arcbox-computer-runtime 0.10.0

//! Write vm-agent into ext4 images through a loop mount.

use super::RootfsBuilder;
use anyhow::{Context, Result, bail};
use std::path::Path;
#[cfg(target_os = "linux")]
use std::{path::PathBuf, sync::Arc};

impl RootfsBuilder {
    /// Inject the configured agent at [`super::VM_AGENT_PATH`] and link
    /// `/etc/resolv.conf` into `/run`. Preserve the image's `/sbin/init`.
    /// An owned task completes injection and cleanup after caller cancellation.
    /// Runtime shutdown or process termination can interrupt resource cleanup.
    pub async fn inject_vm_agent(&self, image: &Path) -> crate::error::Result<()> {
        let builder = Self::new(self.paths.clone(), self.block_tools.clone());
        let image = image.to_owned();
        let (reply, completed) = tokio::sync::oneshot::channel();
        tokio::spawn(async move {
            let result = builder.inject_agent(&image).await;
            if let Err(Err(error)) = reply.send(result) {
                tracing::error!(?error, "cancelled vm-agent injection failed");
            }
        });
        completed
            .await
            .context("vm-agent injection task stopped")
            .and_then(std::convert::identity)
            .map_err(super::rootfs_err)
    }

    /// Inject vm-agent into an ext4 image through a loop mount.
    ///
    /// The image is attached as a loop device through the composer's
    /// [`BlockTools`](arcbox_snapshot::snapshot_cow::BlockTools) and mounted with `mount(2)`; nothing here shells out.
    /// Success means the agent is in the image *and* every resource is
    /// released again: an image whose mount or loop device could not be
    /// torn down is not published, because the caller's rename would cache
    /// a file another mount holds and a leaked `/dev/loopN` per build would
    /// eventually exhaust the pool.
    #[cfg(target_os = "linux")]
    pub(super) async fn inject_agent(&self, image: &Path) -> Result<()> {
        if !self.paths.vm_agent.exists() {
            bail!("vm-agent not found at {}", self.paths.vm_agent.display());
        }

        // Attach first: a failed attach (no free loop device, tooling
        // missing) then leaves nothing behind.
        let loop_dev = self
            .attach_loop(image)
            .await
            .context("failed to attach ext4 image for vm-agent injection")?;
        let mount_dir =
            std::env::temp_dir().join(format!("arcbox-inject-{}", uuid::Uuid::new_v4()));
        let staged = match tokio::fs::create_dir_all(&mount_dir).await {
            Ok(()) => mount_ext4(&loop_dev, &mount_dir)
                .await
                .with_context(|| format!("mount {loop_dev} for vm-agent injection")),
            Err(e) => Err(e).context("failed to create injection mount dir"),
        };
        if let Err(error) = staged {
            // Release the device even when staging fails. Preserve every failure.
            let mut failures = Vec::new();
            self.detach_loop(&loop_dev)
                .await
                .push_failure(&mut failures);
            remove_mount_dir(&mount_dir)
                .await
                .push_failure(&mut failures);
            return finish_cleanup(Err(error), failures);
        }

        let injected = self.write_agent_into(&mount_dir).await;

        // Release everything, collecting rather than short-circuiting so a
        // failed step never skips the ones after it.
        let mut failures = Vec::new();
        unmount(&mount_dir).await.push_failure(&mut failures);
        self.detach_loop(&loop_dev)
            .await
            .push_failure(&mut failures);
        remove_mount_dir(&mount_dir)
            .await
            .push_failure(&mut failures);

        finish_cleanup(injected, failures)
    }

    /// Copy the agent to `/sbin/vm-agent` (mode 755) and point
    /// `/etc/resolv.conf` at the `/run` tmpfs inside the mounted image.
    #[cfg(target_os = "linux")]
    async fn write_agent_into(&self, mount_dir: &Path) -> Result<()> {
        let sbin = mount_dir.join("sbin");
        let dest = mount_dir.join(super::VM_AGENT_PATH.trim_start_matches('/'));
        tokio::fs::create_dir_all(&sbin)
            .await
            .context("failed to create /sbin in rootfs")?;
        tokio::fs::copy(&self.paths.vm_agent, &dest)
            .await
            .context("failed to copy vm-agent into rootfs")?;
        tokio::fs::set_permissions(&dest, std::os::unix::fs::PermissionsExt::from_mode(0o755))
            .await
            .context("failed to chmod vm-agent in rootfs")?;

        // Point resolv.conf at tmpfs, mirroring the default template: DNS
        // rewrites must stay off the CoW block device (CORE-75). Rewriting
        // is safe for Docker images — Docker itself never ships meaningful
        // resolv.conf content (it bind-mounts one at run time).
        let etc = mount_dir.join("etc");
        tokio::fs::create_dir_all(&etc)
            .await
            .context("create /etc in rootfs")?;
        let resolv = etc.join("resolv.conf");
        match tokio::fs::remove_file(&resolv).await {
            Ok(()) => {}
            Err(error) if error.kind() == std::io::ErrorKind::NotFound => {}
            Err(error) => return Err(error).context("replace /etc/resolv.conf in rootfs"),
        }
        tokio::fs::symlink("../run/resolv.conf", &resolv)
            .await
            .context("link /etc/resolv.conf into the run tmpfs")?;
        Ok(())
    }

    /// Loop mounts are a Linux operation; the builder compiles elsewhere but
    /// cannot inject.
    #[cfg(not(target_os = "linux"))]
    pub(super) async fn inject_agent(&self, _image: &Path) -> Result<()> {
        bail!("vm-agent injection needs a Linux loop mount")
    }

    /// [`BlockTools::attach_loop`](arcbox_snapshot::snapshot_cow::BlockTools::attach_loop) on a blocking thread.
    #[cfg(target_os = "linux")]
    async fn attach_loop(&self, image: &Path) -> Result<String> {
        let tools = Arc::clone(&self.block_tools);
        let image = PathBuf::from(image);
        tokio::task::spawn_blocking(move || tools.attach_loop(&image, false))
            .await
            .context("loop attach task panicked")?
            .map_err(anyhow::Error::from)
    }

    /// [`BlockTools::detach_loop`](arcbox_snapshot::snapshot_cow::BlockTools::detach_loop) on a blocking thread.
    ///
    /// The kernel accepts `LOOP_CLR_FD` on a device that still has users
    /// (it flags the device autoclear and frees it when the last one goes),
    /// so this succeeds after a lazy unmount as well as a clean one.
    #[cfg(target_os = "linux")]
    async fn detach_loop(&self, loop_dev: &str) -> Result<()> {
        let tools = Arc::clone(&self.block_tools);
        let dev = loop_dev.to_owned();
        tokio::task::spawn_blocking(move || tools.detach_loop(&dev))
            .await
            .context("loop detach task panicked")?
            .with_context(|| format!("detach {loop_dev}"))
    }
}

#[cfg(target_os = "linux")]
async fn remove_mount_dir(dir: &Path) -> Result<()> {
    match tokio::fs::remove_dir(dir).await {
        Ok(()) => Ok(()),
        // Staging can fail before the directory exists.
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => Ok(()),
        Err(error) => Err(error).with_context(|| format!("remove mount dir {}", dir.display())),
    }
}

#[cfg(target_os = "linux")]
fn finish_cleanup(injected: Result<()>, failures: Vec<String>) -> Result<()> {
    if failures.is_empty() {
        return injected;
    }
    let cleanup = format!(
        "vm-agent injection left resources behind: {}",
        failures.join("; ")
    );
    tracing::error!(error = %cleanup, "vm-agent injection cleanup failed");
    match injected {
        Ok(()) => Err(anyhow::anyhow!(cleanup)),
        Err(error) => Err(error.context(cleanup)),
    }
}

/// `mount(2)` an ext4 image's loop device at `dir`, off the executor —
/// mounting can replay the journal.
#[cfg(target_os = "linux")]
async fn mount_ext4(loop_dev: &str, dir: &Path) -> Result<()> {
    use nix::mount::{MsFlags, mount};

    let dev = loop_dev.to_owned();
    let dir = dir.to_path_buf();
    tokio::task::spawn_blocking(move || {
        mount(
            Some(dev.as_str()),
            &dir,
            Some("ext4"),
            MsFlags::empty(),
            None::<&str>,
        )
    })
    .await
    .context("mount task panicked")?
    .map_err(anyhow::Error::from)
}

/// `umount(2)` `dir`, falling back to a lazy unmount (`MNT_DETACH`) when
/// something still holds the tree, so the following loop detach can at
/// least schedule the device's release instead of stranding it.
#[cfg(target_os = "linux")]
async fn unmount(dir: &Path) -> Result<()> {
    use nix::mount::{MntFlags, umount, umount2};

    let target = dir.to_path_buf();
    tokio::task::spawn_blocking(move || match umount(&target) {
        Ok(()) => Ok(()),
        // Still busy: detach the tree lazily so the loop device can be
        // released once the last user goes, and report the unmount as
        // failed either way — the image is not safe to publish.
        Err(busy) => match umount2(&target, MntFlags::MNT_DETACH) {
            Ok(()) => bail!(
                "umount {}: {busy} (lazily detached instead)",
                target.display()
            ),
            Err(lazy) => bail!("umount {}: {busy}; lazy unmount: {lazy}", target.display()),
        },
    })
    .await
    .context("umount task panicked")?
}

/// Collect a step's failure without stopping the cleanup sequence.
#[cfg(target_os = "linux")]
trait PushFailure {
    fn push_failure(self, failures: &mut Vec<String>);
}

#[cfg(target_os = "linux")]
impl PushFailure for Result<()> {
    fn push_failure(self, failures: &mut Vec<String>) {
        if let Err(e) = self {
            failures.push(format!("{e:#}"));
        }
    }
}

#[cfg(all(test, target_os = "linux"))]
mod tests {
    use super::*;

    #[test]
    fn cleanup_preserves_the_injection_error_and_every_release_failure() {
        let error = finish_cleanup(
            Err(anyhow::anyhow!("agent copy failed")),
            vec!["unmount failed".into(), "loop detach failed".into()],
        )
        .unwrap_err();
        let details = format!("{error:#}");
        assert!(details.contains("agent copy failed"), "{details}");
        assert!(details.contains("unmount failed"), "{details}");
        assert!(details.contains("loop detach failed"), "{details}");
    }

    #[test]
    fn unreceived_injection_cleanup_errors_are_logged() {
        if !super::super::tests::isolated_log_test(
            "rootfs::inject::tests::unreceived_injection_cleanup_errors_are_logged",
        ) {
            return;
        }
        let log = tempfile::NamedTempFile::new().unwrap();
        let writer = log.reopen().unwrap();
        let subscriber = tracing_subscriber::fmt()
            .with_ansi(false)
            .without_time()
            .with_writer(move || writer.try_clone().unwrap())
            .finish();
        tracing::subscriber::with_default(subscriber, || {
            let result = finish_cleanup(Ok(()), vec!["loop detach failed".into()]);
            let (reply, completed) = tokio::sync::oneshot::channel();
            reply.send(result).unwrap();
            drop(completed);
        });
        let logged = std::fs::read_to_string(log.path()).unwrap();
        assert!(logged.contains("loop detach failed"), "{logged}");
    }
}