seed a hyperhive/hyperhive mirror so update_meta finds it on a fresh swarm

swarm-controller/src/meta_flake.rs

@@ -40,6 +40,15 @@ const REV_FILE: &str = "/etc/hyperhive/swarm-controller.json";
 const HYPERHIVE_OWNER: &str = "hyperhive";
 const HYPERHIVE_REPO: &str = "hyperhive";
 
+/// The error for a swarm forge with no readable hyperhive repo, naming the nix
+/// option that mirrors one there.
+fn missing_hyperhive() -> String {
+    format!(
+        "{HYPERHIVE_OWNER}/{HYPERHIVE_REPO} is empty or absent on the swarm forge; \
+         services.hyperhive.deploy.forgejo.hyperhiveUpstream mirrors it there"
+    )
+}
+
 /// What every agent of the swarm gets, the same on every hive.
 #[derive(Debug, Deserialize)]
 #[serde(deny_unknown_fields)]
@@ -365,8 +374,9 @@ async fn hyperhive_pin(
     let Some(flake) = committed_flake else {
         let head = forge
             .head_files(HYPERHIVE_OWNER, HYPERHIVE_REPO, None, &[])
-            .await?
-            .with_context(|| format!("{HYPERHIVE_OWNER}/{HYPERHIVE_REPO} is empty or absent"))?;
+            .await
+            .with_context(missing_hyperhive)?
+            .with_context(missing_hyperhive)?;
         return Ok(HyperhivePin {
             rev: head.commit,
             moved: false,
@@ -841,6 +851,29 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn a_forge_without_hyperhive_names_the_option_that_mirrors_it() {
+        let mut forge = forge_with(&[]);
+        forge.repos.remove("hyperhive/hyperhive");
+        let err = update(
+            &forge,
+            &FakeLocker::locked(),
+            &settings(),
+            None,
+            &argus_and_reviewer(),
+            &[],
+        )
+        .await
+        .unwrap_err();
+        let err = format!("{err:#}");
+        assert!(
+            err.contains("hyperhive/hyperhive is empty or absent")
+                && err.contains("services.hyperhive.deploy.forgejo.hyperhiveUpstream"),
+            "{err}"
+        );
+        assert!(forge.commits.lock().unwrap().is_empty());
+    }
+
     #[tokio::test]
     async fn a_refused_lock_commits_nothing() {
         let forge = forge_with(&[]);