add a per-agent nix builds mode, host or none, carried like privateUsers

hive-c0re/src/attic/closure.rs

@@ -174,6 +174,7 @@ mod tests {
             built_unix: 1,
             cached,
             private_users: false,
+            nix_builds: hive_types::NixBuilds::Host,
         }
     }
 

hive-c0re/src/attic/probe.rs

@@ -246,6 +246,7 @@ mod tests {
             built_unix: 1,
             cached: true,
             private_users: false,
+            nix_builds: hive_types::NixBuilds::Host,
         }
     }
 

hive-c0re/src/job_queue/exec/tests.rs

@@ -275,6 +275,7 @@ fn closure(cached: bool) -> AgentClosure {
         built_unix: 1,
         cached,
         private_users: false,
+        nix_builds: hive_types::NixBuilds::Host,
     }
 }
 

hive-c0re/src/lifecycle/host_config.rs

@@ -7,7 +7,7 @@ use std::path::{Path, PathBuf};
 
 use anyhow::{Context, Result};
 use hive_priv_sock::BindMount;
-use hive_types::Permission;
+use hive_types::{NixBuilds, Permission};
 
 use crate::coordinator::{AgentPaths, HiveEnv};
 
@@ -451,9 +451,11 @@ async fn set_nspawn_flags(
         });
     }
 
-    // Whether the agent's config sets `privateUsers`, as the swarm builder
-    // recorded it. Read from the record, never from the agent's nix.
-    let private_users = private_users_of(&crate::attic::closure::record(agent_name).await);
+    // What the agent's config sets for `privateUsers` and `nixBuilds`, as the
+    // swarm builder recorded it. Read from the record, never from the agent's nix.
+    let record = crate::attic::closure::record(agent_name).await;
+    let private_users = private_users_of(&record);
+    let nix_builds = nix_builds_of(&record);
 
     // Web-socket subdir: bind-mount `/run/hive-agent/<name>/` into the
     // container so the harness can bind `web.sock` there and the host-side
@@ -487,6 +489,7 @@ async fn set_nspawn_flags(
         &load_creds,
         super::hive_facts(hive),
         private_users,
+        nix_builds,
     )
     .await
 }
@@ -529,6 +532,21 @@ fn private_users_of(
     }
 }
 
+/// What hive-priv is told about the agent's nix builds: the record's mode, or
+/// `None` (leave the agent as the last write set it) when there is no record or
+/// it cannot be read, so a swarm outage never turns an agent's host builds on.
+fn nix_builds_of(
+    record: &Result<Option<swarm_wire::agent_closures::AgentClosure>>,
+) -> Option<NixBuilds> {
+    match record {
+        Ok(record) => record.as_ref().map(|r| r.nix_builds),
+        Err(e) => {
+            tracing::warn!(error = %format!("{e:#}"), "closure record unreadable; the agent's nix builds are left as they are");
+            None
+        }
+    }
+}
+
 /// The binds every agent container gets, before its own state, config and
 /// role-specific dirs.
 fn base_binds(runtime_dir: &Path, claude_dir: &Path, claude_mount: String) -> Vec<BindMount> {
@@ -559,8 +577,16 @@ mod tests {
         stage_limits,
     };
     use crate::coordinator::HiveEnv;
+    use hive_types::NixBuilds;
 
     fn record_with(private_users: bool) -> swarm_wire::agent_closures::AgentClosure {
+        record_with_builds(private_users, NixBuilds::Host)
+    }
+
+    fn record_with_builds(
+        private_users: bool,
+        nix_builds: NixBuilds,
+    ) -> swarm_wire::agent_closures::AgentClosure {
         swarm_wire::agent_closures::AgentClosure {
             meta_rev: "a".repeat(40),
             store_path: "/nix/store/x".to_owned(),
@@ -568,6 +594,7 @@ mod tests {
             built_unix: 1,
             cached: true,
             private_users,
+            nix_builds,
         }
     }
 
@@ -609,6 +636,38 @@ mod tests {
         assert_eq!(super::private_users_of(&Ok(Some(old))), Some(false));
     }
 
+    /// The record's mode is what hive-priv is told, both ways. No record and an
+    /// unreadable one say nothing, so a swarm outage changes no agent; a record
+    /// from before the field reads as host builds.
+    #[test]
+    fn the_records_nix_builds_is_what_the_start_path_passes() {
+        for mode in [NixBuilds::Host, NixBuilds::Disabled] {
+            assert_eq!(
+                super::nix_builds_of(&Ok(Some(record_with_builds(false, mode)))),
+                Some(mode)
+            );
+        }
+        assert_eq!(super::nix_builds_of(&Ok(None)), None);
+        assert_eq!(super::nix_builds_of(&Err(anyhow::anyhow!("down"))), None);
+        let old = serde_json::json!({
+            "meta_rev": "a", "store_path": "/nix/store/x",
+            "system": "s", "built_unix": 1,
+        });
+        let old = serde_json::from_value(old).expect("decodes");
+        assert_eq!(super::nix_builds_of(&Ok(Some(old))), Some(NixBuilds::Host));
+        // A mode a newer swarm wrote, which this build does not know, decodes
+        // and builds nothing.
+        let newer = serde_json::json!({
+            "meta_rev": "a", "store_path": "/nix/store/x",
+            "system": "s", "built_unix": 1, "nix_builds": "container",
+        });
+        let newer = serde_json::from_value(newer).expect("decodes");
+        assert_eq!(
+            super::nix_builds_of(&Ok(Some(newer))),
+            Some(NixBuilds::Disabled)
+        );
+    }
+
     /// No agent container gets a cross-agent scratch dir; `/knowledge` is the
     /// control that the lookup finds a bind that is there.
     #[test]

hive-c0re/src/priv_client.rs

@@ -11,6 +11,7 @@ use hive_priv_sock::{
     BindMount, CredentialMount, HiveFacts, InfraAction, InfraContainer, MEASURE_AGENT_DISK_TIMEOUT,
     NetworkIsolation, PRIV_SOCK, PrivEvent, PrivRequest, PrivResponse, PrivStream,
 };
+use hive_types::NixBuilds;
 use std::os::fd::{AsRawFd as _, OwnedFd, RawFd};
 
 use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufReader, Interest};
@@ -320,6 +321,7 @@ pub async fn write_nspawn_flags(
     load_credentials: &[CredentialMount],
     hive_facts: HiveFacts,
     private_users: Option<bool>,
+    nix_builds: Option<NixBuilds>,
 ) -> Result<()> {
     ok(call(&PrivRequest::WriteNspawnFlags {
         container: container.to_owned(),
@@ -328,6 +330,7 @@ pub async fn write_nspawn_flags(
         load_credentials: load_credentials.to_vec(),
         hive_facts: Box::new(hive_facts),
         private_users,
+        nix_builds,
     })
     .await?)
 }