fix(agent99): fail closed host110 runtime replay

scripts/backup/common.sh

@@ -3,14 +3,78 @@
 
 # Production backup entrypoints share this stable gate. Agent99 takes the
 # exclusive side while promoting a complete, digest-bound runtime bundle.
+resolve_backup_runtime_source_path() {
+    local raw_path="$1"
+    local candidate=""
+    local resolved=""
+    local resolved_directory=""
+    [ -n "${raw_path}" ] || return 1
+    case "${raw_path}" in
+        /*) candidate="${raw_path}" ;;
+        *) candidate="${PWD}/${raw_path}" ;;
+    esac
+    if command -v realpath >/dev/null 2>&1; then
+        resolved="$(realpath -e -- "${candidate}" 2>/dev/null || realpath -- "${candidate}" 2>/dev/null)" || resolved=""
+        if [ -n "${resolved}" ]; then
+            printf '%s\n' "${resolved}"
+            return 0
+        fi
+    fi
+    if command -v readlink >/dev/null 2>&1; then
+        resolved="$(readlink -f -- "${candidate}" 2>/dev/null)" || resolved=""
+        if [ -n "${resolved}" ]; then
+            printf '%s\n' "${resolved}"
+            return 0
+        fi
+    fi
+    [ ! -L "${candidate}" ] || return 1
+    resolved_directory="$(cd -P -- "$(dirname -- "${candidate}")" 2>/dev/null && pwd -P)" || return 1
+    [ -e "${resolved_directory}/$(basename -- "${candidate}")" ] || return 1
+    printf '%s/%s\n' "${resolved_directory}" "$(basename -- "${candidate}")"
+}
+
+backup_runtime_inherited_fd_status() {
+    local expected_path="$1"
+    local fd_path=""
+    local fd_target=""
+    if [ -d "/proc/$$/fd" ]; then
+        fd_path="/proc/$$/fd/197"
+        [ -e "${fd_path}" ] || [ -L "${fd_path}" ] || return 1
+        fd_target="$(resolve_backup_runtime_source_path "${fd_path}")" || return 2
+        [ "${fd_target}" = "${expected_path}" ] || return 2
+        return 0
+    fi
+    fd_path="/dev/fd/197"
+    [ -e "${fd_path}" ] || [ -L "${fd_path}" ] || return 1
+    command -v python3 >/dev/null 2>&1 || return 2
+    fd_target="$(python3 - <<'PY'
+import fcntl
+import os
+
+try:
+    target = os.readlink("/dev/fd/197")
+except OSError:
+    value = fcntl.fcntl(197, getattr(fcntl, "F_GETPATH", 50), b"\0" * 1024)
+    target = os.fsdecode(value.split(b"\0", 1)[0])
+print(os.path.realpath(target))
+PY
+    )" || return 2
+    [ "${fd_target}" = "${expected_path}" ] || return 2
+}
+
 acquire_backup_runtime_shared_lock() {
-    local caller_path="${BASH_SOURCE[1]:-${BASH_SOURCE[0]}}"
+    local caller_path=""
     local lock_path="/tmp/agent99-host110-backup-runtime.lock"
+    local lock_canonical=""
+    local inherited_fd_status=0
+    caller_path="$(resolve_backup_runtime_source_path "${BASH_SOURCE[1]:-${BASH_SOURCE[0]:-}}")" || {
+        echo "backup runtime gate unavailable: caller identity unresolved" >&2
+        return 69
+    }
     case "${caller_path}" in
         /backup/scripts/*) ;;
         *) return 0 ;;
     esac
-    [ "${BACKUP_RUNTIME_SHARED_LOCK_HELD:-0}" != "1" ] || return 0
     command -v flock >/dev/null 2>&1 || {
         echo "backup runtime gate unavailable: flock missing" >&2
         return 69
@@ -21,12 +85,25 @@ acquire_backup_runtime_shared_lock() {
     }
     (umask 077; : >> "${lock_path}") || return 69
     [ -f "${lock_path}" ] && [ -O "${lock_path}" ] || return 69
-    exec 197>>"${lock_path}"
+    lock_canonical="$(resolve_backup_runtime_source_path "${lock_path}")" || {
+        echo "backup runtime gate unavailable: lock identity unresolved" >&2
+        return 69
+    }
+    if backup_runtime_inherited_fd_status "${lock_canonical}"; then
+        :
+    else
+        inherited_fd_status=$?
+        if [ "${inherited_fd_status}" -eq 1 ]; then
+            exec 197>>"${lock_path}"
+        else
+            echo "backup runtime gate unsafe: inherited fd 197 identity mismatch" >&2
+            return 69
+        fi
+    fi
     flock -s -w 60 197 || {
         echo "backup runtime deployment is active; retry later" >&2
         return 75
     }
-    export BACKUP_RUNTIME_SHARED_LOCK_HELD=1
 }
 
 acquire_backup_runtime_shared_lock || {