ai-app/server/src/ssh.rs

//! Building the command a driver actually spawns -- locally, or wrapped in
//! `ssh` when the session names a host to run on.
//!
//! A driver speaks JSONL over a child process's stdio and doesn't care what
//! that child is, so a remote session is the identical command with `ssh host …`
//! in front.
//!
//! Uses the system `ssh` client rather than a Rust SSH library, so
//! `~/.ssh/config`, agents and jump hosts all keep working and there is only
//! one place to configure connections.

use std::path::{Path, PathBuf};
use std::process::Command;

use crate::config::SshConfig;

/// Options forced onto every connection. `BatchMode` makes a missing key fail
/// immediately with a readable message instead of hanging on a password prompt
/// nothing can answer; the keepalives turn a silently dropped link into a
/// process exit, which the session reports as `exited` rather than hanging.
const SSH_OPTIONS: [&str; 3] = [
    "BatchMode=yes",
    "ServerAliveInterval=30",
    "ServerAliveCountMax=3",
];

/// Builds the child process for `program args…`, run in `cwd`, either on this
/// machine (`ssh` absent) or on the machine it describes.
///
/// Stdio is left alone: how the streams are connected is the caller's decision
/// and differs by more than the transport does -- a probe wants pipes it will
/// drain, a session wants files that outlive this server.
///
/// A plain [`std::process::Command`], which `tokio` converts from, because not
/// every caller is async: the usage fetch is blocking by nature and should not
/// have to build an ssh invocation of its own.
pub fn command(
    remote: Option<&SshConfig>,
    program: &str,
    args: &[String],
    cwd: Option<&Path>,
) -> Command {
    let Some(ssh) = remote else {
        let mut command = Command::new(program);
        command.args(args);
        if let Some(cwd) = cwd {
            // Expanded here for the same reason `quote_path` expands it on the
            // far side: a working directory typed as `~/repos/ai-app` has to
            // mean the same thing whichever machine runs it. There is no shell
            // in this branch, so nothing else would -- `current_dir` would be
            // handed the literal one-character directory `~`.
            command.current_dir(expand_home(cwd));
        }
        return command;
    };

    let mut command = Command::new("ssh");
    // -T: no pty. This carries JSONL, and a pty would rewrite it (echo, CRLF
    // translation, ^C handling) into something the parser can't read.
    command.arg("-T");
    for option in SSH_OPTIONS {
        command.args(["-o", option]);
    }
    for option in &ssh.options {
        command.args(["-o", option]);
    }
    if let Some(port) = ssh.port {
        command.args(["-p", &port.to_string()]);
    }
    if let Some(identity) = &ssh.identity_file {
        command.arg("-i").arg(identity);
        // Without this, ssh may offer an agent key first and authenticate as
        // somebody else entirely -- silently, and with different permissions.
        command.args(["-o", "IdentitiesOnly=yes"]);
    }
    command.arg(&ssh.address);
    command.arg(remote_script(program, args, cwd));
    command
}

/// The single argument handed to the remote login shell. `exec` so the CLI
/// replaces that shell: the process the connection is attached to is then the
/// CLI itself, and dropping the connection takes it down rather than leaving an
/// orphan behind a live wrapper.
fn remote_script(program: &str, args: &[String], cwd: Option<&Path>) -> String {
    let mut script = String::new();
    if let Some(cwd) = cwd {
        script.push_str("cd ");
        script.push_str(&quote_path(&cwd.to_string_lossy()));
        script.push_str(" && ");
    }
    script.push_str("exec ");
    script.push_str(&quote(program));
    for arg in args {
        script.push(' ');
        script.push_str(&quote(arg));
    }
    script
}

/// A path with a leading `~` replaced by this machine's home directory.
///
/// The local half of the rule [`quote_path`] states for the remote one, and
/// deliberately the same shape: the tilde is expanded, `~user` is not, and
/// nothing else in the path gains a meaning. A machine with no home directory
/// leaves the path alone, which fails with the operating system's own message.
pub(crate) fn expand_home(path: &Path) -> PathBuf {
    let Some(rest) = path.to_str().and_then(|p| {
        if p == "~" {
            Some("")
        } else {
            p.strip_prefix("~/")
        }
    }) else {
        return path.to_path_buf();
    };
    match std::env::home_dir() {
        Some(home) => home.join(rest),
        None => path.to_path_buf(),
    }
}

/// Quotes a path, expanding a leading `~` and nothing else.
///
/// [`quote`] is right for every other word crossing to the remote side and
/// wrong for exactly one character. `~` means "expand me", and single quotes
/// are what stop expansion -- so a working directory typed as `~/repos/ai-app`
/// arrived as the literal four-character directory `~`, and the remote shell
/// said it did not exist, which reads like the path being wrong.
///
/// `"$HOME"` rather than handing the tilde to the shell unquoted: the variable
/// is expanded, the expansion is not re-split or globbed because it is
/// double-quoted, and everything after it stays single-quoted and literal.
/// `$HOME` is set by every shell this can land in, including the fish login
/// shell on the dev VM, so this does not depend on the remote shell being POSIX.
///
/// `~user` is deliberately not handled: there is no portable expansion for it,
/// and inventing one would mean guessing another account's home directory.
pub(crate) fn quote_path(path: &str) -> String {
    if path == "~" {
        return "\"$HOME\"".to_string();
    }
    match path.strip_prefix("~/") {
        Some(rest) => format!("\"$HOME\"/{}", quote(rest)),
        None => quote(path),
    }
}

/// Single-quotes one word for a POSIX shell. Everything crossing to the remote
/// side goes through here: paths, model names and prompts-as-arguments are all
/// attacker-adjacent input in a server whose whole job is running commands, and
/// unquoted they would be shell syntax rather than data.
pub(crate) fn quote(word: &str) -> String {
    // Inside single quotes every character is literal except `'` itself, which
    // is closed, escaped, and reopened.
    format!("'{}'", word.replace('\'', r"'\''"))
}

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

    fn args<const N: usize>(args: [&str; N]) -> Vec<String> {
        args.iter().map(|arg| arg.to_string()).collect()
    }

    /// The rendered argv, for asserting on what would actually run.
    fn argv(command: &Command) -> Vec<String> {
        std::iter::once(command.get_program())
            .chain(command.get_args())
            .map(|arg| arg.to_string_lossy().into_owned())
            .collect()
    }

    /// A host with nothing configured but a name to dial, so `~/.ssh/config`
    /// decides everything else -- the case that proves this adds no flags of its
    /// own when it was not told to.
    fn bare_host() -> SshConfig {
        SshConfig {
            address: "vm".to_string(),
            port: None,
            identity_file: None,
            options: vec![],
            attachments_dir: None,
        }
    }

    #[test]
    fn a_session_with_no_host_runs_the_command_directly() {
        let command = command(
            None,
            "claude",
            &args(["-p", "--verbose"]),
            Some(Path::new("/tmp/x")),
        );
        assert_eq!(argv(&command), ["claude", "-p", "--verbose"]);
        assert_eq!(command.get_current_dir(), Some(Path::new("/tmp/x")));
    }

    #[test]
    fn a_session_with_a_host_wraps_the_same_command_in_ssh() {
        let ssh = SshConfig {
            address: "bob@10.0.2.15".to_string(),
            port: Some(2222),
            identity_file: Some("/home/me/.ssh/id_ai".into()),
            options: vec!["StrictHostKeyChecking=accept-new".to_string()],
            attachments_dir: None,
        };
        let rendered = argv(&command(
            Some(&ssh),
            "claude",
            &args(["-p", "--model", "haiku"]),
            Some(Path::new("/home/bob/work")),
        ));

        assert_eq!(rendered[0], "ssh");
        assert!(rendered.contains(&"-T".to_string()));
        assert!(rendered.contains(&"BatchMode=yes".to_string()));
        assert!(rendered.contains(&"StrictHostKeyChecking=accept-new".to_string()));
        assert!(rendered.contains(&"IdentitiesOnly=yes".to_string()));
        assert!(rendered.contains(&"2222".to_string()));
        assert!(rendered.contains(&"/home/me/.ssh/id_ai".to_string()));
        // The host, then exactly one argument: the remote script.
        assert_eq!(rendered[rendered.len() - 2], "bob@10.0.2.15");
        assert_eq!(
            rendered[rendered.len() - 1],
            "cd '/home/bob/work' && exec 'claude' '-p' '--model' 'haiku'",
        );
    }

    #[test]
    fn a_remote_command_without_a_cwd_just_execs() {
        let ssh = bare_host();
        let rendered = argv(&command(Some(&ssh), "claude", &args(["-p"]), None));
        assert_eq!(rendered.last().unwrap(), "exec 'claude' '-p'");
        // No -i means no IdentitiesOnly: ~/.ssh/config decides instead.
        assert!(!rendered.contains(&"IdentitiesOnly=yes".to_string()));
    }

    /// The one character quoting must not swallow. A working directory typed as
    /// `~/repos/ai-app` was arriving as the literal directory `~`, and the
    /// remote shell reported it missing -- which reads as the path being wrong
    /// rather than the quoting being wrong, and cost an evening.
    #[test]
    fn a_leading_tilde_expands_and_nothing_else_does() {
        assert_eq!(quote_path("~"), "\"$HOME\"");
        assert_eq!(quote_path("~/repos/ai-app"), "\"$HOME\"/'repos/ai-app'");
        // Only leading, and only its own segment: a tilde anywhere else is an
        // ordinary character in a filename, and `~user` has no portable
        // expansion so it stays literal.
        assert_eq!(quote_path("/tmp/~/x"), "'/tmp/~/x'");
        assert_eq!(quote_path("~user/x"), "'~user/x'");

        // And it reaches the script the remote shell is handed.
        assert_eq!(
            remote_script("claude", &args(["-p"]), Some(Path::new("~/repos/ai-app"))),
            "cd \"$HOME\"/'repos/ai-app' && exec 'claude' '-p'",
        );
    }

    /// The same character, on the transport with no shell to expand it. The
    /// local branch runs the program directly, so a working directory of
    /// `~/repos/ai-app` would reach `current_dir` as the literal one-character
    /// directory `~`. The two transports have to agree about what a tilde means
    /// or a path is only portable by accident.
    #[test]
    fn a_local_cwd_expands_its_tilde_the_same_way() {
        let Some(home) = std::env::home_dir() else {
            return;
        };
        assert_eq!(
            expand_home(Path::new("~/repos/ai-app")),
            home.join("repos/ai-app")
        );
        assert_eq!(expand_home(Path::new("~")), home);
        // Leading only, and its own segment only -- `quote_path`'s rule.
        assert_eq!(expand_home(Path::new("/tmp/~/x")), Path::new("/tmp/~/x"));
        assert_eq!(expand_home(Path::new("~user/x")), Path::new("~user/x"));

        let local = command(None, "claude", &args(["-p"]), Some(Path::new("~/work")));
        assert_eq!(local.get_current_dir(), Some(home.join("work").as_path()));
    }

    #[test]
    fn shell_metacharacters_cross_as_data_not_syntax() {
        // Expanding $HOME must not open a door for anything else: the rest stays
        // single-quoted, so this remains one absurd path rather than three
        // commands.
        assert_eq!(
            quote_path("~/'; touch /tmp/pwned; '"),
            r#""$HOME"/''\''; touch /tmp/pwned; '\'''"#,
        );

        assert_eq!(quote("plain"), "'plain'");
        assert_eq!(quote("with space"), "'with space'");
        assert_eq!(quote("; rm -rf /"), "'; rm -rf /'");
        assert_eq!(quote("$(whoami)"), "'$(whoami)'");
        assert_eq!(quote("it's"), r"'it'\''s'");

        // The end-to-end version of the same worry: a working directory that
        // tries to close the quote and start a new command.
        let ssh = bare_host();
        let evil = Path::new("/tmp/'; touch /tmp/pwned; '");
        let rendered = argv(&command(Some(&ssh), "claude", &[], Some(evil)));
        let script = rendered.last().unwrap();
        assert_eq!(
            script,
            r"cd '/tmp/'\''; touch /tmp/pwned; '\''' && exec 'claude'"
        );
        assert!(!script.contains("; touch /tmp/pwned; '\" "));
    }
}