Implement get_downlink_data_delay config.

This config option was present in v3 to set a delay before starting the
downlink flow (Class-A) so that an end-application can enqueue a
downlink to be used within the same uplink / downlink transaction.
However, this option was still missing in v4.

chirpstack/src/cmd/configfile.rs

@@ -166,6 +166,17 @@ pub fn run() {
   # must be less than the (first) receive-window.
   deduplication_delay="{{ network.deduplication_delay }}"
 
+  # Get downlink data delay.
+  #
+  # This is the time that ChirpStack waits between forwarding data to the
+  # integration(s) and reading data from the queue. A higher value means that
+  # an end-application has more time to schedule a downlink queue item which
+  # can be processed within the same uplink / downlink transaction.
+  # Please note that this value has influence on the uplink / downlink
+  # roundtrip time. Setting this value too high means ChirpStack will be
+  # unable to respond to the device within its receive-window.
+  get_downlink_data_delay="{{ network.get_downlink_data_delay }}"
+
   # Mac-commands disabled.
   mac_commands_disabled={{ network.mac_commands_disabled }}
 

chirpstack/src/config.rs

@@ -156,6 +156,8 @@ pub struct Network {
     pub device_session_ttl: Duration,
     #[serde(with = "humantime_serde")]
     pub deduplication_delay: Duration,
+    #[serde(with = "humantime_serde")]
+    pub get_downlink_data_delay: Duration,
     pub mac_commands_disabled: bool,
     pub adr_plugins: Vec<String>,
     pub scheduler: Scheduler,
@@ -169,6 +171,7 @@ impl Default for Network {
             enabled_regions: vec!["eu868".into()],
             device_session_ttl: Duration::from_secs(60 * 60 * 24 * 31),
             deduplication_delay: Duration::from_millis(200),
+            get_downlink_data_delay: Duration::from_millis(100),
             mac_commands_disabled: false,
             adr_plugins: vec![],
             scheduler: Default::default(),

chirpstack/src/uplink/data.rs

@@ -983,6 +983,10 @@ impl Data {
 
     async fn start_downlink_data_flow(&mut self) -> Result<()> {
         trace!("Starting downlink data flow");
+
+        let conf = config::get();
+        tokio::time::sleep(conf.network.get_downlink_data_delay).await;
+
         if let lrwn::Payload::MACPayload(pl) = &self.uplink_frame_set.phy_payload.payload {
             downlink::data::Data::handle_response(
                 self.uplink_frame_set.clone(),