feat(build): fetch kernels from image registry instead of building the kernel (#156)

Now that we have the kernel build infrastructure at https://github.com/edera-dev/kernels
it makes sense to drop building the kernel and download the kernel images directly.

This change introduces a ./hack/kernel/fetch.sh script which is backed by crates/build
We utilize the OCI infrastructure itself to download the kernel image. The DEV guide
has been updated to include calling the fetch script, and the OS builder now uses this
method instead. Due to the lack of need for the kernel build infra to exist here now,
it has also been removed. This should significantly speed up full builds.

This change will also enable us to turn on os build workflows for all PRs. We should
likely make the OS status checks required once this is merged.

crates/build/bin/fetch_kernel.rs

@@ -0,0 +1,120 @@
+use std::{
+    env::{self, args},
+    path::PathBuf,
+};
+
+use anyhow::{anyhow, Result};
+use env_logger::Env;
+use krataoci::{
+    name::ImageName,
+    packer::{service::OciPackerService, OciPackedFormat},
+    progress::OciProgressContext,
+    registry::OciPlatform,
+};
+use oci_spec::image::{Arch, Os};
+use tokio::{
+    fs::{self, File},
+    io::BufReader,
+};
+use tokio_stream::StreamExt;
+use tokio_tar::Archive;
+use uuid::Uuid;
+
+#[tokio::main]
+async fn main() -> Result<()> {
+    env_logger::Builder::from_env(Env::default().default_filter_or("warn")).init();
+    fs::create_dir_all("target/kernel").await?;
+
+    let arch = env::var("TARGET_ARCH").map_err(|_| anyhow!("missing TARGET_ARCH env var"))?;
+    let platform = OciPlatform::new(
+        Os::Linux,
+        match arch.as_str() {
+            "x86_64" => Arch::Amd64,
+            "aarch64" => Arch::ARM64,
+            _ => {
+                return Err(anyhow!("unknown architecture '{}'", arch));
+            }
+        },
+    );
+
+    let image = ImageName::parse(&args().nth(1).unwrap())?;
+    let mut cache_dir = std::env::temp_dir().clone();
+    cache_dir.push(format!("krata-cache-{}", Uuid::new_v4()));
+    fs::create_dir_all(&cache_dir).await?;
+
+    let _delete_cache_dir = scopeguard::guard(cache_dir.clone(), |dir| {
+        let _ = std::fs::remove_dir_all(dir);
+    });
+
+    let (context, _) = OciProgressContext::create();
+    let service = OciPackerService::new(None, &cache_dir, platform).await?;
+    let packed = service
+        .request(image.clone(), OciPackedFormat::Tar, false, context)
+        .await?;
+    let annotations = packed
+        .manifest
+        .item()
+        .annotations()
+        .clone()
+        .unwrap_or_default();
+    let Some(format) = annotations.get("dev.edera.kernel.format") else {
+        return Err(anyhow!(
+            "image manifest missing 'dev.edera.kernel.format' annotation"
+        ));
+    };
+    let Some(version) = annotations.get("dev.edera.kernel.version") else {
+        return Err(anyhow!(
+            "image manifest missing 'dev.edera.kernel.version' annotation"
+        ));
+    };
+    let Some(flavor) = annotations.get("dev.edera.kernel.flavor") else {
+        return Err(anyhow!(
+            "image manifest missing 'dev.edera.kernel.flavor' annotation"
+        ));
+    };
+
+    if format != "1" {
+        return Err(anyhow!("kernel format version '{}' is unknown", format));
+    }
+
+    let file = BufReader::new(File::open(packed.path).await?);
+    let mut archive = Archive::new(file);
+    let mut entries = archive.entries()?;
+
+    let kernel_image_tar_path = PathBuf::from("kernel/image");
+    let kernel_addons_tar_path = PathBuf::from("kernel/addons.squashfs");
+    let kernel_image_out_path = PathBuf::from(format!("target/kernel/kernel-{}", arch));
+    let kernel_addons_out_path = PathBuf::from(format!("target/kernel/addons-{}.squashfs", arch));
+
+    if kernel_image_out_path.exists() {
+        fs::remove_file(&kernel_image_out_path).await?;
+    }
+
+    if kernel_addons_out_path.exists() {
+        fs::remove_file(&kernel_addons_out_path).await?;
+    }
+
+    while let Some(entry) = entries.next().await {
+        let mut entry = entry?;
+        let path = entry.path()?.to_path_buf();
+
+        if !entry.header().entry_type().is_file() {
+            continue;
+        }
+
+        if path == kernel_image_tar_path {
+            entry.unpack(&kernel_image_out_path).await?;
+        } else if path == kernel_addons_tar_path {
+            entry.unpack(&kernel_addons_out_path).await?;
+        }
+    }
+
+    if !kernel_image_out_path.exists() {
+        return Err(anyhow!("image did not contain a file named /kernel/image"));
+    }
+
+    println!("kernel version: v{}", version);
+    println!("kernel flavor: {}", flavor);
+
+    Ok(())
+}