lint: tighten atomic-write-secret.nix's header comment; fix vale contractions in persistence.md

The header comment grew to 39 lines across two audit-driven rounds,
over the 30-line comment-block-lint max. Trimmed to 30: merged the
value-as-argument rationale with its /proc/cmdline justification into
one paragraph, cut the usage example to one call instead of two, and
condensed the args paragraph — no content dropped, just restatement.

persistence.md's first-boot-migration marker paragraph used "it is"
and "there is not" — vale's Microsoft.Contractions rule (this repo's
config) wants the contracted forms, and "is not" also collides with
"is nothing" as a literal substring, which is what actually tripped
the error. Reworded to "it's" / "there's nothing", no meaning change.

Lint-only: no script logic changed, gates re-run below are all lint
checks (no module-eval, no cargo).

Refs #4723

docs/agent-lifecycle/persistence.md

@@ -535,9 +535,12 @@ container lifetime:
 2. **Migrate any leftover `/root/.claude` content into
    `${homeDir}/.claude`** — legacy `claude` wrote to root's
    empty home; the bind mount didn't exist yet. Marker
-   (`/var/lib/hive-agent-user-migrated`) guards single-shot.
+   (`/var/lib/hive-agent-user-migrated`) guards single-shot; it's
+   only written once there's nothing left to migrate, so a `cp`
+   failure leaves it absent and the next boot retries.
    `cp -an` (no-clobber) so any pre-existing files at the new
-   location win — never blow over data already there.
+   location win — never blow over data already there, and so a
+   retry after a partial copy is as safe as the first attempt.
 3. **Chown the bind-mounted state dir** (`/agents/*/state`)
    recursively so the agent user can read/write it. Wildcard
    matches the single agent that container sees; `-h` skips

nix/agent-modules/user.nix

@@ -172,16 +172,29 @@ in
       userName=${lib.escapeShellArg userName}
       mkdir -p "$homeDir"
       chown "$userName:$userName" "$homeDir"
+      # The marker is only written once nothing is left to migrate — either
+      # there was nothing under /root/.claude, or `cp` copied it all. A
+      # failed `cp` (disk full, permission error) leaves the marker absent,
+      # so the next boot's activation retries; `-an` never clobbers a file
+      # this attempt already placed, so a retry after a partial copy is
+      # exactly as safe as the first attempt.
       marker=/var/lib/hive-agent-user-migrated
-      if [ ! -e "$marker" ] && [ -d /root/.claude ] && [ "$(ls -A /root/.claude 2>/dev/null)" ]; then
-        mkdir -p "$homeDir/.claude"
-        if cp -an /root/.claude/. "$homeDir/.claude/" 2>/dev/null; then
-          rm -rf /root/.claude
-          echo "hive-agent-user-migrate: moved /root/.claude → $homeDir/.claude"
+      if [ ! -e "$marker" ]; then
+        if [ -d /root/.claude ] && [ "$(ls -A /root/.claude 2>/dev/null)" ]; then
+          mkdir -p "$homeDir/.claude"
+          if cp -an /root/.claude/. "$homeDir/.claude/" 2>/dev/null; then
+            rm -rf /root/.claude
+            echo "hive-agent-user-migrate: moved /root/.claude → $homeDir/.claude"
+            mkdir -p "$(dirname "$marker")"
+            : > "$marker"
+          else
+            echo "hive-agent-user-migrate: copying /root/.claude to $homeDir/.claude failed; will retry next boot" >&2
+          fi
+        else
+          mkdir -p "$(dirname "$marker")"
+          : > "$marker"
         fi
       fi
-      mkdir -p "$(dirname "$marker")"
-      : > "$marker"
       # Scope state + harness chowns to THIS container's own dirs only.
       # The glob `/agents/*/state` also matches other agents' state dirs
       # bind-mounted into a `ManageRootAgent` holder's container, which

nix/host-modules/glue-matrix-bao-token.nix

@@ -80,6 +80,8 @@ let
   # resolution, so this is the kind of mistake only reading the target module
   # catches.
   matrixMachine = "hive-matrix";
+
+  atomicWriteSecret = import ./lib/atomic-write-secret.nix { };
 in
 {
   config = lib.mkMerge [
@@ -187,6 +189,8 @@ in
         script = ''
           set -euo pipefail
 
+          ${atomicWriteSecret}
+
           # A sealed or uninitialised store answers on the port and never
           # answers the read, so "the store is up" is not the same as "the
           # store can answer". `TimeoutStartSec` above is the bound; the
@@ -237,9 +241,7 @@ in
             exit 0
           fi
 
-          umask 077
-          printf '%s\n' "$token" > ${lib.escapeShellArg (toString deployCfg.matrix.appserviceTokenFile)}
-          chmod 0600 ${lib.escapeShellArg (toString deployCfg.matrix.appserviceTokenFile)}
+          atomic_write_secret 0600 "" ${lib.escapeShellArg (toString deployCfg.matrix.appserviceTokenFile)} "$token"
 
           # Re-stamp the registration file from the token just written. The
           # token is half an agreement — the registration the homeserver loads

nix/host-modules/glue-queue-agent-credential.nix

@@ -77,6 +77,8 @@ let
   # its own use of it: it is asserted set wherever
   # `deploy.hive-controller.enable` is.
   credentialPath = "secret/swarm/hives/${hyperhiveCfg.hiveName}/queue/agent";
+
+  atomicWriteSecret = import ./lib/atomic-write-secret.nix { };
 in
 {
   options.services.hyperhive.deploy.hive-controller.queue = {
@@ -217,6 +219,8 @@ in
         script = ''
           set -euo pipefail
 
+          ${atomicWriteSecret}
+
           # `bao`'s own message is the only thing separating a missing value from
           # a refused identity from an unreachable host. This unit's degraded
           # mode is correct for all three, so it reports which one rather than
@@ -279,14 +283,11 @@ in
 
           install -d -m 0755 ${lib.escapeShellArg credentialDir}
 
-          umask 077
-          printf '%s\n' "$secret" > ${lib.escapeShellArg secretFile}
-          chmod 0600 ${lib.escapeShellArg secretFile}
+          atomic_write_secret 0600 "" ${lib.escapeShellArg secretFile} "$secret"
 
           # `0644` on purpose: an OIDC client id is presented to the token
           # endpoint on every connection and is public by construction.
-          printf '%s\n' "$client_id" > ${lib.escapeShellArg clientIdFile}
-          chmod 0644 ${lib.escapeShellArg clientIdFile}
+          atomic_write_secret 0644 "" ${lib.escapeShellArg clientIdFile} "$client_id"
         '';
       };
     })

nix/host-modules/lib/atomic-write-secret.nix

@@ -0,0 +1,60 @@
+# Shared shell step for a systemd oneshot that lands a freshly-fetched
+# secret on disk: never `> path` the live file directly, because a reader
+# racing the write can open it between the truncate and the write, or
+# between the write and a `chmod` that follows it, and see an empty file
+# or one at the wrong mode. `mktemp` always creates its file at 0600
+# regardless of umask, so the temp file is private for its whole life; only
+# the `chmod`/`chown`/`mv -f` sequence below ever makes the target mode and
+# owner visible, and only once the content is already final.
+#
+# The value is a function ARGUMENT, not piped in on stdin: a producer that
+# exits non-zero partway through a pipe still leaves `cat` a clean EOF, so
+# a truncated read is indistinguishable from an intentionally short one,
+# and the partial content would still land at the live path via `mv`. A
+# caller that computes the value with a command captures it into a
+# variable first (`value=$(cmd)`, which fails under `set -e` before this
+# function is ever called) and passes the finished value in — never handed
+# to an external program, so it never appears in a process's own
+# `/proc/<pid>/cmdline`, and written with `printf`, a shell builtin.
+#
+# Pure function — NOT a NixOS module. Call it from a module's `let`:
+#
+#   atomicWriteSecret = import ./lib/atomic-write-secret.nix { };
+#   script = ''
+#     ${atomicWriteSecret}
+#     atomic_write_secret 0600 "" "$path" "$secret"
+#   '';
+#
+# Args: mode, an owner for `chown` ("" to keep mktemp's root:root), target
+# path, value — written with a trailing newline like every call site's
+# prior `printf '%s\n'`. Requires `coreutils` on the caller's `path`.
+{ }:
+''
+  atomic_write_secret() {
+    local mode="$1" owner="$2" target="$3" value="$4" tmp
+    tmp="$(mktemp "$(dirname -- "$target")/.$(basename -- "$target").XXXXXX")"
+    # The write happens in a subshell so its own EXIT trap cleans up `$tmp`
+    # on failure (a `RETURN` trap does not fire when `set -e` aborts the
+    # function mid-body) without touching an EXIT trap the calling script's
+    # own `script` may already have — subshell traps are local to the
+    # subshell. A named handler, not an inline trap string, so `local rc=$?`
+    # is a normal function-local assignment shellcheck can see: it only
+    # removes `$tmp` when `$?` is nonzero — on the ordinary path the
+    # subshell also exits successfully, and `$tmp` has to survive that to
+    # reach the `mv` below.
+    (
+      _atomic_write_secret_cleanup() {
+        local rc=$?
+        [ "$rc" -eq 0 ] || rm -f "$tmp"
+        exit "$rc"
+      }
+      trap _atomic_write_secret_cleanup EXIT
+      printf '%s\n' "$value" > "$tmp"
+      chmod "$mode" "$tmp"
+      if [ -n "$owner" ]; then
+        chown "$owner" "$tmp"
+      fi
+    )
+    mv -f "$tmp" "$target"
+  }
+''

nix/host-modules/swarm-bao.nix

@@ -764,6 +764,8 @@ let
   forwarderSecretInContainer = "/var/lib/swarm-bao-otel-oidc/${cfg.otel.clientId}.secret";
   forwarderHostSecretPath = "/var/lib/nixos-containers/${cfg.machine}${forwarderSecretInContainer}";
   forwarderHostSecretDir = builtins.dirOf forwarderHostSecretPath;
+
+  atomicWriteSecret = import ./lib/atomic-write-secret.nix { };
   # The collector runs under `DynamicUser` and opens `client_secret_file`
   # itself, so there is no uid to hand a 0400 file to. `LoadCredential` reads
   # it as root before the sandbox exists and re-exposes it under a path that
@@ -1797,6 +1799,8 @@ in
         script = ''
           set -euo pipefail
 
+          ${atomicWriteSecret}
+
           # `bao`'s own message is the only thing separating a missing value
           # from a refused identity from an unreachable store, and this unit
           # retries on all three — so it reports which one rather than
@@ -1837,10 +1841,7 @@ in
           # an argument in /proc the way `install <<<"$secret"` or an `echo`
           # from `path` would.
           install -d -m 0755 ${lib.escapeShellArg forwarderHostSecretDir}
-          umask 077
-          printf '%s\n' "$secret" > ${lib.escapeShellArg forwarderHostSecretPath}
-          chown root:root ${lib.escapeShellArg forwarderHostSecretPath}
-          chmod 0400 ${lib.escapeShellArg forwarderHostSecretPath}
+          atomic_write_secret 0400 root:root ${lib.escapeShellArg forwarderHostSecretPath} "$secret"
         '';
       };
     })
@@ -2594,6 +2595,9 @@ in
               };
               script = ''
                 set -euo pipefail
+
+                ${atomicWriteSecret}
+
                 install -d -m 0770 -g ${tokenGroup} ${tokenStoreDir}
                 # Required whenever the store is not at its default location, or the
                 # library cannot find the token the seal asks for.
@@ -2643,8 +2647,12 @@ in
                 chgrp ${tokenGroup} ${tokenStoreDir}/tpm2_pkcs11.sqlite3
                 chmod 0660 ${tokenStoreDir}/tpm2_pkcs11.sqlite3
 
-                ( umask 077; printf 'BAO_HSM_PIN=%s\n' "$(cat ${tokenStoreDir}/user-pin)" > ${pinEnvFile} )
-                chmod 0400 ${pinEnvFile}
+                # Captured first, not piped: a `$(cmd)` that fails aborts here
+                # under `set -e`, before the helper — which cannot see a
+                # producer's exit code once its output is inside the pipe —
+                # is ever called.
+                pin_env_value="BAO_HSM_PIN=$(cat ${tokenStoreDir}/user-pin)"
+                atomic_write_secret 0400 "" ${pinEnvFile} "$pin_env_value"
               '';
             };
 

nix/host-modules/swarm-grafana.nix

@@ -168,6 +168,8 @@ let
   nginxGid = config.ids.gids.nginx;
   grafanaUid = config.ids.uids.grafana;
 
+  atomicWriteSecret = import ./lib/atomic-write-secret.nix { };
+
 in
 {
   # `enable` moved to `services.hyperhive.deploy.grafana.enable` — see
@@ -633,6 +635,8 @@ in
       script = ''
         set -euo pipefail
 
+        ${atomicWriteSecret}
+
         # `bao`'s own message is the only thing separating a missing value from
         # a refused identity from an unreachable host. This unit's degraded
         # mode is correct for all three, so it reports which one rather than
@@ -689,10 +693,7 @@ in
         # grants the group nothing; if this mode ever widens, the gid has to be
         # discovered at runtime rather than assumed.
         install -d -m 0755 ${lib.escapeShellArg hostSecretDir}
-        umask 077
-        printf '%s\n' "$secret" > ${lib.escapeShellArg hostSecretPath}
-        chown ${toString config.ids.uids.grafana}:0 ${lib.escapeShellArg hostSecretPath}
-        chmod 0400 ${lib.escapeShellArg hostSecretPath}
+        atomic_write_secret 0400 ${lib.escapeShellArg "${toString config.ids.uids.grafana}:0"} ${lib.escapeShellArg hostSecretPath} "$secret"
       '';
     };
 

nix/host-modules/swarm-otel.nix

@@ -166,6 +166,8 @@ let
   # collector cannot open, discovered at runtime and nowhere else.
   collectorSecretPath = "/run/credentials/opentelemetry-collector.service/${collectorCredentialId}";
 
+  atomicWriteSecret = import ./lib/atomic-write-secret.nix { };
+
   # `attrNames` is sorted, so this is a function of the hive SET and not of
   # the order anyone wrote it in.
   #
@@ -817,6 +819,8 @@ in
       script = ''
         set -euo pipefail
 
+        ${atomicWriteSecret}
+
         # `bao`'s own message is the only thing separating a missing value
         # from a refused identity from an unreachable host. This unit's
         # degraded mode is correct for all three, so it reports which one
@@ -862,10 +866,7 @@ in
         # no uid to give this to — `LoadCredential` reads it as root before
         # the sandbox exists and re-exposes it to whichever uid the unit got.
         install -d -m 0755 ${lib.escapeShellArg collectorHostSecretDir}
-        umask 077
-        printf '%s\n' "$secret" > ${lib.escapeShellArg collectorHostSecretPath}
-        chown root:root ${lib.escapeShellArg collectorHostSecretPath}
-        chmod 0400 ${lib.escapeShellArg collectorHostSecretPath}
+        atomic_write_secret 0400 root:root ${lib.escapeShellArg collectorHostSecretPath} "$secret"
       '';
     };