refactor(#2754): make the container weights Option, not a 0 sentinel

Encoding "not configured" as weight 0 worked (the writer omitted the
line) but the type lied: 0 is not a legal cgroup v2 weight, and every
reader had to know the sentinel. Use Option<u32> end to end instead —
wire type, priv_client, HiveEnv, drop-in writer — so "unset" is a state
of the type rather than a magic value.

The nix options become nullOr, keeping their default of 80; null now
expresses "leave the setting out of the drop-in entirely" declaratively,
which is the useful shape on a host whose IO scheduler ignores
io.weight anyway.

Backward compat is unchanged: the fields stay #[serde(default)], so a
request from an older hive-c0re deserialises to None and reproduces the
pre-weights drop-in byte for byte. The test that pins that now passes
None instead of 0.

hive-c0re/src/coordinator.rs

@@ -87,12 +87,14 @@ pub struct Coordinator {
     pub agent_memory_max: String,
     /// Per-agent systemd `CPUWeight=` (cgroup v2 `cpu.weight`, 1..=10000).
     /// Same drop-in. A *relative share* under contention, not a cap — see
-    /// the `agentCpuWeight` NixOS option for the full semantics.
+    /// the `agentCpuWeight` NixOS option for the full semantics. `None`
-    pub agent_cpu_weight: u32,
+    /// (option set to `null`) omits the setting: kernel default, and the
+    /// drop-in is byte-identical to the one written before weights existed.
+    pub agent_cpu_weight: Option<u32>,
     /// Per-agent systemd `IOWeight=` (cgroup v2 `io.weight`, 1..=10000).
-    /// Same drop-in, same relative-share semantics as
+    /// Same drop-in, same relative-share and `None` semantics as
     /// [`Self::agent_cpu_weight`].
-    pub agent_io_weight: u32,
+    pub agent_io_weight: Option<u32>,
     /// Operator-tunable model→price table backing the hive-wide cost
     /// estimate on the ST4TS tab. Set via `services.hyperhive.modelPrices`
     /// and passed to `hive-c0re serve --model-prices <json>`. Models not
@@ -220,10 +222,11 @@ pub struct HiveEnv {
     pub agent_memory_max: String,
     /// Per-agent systemd `CPUWeight=` — cgroup v2 `cpu.weight`, 1..=10000.
     /// Relative share under contention, not a cap; see `agentCpuWeight`.
-    pub agent_cpu_weight: u32,
+    /// `None` = leave the setting out of the drop-in (kernel default).
+    pub agent_cpu_weight: Option<u32>,
     /// Per-agent systemd `IOWeight=` — cgroup v2 `io.weight`, 1..=10000.
     /// Same semantics as [`Self::agent_cpu_weight`]; see `agentIoWeight`.
-    pub agent_io_weight: u32,
+    pub agent_io_weight: Option<u32>,
 }
 
 impl Default for HiveEnv {
@@ -244,8 +247,8 @@ impl Default for HiveEnv {
             // Slightly below the kernel default of 100, so agent
             // containers yield to host services + the infra containers
             // (which carry no drop-in and stay at 100) under contention.
-            agent_cpu_weight: 80,
+            agent_cpu_weight: Some(80),
-            agent_io_weight: 80,
+            agent_io_weight: Some(80),
         }
     }
 }

hive-c0re/src/lifecycle/host_config.rs

@@ -45,12 +45,14 @@ pub async fn write_dropins(name: &str, hive: &HiveEnv, paths: &AgentPaths) -> Re
 /// The weights are hive-wide (`services.hyperhive.agentCpuWeight` /
 /// `agentIoWeight`) — unlike the caps they have no per-agent override in
 /// `meta/resource-limits.json`, so they come straight off [`HiveEnv`].
+/// `None` (the nix option set to `null`) means the weight line is left out
+/// and the container keeps the kernel default.
 async fn set_resource_limits(
     container: &str,
     cpu_quota: &str,
     memory_max: &str,
-    cpu_weight: u32,
+    cpu_weight: Option<u32>,
-    io_weight: u32,
+    io_weight: Option<u32>,
 ) -> Result<()> {
     crate::priv_client::write_resource_limits(
         container, memory_max, cpu_quota, cpu_weight, io_weight,

hive-c0re/src/main.rs

@@ -197,11 +197,15 @@ async fn main() -> Result<()> {
             if let Some(v) = agent_memory_max {
                 sc.env.agent_memory_max = v;
             }
+            // Passing the flag sets a weight; omitting it keeps whatever the
+            // config file says (including `null` = don't emit the setting).
+            // There's deliberately no flag spelling for "clear it" — that's
+            // what the nix option's `null` is for.
             if let Some(v) = agent_cpu_weight {
-                sc.env.agent_cpu_weight = v;
+                sc.env.agent_cpu_weight = Some(v);
             }
             if let Some(v) = agent_io_weight {
-                sc.env.agent_io_weight = v;
+                sc.env.agent_io_weight = Some(v);
             }
             if let Some(v) = model_prices {
                 sc.model_prices =

hive-c0re/src/priv_client.rs

@@ -174,8 +174,8 @@ pub async fn write_resource_limits(
     container: &str,
     memory_max: &str,
     cpu_quota: &str,
-    cpu_weight: u32,
+    cpu_weight: Option<u32>,
-    io_weight: u32,
+    io_weight: Option<u32>,
 ) -> Result<()> {
     ok(call(&PrivRequest::WriteResourceLimits {
         container: container.to_owned(),

hive-priv-sock/src/lib.rs

@@ -343,15 +343,17 @@ pub enum PrivRequest {
         container: String,
         memory_max: String,
         cpu_quota: String,
-        /// cgroup v2 `cpu.weight`, 1..=10000. `#[serde(default)]` so a
+        /// cgroup v2 `cpu.weight`, 1..=10000. `None` means "not
-        /// hive-priv built before this field still deserialises new
+        /// configured" — the writer omits the line entirely, leaving the
-        /// requests; 0 is treated as "omit the line" by the writer.
+        /// kernel default. `#[serde(default)]` so a request from a
+        /// hive-c0re built before this field existed deserialises to
+        /// `None` and reproduces the pre-weights drop-in.
         #[serde(default)]
-        cpu_weight: u32,
+        cpu_weight: Option<u32>,
-        /// cgroup v2 `io.weight`, 1..=10000. Same default/omit rule as
+        /// cgroup v2 `io.weight`, 1..=10000. Same `None` = omit rule as
         /// `cpu_weight`.
         #[serde(default)]
-        io_weight: u32,
+        io_weight: Option<u32>,
     },
 
     /// Remove `/run/systemd/system/container@<container>.service.d/` if present.

hive-priv/src/main.rs

@@ -537,8 +537,8 @@ fn chmod_socket_dir(agent_name: &str, mode: u32) -> Result<(String, String)> {
 /// `CPUQuota=` are hard caps that throttle even on an idle host;
 /// `CPUWeight=` / `IOWeight=` are cgroup v2 relative shares that only
 /// decide who yields *under contention*. A zero weight means "not
-/// configured" and omits the line, so a hive-c0re built before the weights
+/// configured" (`None`) and omits the line, so a hive-c0re built before the
-/// existed keeps producing the old two-line drop-in.
+/// weights existed keeps producing the old two-line drop-in.
 ///
 /// The condition causes systemd to *skip* (not *fail*) the unit when the
 /// bind-mount source dir is absent — result is `condition`, which does not
@@ -550,8 +550,8 @@ fn write_resource_limits(
     container: &str,
     memory_max: &str,
     cpu_quota: &str,
-    cpu_weight: u32,
+    cpu_weight: Option<u32>,
-    io_weight: u32,
+    io_weight: Option<u32>,
 ) -> Result<(String, String)> {
     validate_container_system_name(container)?;
     // Derive the logical agent name (strip h- prefix) to form the runtime
@@ -572,29 +572,21 @@ fn write_resource_limits(
 /// `[Unit]`: the condition is checked at start time — it skips (not fails)
 /// the unit when the MCP socket dir is absent, avoiding restart loops.
 /// `[Service]`: the hard caps first, then the relative weights. A weight of
-/// `0` means "not configured" and omits its line entirely, so a request from
+/// `None` means "not configured" and omits its line entirely, so a request
-/// a hive-c0re built before the weights existed reproduces the old
+/// from a hive-c0re built before the weights existed — or one whose nix
-/// two-setting drop-in byte for byte.
+/// option is `null` — reproduces the old two-setting drop-in byte for byte.
 fn limits_dropin_body(
     runtime_dir: &str,
     memory_max: &str,
     cpu_quota: &str,
-    cpu_weight: u32,
+    cpu_weight: Option<u32>,
-    io_weight: u32,
+    io_weight: Option<u32>,
 ) -> String {
     // Built as two possibly-empty lines rather than pushed onto the
     // string: `format!` appended to a `String` trips clippy::pedantic's
     // `format_push_string`, and a `write!` would need an unwrap.
-    let cpu_weight_line = if cpu_weight > 0 {
+    let cpu_weight_line = cpu_weight.map_or_else(String::new, |w| format!("CPUWeight={w}\n"));
-        format!("CPUWeight={cpu_weight}\n")
+    let io_weight_line = io_weight.map_or_else(String::new, |w| format!("IOWeight={w}\n"));
-    } else {
-        String::new()
-    };
-    let io_weight_line = if io_weight > 0 {
-        format!("IOWeight={io_weight}\n")
-    } else {
-        String::new()
-    };
     format!(
         "[Unit]\n\
          ConditionPathIsDirectory={runtime_dir}\n\
@@ -2231,13 +2223,13 @@ mod tests {
     use std::path::PathBuf;
     use std::sync::atomic::{AtomicU32, Ordering};
 
-    /// A zero weight is "not configured": the drop-in must come out
+    /// An unset weight is "not configured": the drop-in must come out
     /// byte-identical to the pre-weights two-setting body, so a hive-c0re
     /// older than this field can't change what lands on disk.
     #[test]
-    fn zero_weights_reproduce_the_pre_weights_dropin() {
+    fn unset_weights_reproduce_the_pre_weights_dropin() {
         assert_eq!(
-            limits_dropin_body("/run/hyperhive/agents/iris", "4G", "200%", 0, 0),
+            limits_dropin_body("/run/hyperhive/agents/iris", "4G", "200%", None, None),
             "[Unit]\n\
              ConditionPathIsDirectory=/run/hyperhive/agents/iris\n\
              \n\
@@ -2248,23 +2240,23 @@ mod tests {
     }
 
     /// Weights are appended to the `[Service]` section, each omitted
-    /// independently when zero.
+    /// independently when `None`.
     #[test]
     fn weights_are_emitted_only_when_set() {
-        let both = limits_dropin_body("/rt/x", "4G", "200%", 80, 80);
+        let both = limits_dropin_body("/rt/x", "4G", "200%", Some(80), Some(80));
         assert!(
             both.ends_with("CPUQuota=200%\nCPUWeight=80\nIOWeight=80\n"),
             "{both}"
         );
 
-        let cpu_only = limits_dropin_body("/rt/x", "4G", "200%", 80, 0);
+        let cpu_only = limits_dropin_body("/rt/x", "4G", "200%", Some(80), None);
         assert!(
             cpu_only.ends_with("CPUQuota=200%\nCPUWeight=80\n"),
             "{cpu_only}"
         );
         assert!(!cpu_only.contains("IOWeight"), "{cpu_only}");
 
-        let io_only = limits_dropin_body("/rt/x", "4G", "200%", 0, 80);
+        let io_only = limits_dropin_body("/rt/x", "4G", "200%", None, Some(80));
         assert!(
             io_only.ends_with("CPUQuota=200%\nIOWeight=80\n"),
             "{io_only}"

nix/host-modules/hive-c0re/options.nix

@@ -302,9 +302,9 @@
     };
 
     agentCpuWeight = lib.mkOption {
-      type = lib.types.ints.between 1 10000;
+      type = lib.types.nullOr (lib.types.ints.between 1 10000);
       default = 80;
-      example = 50;
+      example = null;
       description = ''
         systemd `CPUWeight=` applied to every agent container via the
         same drop-in as `agentCpuQuota`. This is the cgroup v2
@@ -321,13 +321,17 @@
         `hive-matrix`), which stay at `100`. Note this is a hive-wide
         value, so it does not rank agents against *each other*: they all
         share one weight.
+
+        Set to `null` to leave `CPUWeight=` out of the drop-in entirely —
+        the container then inherits the kernel default and the generated
+        unit file is identical to one from before this option existed.
       '';
     };
 
     agentIoWeight = lib.mkOption {
-      type = lib.types.ints.between 1 10000;
+      type = lib.types.nullOr (lib.types.ints.between 1 10000);
       default = 80;
-      example = 50;
+      example = null;
       description = ''
         systemd `IOWeight=` applied to every agent container via the
         same drop-in as `agentCpuQuota` — the block-IO counterpart of
@@ -340,7 +344,8 @@
         scheduler. On a host running `none`/`mq-deadline`/`kyber` without
         iocost QoS configured, systemd writes the value and the kernel
         ignores it — harmless, but it will measure as a no-op. Check with
-        `cat /sys/fs/cgroup/io.cost.qos` on the host.
+        `cat /sys/fs/cgroup/io.cost.qos` on the host, and set this to
+        `null` to omit the setting rather than write one nothing reads.
       '';
     };