various: report fs-verity IDs as command results

When creating stream or images, report the fs-verity of the resulting
object:

  - as a return value from the function on `Repository`
  - via stdout for commands run from `cfsctl`

Closes #4

src/bin/cfsctl.rs

@@ -90,24 +90,27 @@ fn main() -> Result<()> {
             }
         },
         Command::Cat { name } => {
-            repo.merge_splitstream(&name, &mut std::io::stdout())
+            repo.merge_splitstream(&name, &mut std::io::stdout())?;
         },
         Command::ImportImage { reference, } => {
-            repo.import_image(&reference, &mut std::io::stdin())
+            let image_id = repo.import_image(&reference, &mut std::io::stdin())?;
+            println!("{}", hex::encode(image_id));
         },
         Command::Oci{ cmd: oci_cmd } => match oci_cmd {
             OciCommand::ImportLayer { name } => {
-                oci::import_layer(&repo, &name, &mut std::io::stdin())
+                let stream_id = oci::import_layer(&repo, &name, &mut std::io::stdin())?;
+                println!("{}", hex::encode(stream_id));
             },
             OciCommand::LsLayer { name } => {
-                oci::ls_layer(&repo, &name)
+                oci::ls_layer(&repo, &name)?;
             },
         }
         Command::Mount { name, mountpoint } => {
-            repo.mount(&name, &mountpoint)
+            repo.mount(&name, &mountpoint)?;
         },
         Command::GC => {
-            repo.gc()
+            repo.gc()?;
         }
     }
+    Ok(())
 }

src/oci/mod.rs

@@ -9,7 +9,7 @@ use crate::{
     repository::Repository
 };
 
-pub fn import_layer<R: Read>(repo: &Repository, name: &str, tar_stream: &mut R) -> Result<()> {
+pub fn import_layer<R: Read>(repo: &Repository, name: &str, tar_stream: &mut R) -> Result<Sha256HashValue> {
     let mut split_stream = zstd::stream::write::Encoder::new(vec![], 0)?;
 
     tar::split(

src/repository.rs

@@ -197,7 +197,7 @@ impl Repository {
     }
 
     /// this function is not safe for untrusted users
-    pub fn import_image<R: Read>(&self, name: &str, image: &mut R) -> Result<()> {
+    pub fn import_image<R: Read>(&self, name: &str, image: &mut R) -> Result<Sha256HashValue> {
         let mut data = vec![];
         image.read_to_end(&mut data)?;
         let object_id = self.ensure_object(&data)?;
@@ -212,14 +212,14 @@ impl Repository {
 
     pub fn link_ref(
         &self, name: &str, category: &str, object_id: Sha256HashValue
-    ) -> Result<()> {
+    ) -> Result<Sha256HashValue> {
         let object_path = format!("objects/{:02x}/{}", object_id[0], hex::encode(&object_id[1..]));
         let category_path = format!("{}/{}", category, hex::encode(object_id));
         let ref_path = format!("{}/refs/{}", category, name);
 
         self.symlink(&ref_path, &category_path)?;
         self.symlink(&category_path, &object_path)?;
-        Ok(())
+        Ok(object_id)
     }
 
     fn symlink<P: AsRef<Path>>(&self, name: P, target: &str) -> Result<()> {