {}\n",
                        executable, format_bytes::Utf8(error)
                    ));
                    on_unsupported = OnUnsupported::Abort
                }
            }
        }
    }
    match &result {
        Ok(_) => {}
        Err(CommandError::Abort { message }) => {
            if !message.is_empty() {
                // Ignore errors when writing to stderr, we’re already exiting

                        b"unsupported feature: {}\n",
                        message
                    ));
                }
                OnUnsupported::AbortSilent => {}
                OnUnsupported::Fallback { .. } => unreachable!(),
            }
        }
    }
    std::process::exit(exit_code(&result))
}

    /// Print an error message describing what feature is not supported,
    /// and exit with code 252.
    Abort,
    /// Silently exit with code 252.
    AbortSilent,
    /// Try running a Python implementation
    Fallback { executable: Vec<u8> },
}

impl OnUnsupported {
    const DEFAULT: Self = OnUnsupported::Abort;
    const DEFAULT_FALLBACK_EXECUTABLE: &'static [u8] = b"hg";

    fn from_config(config: &Config) -> Self {
        match config
            .get(b"rhg", b"on-unsupported")
            .map(|value| value.to_ascii_lowercase())
            .as_deref()
        {
            Some(b"abort") => OnUnsupported::Abort,
            Some(b"abort-silent") => OnUnsupported::AbortSilent,
            Some(b"fallback") => OnUnsupported::Fallback {
                executable: config
                    .get(b"rhg", b"fallback-executable")
                    .unwrap_or(Self::DEFAULT_FALLBACK_EXECUTABLE)
                    .to_owned(),
            },
            None => Self::DEFAULT,
            Some(_) => {
                // TODO: warn about unknown config value
                Self::DEFAULT
            }
        }
    }
}