examples: Add example that injects arbitrary tar as OCI

I wanted to test out how container runtimes behaved with
a tarball that had a filled in user/group name. For convenience
of testing arbitrary tarballs in container images, add this
example which takes an externally generated tar and just wraps
it with OCI metadata.

Signed-off-by: Colin Walters <[email protected]>

Author: cgwalters

examples/ocidir-import-tar.rs

@@ -0,0 +1,80 @@
+//! # Import a pre-generated tarball, wrapping with OCI metadata into an OCI directory.
+//!
+//! This little exmaple shows a bit of how to use the ocidir API. But it has a use case
+//! for low-level testing of OCI runtimes by injecting arbitrary tarballs.
+
+use std::io::BufReader;
+use std::{fs::File, path::Path};
+
+use anyhow::Context;
+use cap_tempfile::cap_std;
+use chrono::Utc;
+use oci_spec::image::{MediaType, Platform};
+use ocidir::OciDir;
+
+fn import(oci_dir: &OciDir, name: &str, src: File) -> anyhow::Result<()> {
+    let mtime = src.metadata()?.modified()?;
+    let mut input_tar = BufReader::new(src);
+    let created = chrono::DateTime::<Utc>::from(mtime);
+
+    let mut manifest = oci_dir.new_empty_manifest().unwrap().build().unwrap();
+    let mut config = ocidir::oci_spec::image::ImageConfigurationBuilder::default()
+        .build()
+        .unwrap();
+
+    // Add the src as a layer
+    let mut writer = oci_dir.create_blob().unwrap();
+    std::io::copy(&mut input_tar, &mut writer)?;
+
+    let blob = writer.complete()?;
+    let descriptor = blob
+        .descriptor()
+        .media_type(MediaType::ImageLayer)
+        .build()
+        .unwrap();
+    let blob_digest = descriptor.digest().to_string();
+    manifest.layers_mut().push(descriptor);
+    let mut rootfs = config.rootfs().clone();
+    rootfs.diff_ids_mut().push(blob_digest);
+    config.set_rootfs(rootfs);
+    let h = oci_spec::image::HistoryBuilder::default()
+        .created(created.to_rfc3339_opts(chrono::SecondsFormat::Secs, true))
+        .created_by(name.to_string())
+        .build()
+        .unwrap();
+    config.history_mut().push(h);
+
+    println!(
+        "Created image with manifest: {}",
+        manifest.to_string_pretty().unwrap()
+    );
+
+    // Add the image manifest
+    let _descriptor = oci_dir
+        .insert_manifest_and_config(
+            manifest.clone(),
+            config,
+            Some("latest"),
+            Platform::default(),
+        )
+        .unwrap();
+
+    Ok(())
+}
+
+fn main() -> anyhow::Result<()> {
+    let args = std::env::args().collect::<Vec<_>>();
+    let ocidir = args[1].as_str();
+    let path = Path::new(args[2].as_str());
+    let Some(name) = path.file_stem().and_then(|v| v.to_str()) else {
+        anyhow::bail!("Invalid path: {path:?}");
+    };
+    let f = File::open(path).with_context(|| format!("Opening {path:?}"))?;
+
+    let dir = &cap_std::fs::Dir::open_ambient_dir(ocidir, cap_std::ambient_authority())
+        .with_context(|| format!("Opening {ocidir}"))?;
+    let oci_dir = OciDir::ensure(dir.try_clone()?)?;
+
+    import(&oci_dir, name, f)?;
+    Ok(())
+}