rabbit_env: Enable disksup in os_mon but set threshold to 1.0

Previously we set `start_disksup` to `false` to avoid OTP's automatic
monitoring of disk space. `disksup`'s gen_server starts a port (which
runs `df` on Unix) which measures disk usage and sets an alarm through
OTP's `alarm_handler` when usage exceeds the configured
`disk_almost_full_threshold`. We can set this threshold to 1.0 to
effectively turn off disksup's monitoring (i.e. the alarm will never be
set).

By enabling disksup we have access to `get_disk_data/0` and
`get_disk_info/0,1` which can be used to replace the copied versions in
`rabbit_disk_monitor`.

Author: the-mikedavis

deps/rabbit_common/src/rabbit_env.erl

@@ -306,7 +306,12 @@ context_to_app_env_vars1(
       [{kernel, inet_default_connect_options, [{nodelay, true}]},
        {sasl, errlog_type, error},
        {os_mon, start_cpu_sup, false},
-       {os_mon, start_disksup, false},
+       %% Start disksup but configure the threshold high enough that it will
+       %% never alarm. `disksup' must be started to call `get_disk_info/0,1'
+       %% and `get_disk_data/0' but we don't want it polluting the logs with
+       %% its alarms. Alarming is done by `rabbit_disk_monitor' instead.
+       {os_mon, start_disksup, true},
+       {os_mon, disk_almost_full_threshold, 1.0},
        {os_mon, start_memsup, false},
        {mnesia, dir, DataDir},
        {ra, data_dir, QuorumQueueDir},