swarm-archive: forget future-dated snapshots before each prune; fix the grants case

A client chooses a snapshot's time, so snapshots dated ahead could fill
--keep-last and push the real ones out of a prune. The prune now forgets
snapshots dated more than an hour after now first, and logs their ids. The
VM test forges one and checks the real snapshot stays. The bao-grants case
that read a + path as a glob now names the literal path, and two cases that
pinned 'nothing else' list the archive grants. The ciRunner stanza is back
at its original indent.

nix/host-modules/lib/archive-server.nix

@@ -33,6 +33,9 @@ let
   resticPrefix = "secret/swarm/services/archive/restic";
   readyPath = "secret/swarm/services/archive/ready";
 
+  # Slack for the clocks of the hives that stamp snapshots.
+  futureSkewSeconds = 3600;
+
   keepFlags = lib.concatStringsSep " " (
     [ "--keep-last ${toString retention.keepLast}" ]
     ++ lib.optional (
@@ -130,10 +133,37 @@ in
   # `restic forget --prune` on every repository, with its password read from
   # the store at run time. The server refuses deletes from clients, so this is
   # the only place history is dropped.
+  #
+  # restic ranks snapshots by their time, which the client chooses (`restic
+  # backup --time`). Snapshots dated years ahead would fill `--keep-last` and
+  # push the real ones out, so each prune first forgets the snapshots dated more
+  # than `futureSkewSeconds` after now. restic has no rule relative to the
+  # clock for `--keep-last`.
   pruneScript = ''
     set -euo pipefail
     ${storeSeededSecret}
 
+    # Forgets the snapshots of $repo dated after the limit, naming them in the
+    # log; returns non-zero when it cannot read the list or a time.
+    forget_future() {
+      local repo="$1" agent="$2" snaps id time at future="" limit
+      limit=$(( $(date +%s) + ${toString futureSkewSeconds} ))
+      snaps="$(${restic}/bin/restic -r "$repo" --retry-lock 10m snapshots --json \
+        | jq -r '.[] | "\(.id) \(.time)"')" || return 1
+      while read -r id time; do
+        [ -n "$id" ] || continue
+        at="$(date -d "$time" +%s)" || return 1
+        if [ "$at" -gt "$limit" ]; then
+          future+=" $id"
+        fi
+      done <<<"$snaps"
+      if [ -n "$future" ]; then
+        echo "archive prune warning: $agent has snapshots dated after now, forgetting them:$future" >&2
+        # shellcheck disable=SC2086
+        ${restic}/bin/restic -r "$repo" --retry-lock 10m forget $future || return 1
+      fi
+    }
+
     store_init
     store_login
     if [ "$STORE_UP" != 1 ]; then
@@ -151,7 +181,8 @@ in
         continue
       fi
       export RESTIC_PASSWORD_COMMAND="bao kv get -field=password ${resticPrefix}/$agent"
-      if ${restic}/bin/restic -r "$repo" --retry-lock 10m forget --group-by host ${keepFlags} --prune; then
+      if forget_future "$repo" "$agent" \
+        && ${restic}/bin/restic -r "$repo" --retry-lock 10m forget --group-by host ${keepFlags} --prune; then
         echo "pruned $agent"
       else
         echo "archive prune failed: $agent" >&2