swarm-builder: give the check flake the hive's docs source

The builder's hyperhive input is the docs-stripped flake source, so an
agent config with docs.enable = true failed to evaluate: docs.source
defaults to reference-docs, built from the missing docs/. Pass the host's
c0re.hyperhiveDocs as a hyperhive-docs input and set docs.source from it,
as the hive's meta flake does.

Fixes #5411

docs/swarm/builder.md

@@ -26,6 +26,10 @@ Where it runs and how it's enabled: [`services.md`](services.md#builder).
    - `hyperhive` and `nixpkgs`: the sources the builder's own host was
      deployed with, the same two a hive on that host renders into its meta
      flake (`c0re.hyperhiveFlake`, `c0re.nixpkgsFlake`).
+   - `hyperhive-docs`: the `docs/` source a hive on that host renders into its
+     meta flake (`c0re.hyperhiveDocs`). The check sets
+     `services.hyperhive.agent.docs.source` from it, as the meta flake does,
+     since the `hyperhive` source has no `docs/`.
    - `agent`: `git+https://<forge>/agent-configs/<agent>?ref=refs/pull/<pr>/head&rev=<head>`.
      A build has one hour.
 4. The reply is the system closure's store path, or the last lines `nix`
@@ -48,7 +52,7 @@ Where it runs and how it's enabled: [`services.md`](services.md#builder).
 
 - It composes the config the way a hive's meta flake does
   (`agent-base.extendModules` with the config's `nixosModules.default`), minus
-  what a hive adds: its CA bundle, docs source, `claude-code` path, ports and
+  what a hive adds: its CA bundle, `claude-code` path, ports and
   OTEL settings, and the `follows` that point the config's own inputs at the
   hive's. A green check can therefore still fail on a hive, and the reverse.
 - The agent base is the builder's host's, not the hive's. Hives on another

nix/builder/check-flake/flake.nix

@@ -1,6 +1,8 @@
 # The swarm builder's build check: one agent config on hyperhive's agent base,
 # composed the way a hive's meta flake composes it, minus the values a hive
-# adds (its CA, docs source, claude-code path and ports).
+# adds (its CA, claude-code path and ports). The docs source is set the way the
+# meta flake sets it: `hyperhive`'s source has no `docs/`, so the default
+# `docs.source` fails to evaluate.
 #
 # Every input is replaced at build time with `--override-input`
 # (swarm-builder/src/nix_build.rs), so the URLs below are never fetched and
@@ -12,15 +14,25 @@
     nixpkgs.url = "flake:nixpkgs";
     hyperhive.url = "flake:hyperhive";
     hyperhive.inputs.nixpkgs.follows = "nixpkgs";
+    hyperhive-docs.url = "flake:hyperhive-docs";
+    hyperhive-docs.flake = false;
     agent.url = "flake:agent";
   };
 
   outputs =
-    { hyperhive, agent, ... }:
+    {
+      hyperhive,
+      hyperhive-docs,
+      agent,
+      ...
+    }:
     {
       packages.x86_64-linux.check =
         (hyperhive.nixosConfigurations.agent-base.extendModules {
-          modules = [ agent.nixosModules.default ];
+          modules = [
+            agent.nixosModules.default
+            { services.hyperhive.agent.docs.source = hyperhive-docs.outPath; }
+          ];
         }).config.system.build.toplevel;
     };
 }

nix/host-modules/swarm-builder.nix

@@ -89,10 +89,11 @@ let
   # The `nix` the host daemon runs, so client and daemon speak one protocol.
   nix = config.nix.package;
 
-  # Strings, not paths: both are already store paths, and the flake reference
+  # Strings, not paths: all three are already store paths, and the flake
-  # the daemon passes must be the same one a hive's meta flake names.
+  # references the daemon passes must be the ones a hive's meta flake names.
   hyperhiveFlake = hyperhiveCfg.c0re.hyperhiveFlake;
   nixpkgsFlake = hyperhiveCfg.c0re.nixpkgsFlake;
+  docsFlake = hyperhiveCfg.c0re.hyperhiveDocs;
 
   privateNetwork = true;
 in
@@ -253,6 +254,7 @@ in
               SWARM_BUILDER_CHECK_FLAKE = "${../builder/check-flake}";
               SWARM_BUILDER_HYPERHIVE_FLAKE = hyperhiveFlake;
               SWARM_BUILDER_NIXPKGS_FLAKE = nixpkgsFlake;
+              SWARM_BUILDER_HYPERHIVE_DOCS_FLAKE = docsFlake;
               SWARM_BUILDER_FORGE_URL = "https://${forgeCfg.domain}";
               SWARM_BUILDER_CONFIG_ORG = "agent-configs";
             }

nix/module-eval/builder.nix

@@ -49,6 +49,30 @@ let
   svc = inner.systemd.services.swarm-builder;
   env = svc.environment;
 
+  # The check flake on the inputs the builder's unit names, called as
+  # `nix build` calls it. `hyperhive` is entered through the filtered source's
+  # own `flake.nix`, so its agent base sees the tree without `docs/` that a
+  # real build gets; `self`'s own agent base would find `docs/` here.
+  builderCheck =
+    agentModule:
+    let
+      src = env.SWARM_BUILDER_HYPERHIVE_FLAKE;
+      hyperhive = (import "${src}/flake.nix").outputs (
+        self.inputs
+        // {
+          self = hyperhive // {
+            outPath = src;
+            inherit (self) inputs;
+          };
+        }
+      );
+    in
+    ((import "${env.SWARM_BUILDER_CHECK_FLAKE}/flake.nix").outputs {
+      inherit hyperhive;
+      hyperhive-docs.outPath = env.SWARM_BUILDER_HYPERHIVE_DOCS_FLAKE;
+      agent.nixosModules.default = agentModule;
+    }).packages.x86_64-linux.check;
+
   cases = [
     {
       name = "the swarm builder's container exists only where its host enables it";
@@ -113,9 +137,10 @@ let
     {
       # The check builds on the sources a hive on this host would render into
       # its meta flake, so a green check means the same base.
-      name = "the builder builds on the hyperhive and nixpkgs this host's hives use";
+      name = "the builder builds on the hyperhive, docs and nixpkgs this host's hives use";
       ok =
         env.SWARM_BUILDER_HYPERHIVE_FLAKE == withBuilder.services.hyperhive.c0re.hyperhiveFlake
+        && env.SWARM_BUILDER_HYPERHIVE_DOCS_FLAKE == withBuilder.services.hyperhive.c0re.hyperhiveDocs
         && env.SWARM_BUILDER_NIXPKGS_FLAKE == withBuilder.services.hyperhive.c0re.nixpkgsFlake
         && lib.hasSuffix "-check-flake" env.SWARM_BUILDER_CHECK_FLAKE
         && env.SWARM_BUILDER_CONFIG_ORG == "agent-configs"
@@ -150,6 +175,18 @@ let
         && lib.elem "execution" reviewer
         && !(lib.elem "meta" reviewer);
     }
+    {
+      # `docs.enable` reads `docs.source`, whose default builds from the
+      # `docs/` the `hyperhive` source has none of; an eval error there fails
+      # this whole group.
+      name = "the builder's check evaluates a config with the reference docs on";
+      ok =
+        lib.hasSuffix ".drv"
+          (builderCheck {
+            services.hyperhive.agent.user.name = "a1";
+            services.hyperhive.agent.docs.enable = true;
+          }).drvPath;
+    }
     {
       # The responder denies a client id it does not know, and a NATS denial
       # arrives as a timeout.

swarm-builder/src/main.rs

@@ -49,6 +49,7 @@ async fn main() -> Result<()> {
         check_flake: var("CHECK_FLAKE")?,
         hyperhive: var("HYPERHIVE_FLAKE")?,
         nixpkgs: var("NIXPKGS_FLAKE")?,
+        hyperhive_docs: var("HYPERHIVE_DOCS_FLAKE")?,
         forge_url: forge_url()?,
         config_org: var("CONFIG_ORG")?,
         deadline: BUILD_DEADLINE,

swarm-builder/src/nix_build.rs

@@ -1,5 +1,6 @@
-//! One build: `nix build` of the check flake with its three inputs pointed at
+//! One build: `nix build` of the check flake with its four inputs pointed at
-//! this host's hyperhive and nixpkgs and at the agent's config commit.
+//! this host's hyperhive, hyperhive docs and nixpkgs and at the agent's config
+//! commit.
 //!
 //! The config repo is private, so `git` fetches it with the builder's forge
 //! token. The token reaches `git` through a credential helper that reads it
@@ -44,6 +45,8 @@ pub struct Builder {
     pub hyperhive: String,
     /// The nixpkgs source this host was deployed from.
     pub nixpkgs: String,
+    /// The hyperhive `docs/` source this host was deployed from.
+    pub hyperhive_docs: String,
     /// `https://<forge>`, with no trailing slash and no path.
     pub forge_url: String,
     /// The org the config repos are in.
@@ -99,6 +102,9 @@ impl Builder {
             "nixpkgs".to_owned(),
             self.nixpkgs.clone(),
             "--override-input".to_owned(),
+            "hyperhive-docs".to_owned(),
+            self.hyperhive_docs.clone(),
+            "--override-input".to_owned(),
             "agent".to_owned(),
             self.agent_url(job),
         ])
@@ -302,6 +308,7 @@ mod tests {
             check_flake: "/nix/store/c-check".to_owned(),
             hyperhive: "/nix/store/h-source".to_owned(),
             nixpkgs: "/nix/store/n-source".to_owned(),
+            hyperhive_docs: "/nix/store/d-docs-source".to_owned(),
             forge_url: "https://forge.example".to_owned(),
             config_org: "agent-configs".to_owned(),
             nix: PathBuf::from("nix"),
@@ -326,7 +333,14 @@ mod tests {
             .filter(|w| w[0] == "--override-input")
             .map(|w| w[1].as_str())
             .collect();
-        assert_eq!(overridden, ["hyperhive", "nixpkgs", "agent"]);
+        assert_eq!(
+            overridden,
+            ["hyperhive", "nixpkgs", "hyperhive-docs", "agent"]
+        );
+        assert!(
+            args.windows(2)
+                .any(|w| w[0] == "hyperhive-docs" && w[1] == "/nix/store/d-docs-source")
+        );
         assert!(args.iter().any(|a| a == "/nix/store/c-check#check"));
         assert!(args.iter().any(|a| a == "--no-write-lock-file"));
     }