+##
+# wait for a command to return the expected result
+#
+# This will run @check on @node repeatedly until the output matches @expect
+# based on the supplied condition, or until @max_wait seconds have elapsed,
+# whichever comes first. @cond may be one of the normal bash operators,
+# "-gt", "-ge", "-eq", "-le", "-lt", "==", "!=", or "=~", and must be quoted
+# in the caller to avoid unintentional evaluation by the shell in the caller.
+#
+# If @max_wait is not specified, the condition will be checked for up to 90s.
+#
+# If --verbose is passed as the first argument, the result is printed on each
+# value change, otherwise it is only printed after every 10s interval.
+#
+# Using wait_update_cond() or related helper function is preferable to adding
+# a "long enough" wait for some state to change in the background, since
+# "long enough" may be too short due to tunables, system config, or running in
+# a VM, and must by necessity wait too long for most cases or risk failure.
+#
+# usage: wait_update_cond [--verbose] node check cond expect [max_wait]
+wait_update_cond() {