magi-cli 0.8.0

//! Spawning child processes without putting a window on the operator's screen.
//!
//! Every external program magi runs - the agent CLIs, `git`, `gh`, the
//! configured verification commands - is a console application. What happens
//! when one is spawned depends on whether the *parent* has a console, and
//! magi has two kinds of parent:
//!
//! - `magi run` / `magi review` in a terminal. The child inherits that
//!   console, writes nowhere visible because its pipes are redirected, and
//!   nothing appears.
//! - `magi web`, which serves the deck. Its successor is spawned
//!   `DETACHED_PROCESS` on purpose (see [`crate::web`]): it has to outlive the
//!   process that started it and must not hold a pipe a terminal is waiting
//!   on. **That process has no console at all**, so Windows allocates a brand
//!   new one for each console child - and draws it. An implement wave is
//!   three agents, so three black windows opened over whatever the operator
//!   was doing, in front of the browser they were reading the deck in.
//!
//! `CREATE_NO_WINDOW` is the answer to exactly that: the child still gets a
//! console for its standard handles, and that console is never shown. It is
//! not the same as `DETACHED_PROCESS`, which gives the child no console and
//! would make a grandchild pop a window of its own for the same reason.
//!
//! Nothing here is conditional on how magi was started. A hidden console is
//! correct in a terminal too: the pipes are redirected either way, so there
//! was never anything to look at.

/// `CREATE_NO_WINDOW` - run the child's console, but never draw it.
///
/// From `processthreadsapi.h`. Spelled out rather than pulled in from a
/// bindings crate: it is one number that has been stable since Windows 2000,
/// and the alternative is a dependency for it.
#[cfg(windows)]
const CREATE_NO_WINDOW: u32 = 0x0800_0000;

/// Spawn without a visible console window.
///
/// Implemented for both `Command` types magi uses - `std` for the few
/// synchronous calls, `tokio` for everything else - so a call site does not
/// have to know which one it is holding, and so no call site has to repeat a
/// `#[cfg(windows)]` block to get it.
///
/// A no-op off Windows, where a spawned process has no window to begin with.
pub trait Quiet {
    /// Apply it, and hand the command back for further building.
    fn quiet(&mut self) -> &mut Self;
}

impl Quiet for std::process::Command {
    fn quiet(&mut self) -> &mut Self {
        #[cfg(windows)]
        {
            use std::os::windows::process::CommandExt as _;
            self.creation_flags(CREATE_NO_WINDOW);
        }
        self
    }
}

impl Quiet for tokio::process::Command {
    fn quiet(&mut self) -> &mut Self {
        #[cfg(windows)]
        {
            self.creation_flags(CREATE_NO_WINDOW);
        }
        self
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    /// The flag is the one Windows documents, and not one of the two it is
    /// easily confused with.
    ///
    /// `DETACHED_PROCESS` (0x8) is what leaves a process without a console -
    /// which is what caused the windows this module exists to stop, because a
    /// child of such a process gets a fresh console *with* a window.
    /// `CREATE_NEW_CONSOLE` (0x10) asks for the window outright.
    #[cfg(windows)]
    #[test]
    fn the_flag_hides_a_console_rather_than_removing_or_creating_one() {
        assert_eq!(CREATE_NO_WINDOW, 0x0800_0000);
        assert_ne!(CREATE_NO_WINDOW, 0x0000_0008, "DETACHED_PROCESS");
        assert_ne!(CREATE_NO_WINDOW, 0x0000_0010, "CREATE_NEW_CONSOLE");
    }

    /// Applying it does not disturb the command being built.
    ///
    /// The trait returns `&mut Self` so it can sit in the middle of a builder
    /// chain, and a call site that put it there must not lose its program or
    /// arguments to it.
    #[test]
    fn quiet_leaves_the_command_it_was_handed_intact() {
        let mut cmd = tokio::process::Command::new("git");
        cmd.args(["status", "--short"]).quiet();
        let built = cmd.as_std();
        assert_eq!(built.get_program(), "git");
        let args: Vec<_> = built.get_args().collect();
        assert_eq!(args, ["status", "--short"]);
    }
}