yash_env/semantics/

command.rs

// This file is part of yash, an extended POSIX shell.
// Copyright (C) 2025 WATANABE Yuki
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <https://www.gnu.org/licenses/>.

//! Command execution components
//!
//! This module provides functionality related to command execution semantics.

pub mod search;

use crate::Env;
use crate::function::Function;
use crate::job::{RunBlocking, RunUnblocking, handle_job_status};
use crate::semantics::{ExitStatus, Field, Result};
use crate::source::Location;
use crate::source::pretty::{Report, ReportType, Snippet};
use crate::subshell::{BlockSignals, Config};
use crate::system::concurrency::WaitForSignals;
use crate::system::resource::SetRlimit;
use crate::system::{
    Close, Dup, Errno, Exec, Exit, Fork, GetPid, Open, SendSignal, SetPgid, ShellPath, TcSetPgrp,
    Wait,
};
use crate::trap::SignalSystem;
use itertools::Itertools as _;
use std::convert::Infallible;
use std::ffi::CString;
use std::ops::ControlFlow::Continue;
use std::pin::Pin;
use std::rc::Rc;
use thiserror::Error;

type PinFuture<'a, T = ()> = Pin<Box<dyn Future<Output = T> + 'a>>;
type FutureResult<'a, T = ()> = PinFuture<'a, Result<T>>;

type EnvPrepHook<S> = fn(&mut Env<S>) -> PinFuture<'_, ()>;

/// Wrapper for a function that runs a shell function
///
/// This struct declares a function type that runs a shell function.
/// It is used to inject command execution behavior into the shell environment.
/// An instance of this struct can be stored in the shell environment
/// ([`Env::any`]) and used by modules that need to run shell functions.
///
/// The wrapped function takes the following arguments:
///
/// 1. A mutable reference to the shell environment (`&'a mut Env`)
/// 2. A reference-counted pointer to the shell function to be executed (`Rc<Function>`)
/// 3. A vector of fields representing the arguments to be passed to the function (`Vec<Field>`)
///     - This should not be empty; the first element is the function name and
///       the rest are the actual arguments.
/// 4. An optional environment preparation hook
///    (`Option<fn(&mut Env) -> Pin<Box<dyn Future<Output = ()>>>>`)
///     - This hook is called after setting up the local variable context. It can inject
///       additional setup logic or modify the environment before the function is executed.
///
/// The function returns a future that resolves to a [`Result`] indicating the
/// outcome of the function execution.
///
/// The most standard implementation of this type is provided in the
/// [`yash-semantics` crate](https://crates.io/crates/yash-semantics):
///
/// ```
/// # use yash_env::Env;
/// # use yash_env::semantics::command::RunFunction;
/// fn register_run_function<S: 'static>(env: &mut Env<S>) {
///     env.any.insert(Box::new(RunFunction::<S>(|env, function, fields, env_prep_hook| {
///         Box::pin(async move {
///             yash_semantics::command::simple_command::execute_function_body(
///                 env, function, fields, env_prep_hook
///             ).await
///         })
///     })));
/// }
/// # register_run_function(&mut Env::new_virtual());
/// ```
pub struct RunFunction<S>(
    #[allow(clippy::type_complexity, reason = "we can't make this simpler")]
    pub  for<'a> fn(
        &'a mut Env<S>,
        Rc<Function<S>>,
        Vec<Field>,
        Option<EnvPrepHook<S>>,
    ) -> FutureResult<'a>,
);

// Not derived automatically because S may not implement Clone, Copy or Debug.
impl<S> Clone for RunFunction<S> {
    fn clone(&self) -> Self {
        *self
    }
}

impl<S> Copy for RunFunction<S> {}

impl<S> std::fmt::Debug for RunFunction<S> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_tuple("RunFunction").field(&self.0).finish()
    }
}

/// Error returned when [replacing the current process](replace_current_process) fails
#[derive(Clone, Debug, Error)]
#[error("cannot execute external utility {path:?}: {errno}")]
pub struct ReplaceCurrentProcessError {
    /// Path of the external utility attempted to be executed
    pub path: CString,
    /// Error returned by the [`execve`](Exec::execve) system call
    pub errno: Errno,
}

/// Substitutes the currently executing shell process with the external utility.
///
/// This function performs the very last step of the simple command execution.
/// It disables the internal signal dispositions and calls the
/// [`execve`](Exec::execve) system call. If the call fails, it updates
/// `env.exit_status` and returns an error, in which case the caller should
/// print an error message and terminate the current process with the exit
/// status.
///
/// If the `execve` call fails with [`ENOEXEC`](Errno::ENOEXEC), this function
/// falls back on invoking the shell with the given arguments, so that the shell
/// can interpret the script. The path to the shell executable is taken from
/// [`ShellPath::shell_path`].
///
/// If the `execve` call succeeds, the future returned by this function never
/// resolves.
///
/// This function is for implementing the simple command execution semantics and
/// the `exec` built-in utility.
pub async fn replace_current_process<S: Exec + ShellPath + SignalSystem>(
    env: &mut Env<S>,
    path: CString,
    args: Vec<Field>,
) -> std::result::Result<Infallible, ReplaceCurrentProcessError> {
    env.traps
        .disable_internal_dispositions(&env.system)
        .await
        .ok();

    let args = to_c_strings(args);
    let envs = env.variables.env_c_strings();
    let Err(errno) = env
        .system
        .execve(path.as_c_str(), args.as_slice(), envs.as_slice())
        .await;
    env.exit_status = match errno {
        Errno::ENOEXEC => {
            fall_back_on_sh(&env.system, path.clone(), args, envs).await;
            ExitStatus::NOEXEC
        }
        Errno::ENOENT | Errno::ENOTDIR => ExitStatus::NOT_FOUND,
        _ => ExitStatus::NOEXEC,
    };
    Err(ReplaceCurrentProcessError { path, errno })
}

/// Converts fields to C strings.
fn to_c_strings(s: Vec<Field>) -> Vec<CString> {
    s.into_iter()
        .filter_map(|f| {
            let bytes = f.value.into_bytes();
            // TODO Handle interior null bytes more gracefully
            CString::new(bytes).ok()
        })
        .collect()
}

/// Invokes the shell with the given arguments.
async fn fall_back_on_sh<S: ShellPath + Exec>(
    system: &S,
    mut script_path: CString,
    mut args: Vec<CString>,
    envs: Vec<CString>,
) {
    // Prevent the path to be regarded as an option
    if script_path.as_bytes().starts_with("-".as_bytes()) {
        let mut bytes = script_path.into_bytes();
        bytes.splice(0..0, "./".bytes());
        script_path = CString::new(bytes).unwrap();
    }

    args.insert(1, script_path);

    // Some shells change their behavior depending on args[0].
    // We set it to "sh" for the maximum portability.
    c"sh".clone_into(&mut args[0]);

    let sh_path = system.shell_path();
    system.execve(&sh_path, args, envs).await.ok();
}

/// Error returned when starting a subshell fails in [`run_external_utility_in_subshell`]
#[derive(Clone, Debug, Error)]
#[error("cannot start subshell for utility {utility:?}: {errno}")]
pub struct StartSubshellError {
    pub utility: Field,
    pub errno: Errno,
}

impl<'a> From<&'a StartSubshellError> for Report<'a> {
    fn from(error: &'a StartSubshellError) -> Self {
        let mut report = Report::new();
        report.r#type = ReportType::Error;
        report.title = format!(
            "cannot start subshell for utility {:?}",
            error.utility.value
        )
        .into();
        report.snippets = Snippet::with_primary_span(
            &error.utility.origin,
            format!("{:?}: {}", error.utility.value, error.errno).into(),
        );
        report
    }
}

/// Starts an external utility in a subshell and waits for it to finish.
///
/// `path` is the path to the external utility. `args` are the command line
/// words of the utility. The first field must exist and be the name of the
/// utility as it may be used in error messages.
///
/// This function starts the utility in a subshell and waits for it to finish.
/// The subshell will be a foreground job if job control is enabled.
///
/// This function returns the exit status of the utility. In case of an error,
/// one of the error handling functions will be called before returning an
/// appropriate exit status. `handle_start_subshell_error` is called in the
/// parent shell if starting the subshell fails.
/// `handle_replace_current_process_error` is called in the subshell if
/// replacing the subshell process with the utility fails. Both functions
/// should print appropriate error messages.
///
/// This function is for implementing the simple command execution semantics and
/// the `command` built-in utility. This function internally uses
/// [`replace_current_process`] to execute the utility in the subshell.
pub async fn run_external_utility_in_subshell<S>(
    env: &mut Env<S>,
    path: CString,
    args: Vec<Field>,
    handle_start_subshell_error: fn(&mut Env<S>, StartSubshellError) -> PinFuture<'_>,
    handle_replace_current_process_error: fn(
        &mut Env<S>,
        ReplaceCurrentProcessError,
        Location,
    ) -> PinFuture<'_>,
) -> Result<ExitStatus>
where
    S: BlockSignals
        + Close
        + Dup
        + Exec
        + Exit
        + Fork
        + GetPid
        + Open
        + RunBlocking
        + RunUnblocking
        + SendSignal
        + SetPgid
        + SetRlimit
        + ShellPath
        + SignalSystem
        + TcSetPgrp
        + Wait
        + WaitForSignals
        + 'static,
{
    let utility = args[0].clone();

    let job_name = if env.controls_jobs() {
        to_job_name(&args)
    } else {
        String::new()
    };
    let subshell_result =
        Config::foreground().start_and_wait(env, async move |env, _job_control| {
            let location = args[0].origin.clone();
            let Err(e) = replace_current_process(env, path, args).await;
            handle_replace_current_process_error(env, e, location).await;
        });

    match subshell_result.await {
        Ok((pid, result)) => handle_job_status(env, pid, result, || job_name),
        Err(errno) => {
            handle_start_subshell_error(env, StartSubshellError { utility, errno }).await;
            Continue(ExitStatus::NOEXEC)
        }
    }
}

fn to_job_name(fields: &[Field]) -> String {
    fields
        .iter()
        .format_with(" ", |field, f| f(&format_args!("{}", field.value)))
        .to_string()
}