swarm: refuse swarm.hives keys that are not valid hive identifiers

A hive key becomes a certificate subject and a directory name, but only the
reserved-name and fragment guards looked at it, so `a/b` or `../x` evaluated
and misbehaved later. Assert at eval time that every key is 1-63 characters
of [a-z0-9-], the rule `Ident` applies in hive-types, and name the offending
keys in the message.

nix/host-modules/lib/name-guards.nix

@@ -63,4 +63,27 @@ in
       problem = "has name(s) containing a reserved word";
       forbiddenLabel = "Forbidden as substrings";
     };
+
+  # Every name must be a valid hive `Ident` (hive-types/src/lib.rs): 1-63
+  # characters of `[a-z0-9-]`, with no leading-character rule. A hive name is
+  # spliced into certificate subjects and filesystem paths, where `/` and `..`
+  # change meaning.
+  mustBeIdent =
+    { option, names }:
+    let
+      offenders = lib.filter (n: builtins.match "[a-z0-9-]+" n == null || lib.stringLength n > 63) names;
+    in
+    {
+      assertion = offenders == [ ];
+      message = ''
+        ${option} has name(s) that are not valid identifiers: ${
+          lib.concatMapStringsSep ", " (x: "'${x}'") offenders
+        }
+        Required: 1-63 characters, each one of [a-z0-9-]
+
+        A hive name becomes a certificate subject and a directory name, so a
+        `/` or `..` in it names something else. `Ident` in
+        hive-types/src/lib.rs applies the same rule. Rename the hive.
+      '';
+    };
 }

nix/host-modules/swarm.nix

@@ -369,6 +369,10 @@ in
           designed for it.
         '';
       }
+      (nameGuards.mustBeIdent {
+        option = "services.hyperhive.swarm.hives";
+        names = hiveNames;
+      })
       (nameGuards.mustNotEqual {
         option = "services.hyperhive.swarm.hives";
         names = hiveNames;

nix/module-eval/name-guards.nix

@@ -126,7 +126,38 @@ let
     swarm.hives."h1-agent".domain = "a.t.local";
   };
 
-  # Markers from `lib/name-guards.nix`'s two `problem` strings. Matching the
+  # The key characters are checked by the same module, so these run with the
+  # collector off too. `a/b` adds a certificate-subject component; `../x`
+  # leaves a directory named after the hive.
+  hiveNameWithSlash = hive {
+    deploy.swarm-otel.enable = false;
+    swarm.hives."a/b".domain = "a.t.local";
+  };
+
+  hiveNameWithParentDir = hive {
+    deploy.swarm-otel.enable = false;
+    swarm.hives."../x".domain = "x.t.local";
+  };
+
+  # Both sides of the length cap: 63 characters passes, 64 does not.
+  hiveNameAtLengthCap = hive {
+    deploy.swarm-otel.enable = false;
+    swarm.hives.${lib.concatStrings (lib.replicate 63 "a")}.domain = "l.t.local";
+  };
+
+  hiveNameOverLengthCap = hive {
+    deploy.swarm-otel.enable = false;
+    swarm.hives.${lib.concatStrings (lib.replicate 64 "a")}.domain = "l.t.local";
+  };
+
+  # Digits and hyphens in every position: the rule has no leading-character
+  # restriction.
+  hiveNameWithDigitsAndHyphens = hive {
+    deploy.swarm-otel.enable = false;
+    swarm.hives."0-h-9".domain = "h.t.local";
+  };
+
+  # Markers from `lib/name-guards.nix`'s `problem` strings. Matching the
   # problem rather than the `why` prose keeps the messages rewordable.
   equalityGuardFired =
     h: lib.any (a: !a.assertion && lib.hasInfix "has reserved name(s)" a.message) h.assertions;
@@ -136,7 +167,39 @@ let
     lib.any (
       a: !a.assertion && lib.hasInfix "has name(s) containing a reserved word" a.message
     ) h.assertions;
+
+  # True when the identifier guard refuses `key`.
+  identGuardRefuses =
+    h: key:
+    lib.any (
+      a:
+      !a.assertion && lib.hasInfix "not valid identifiers" a.message && lib.hasInfix "'${key}'" a.message
+    ) h.assertions;
+
+  identGuardFired =
+    h: lib.any (a: !a.assertion && lib.hasInfix "not valid identifiers" a.message) h.assertions;
   cases = [
+    {
+      name = "a hive key containing a slash is refused, naming the key";
+      ok = identGuardRefuses hiveNameWithSlash "a/b";
+    }
+    {
+      name = "a hive key that climbs out of its directory is refused, naming the key";
+      ok = identGuardRefuses hiveNameWithParentDir "../x";
+    }
+    {
+      name = "a hive key over 63 characters is refused";
+      ok = identGuardFired hiveNameOverLengthCap;
+    }
+    {
+      # Controls: without them an identifier guard that refuses everything
+      # passes the three cases above.
+      name = "63-character and digit-and-hyphen hive keys pass the identifier guard";
+      ok =
+        !(identGuardFired hiveNameAtLengthCap)
+        && !(identGuardFired hiveNameWithDigitsAndHyphens)
+        && !(identGuardFired hiveNamesAllLegal);
+    }
     {
       # `ctl` is in no deny list — it is reserved *because it is the subject a
       # cert-auth role accepts*, which is a value an operator sets, so a