refactor: update logic of round-robin

src/backend/load_balance.rs

@@ -1,3 +1,9 @@
+use derive_builder::Builder;
+use std::sync::{
+  atomic::{AtomicUsize, Ordering},
+  Arc,
+};
+
 /// Constants to specify a load balance option
 pub(super) mod load_balance_options {
   pub const FIX_TO_FIRST: &str = "none";
@@ -6,13 +12,48 @@ pub(super) mod load_balance_options {
   pub const STICKY_ROUND_ROBIN: &str = "sticky";
 }
 
+//
+//   /// Counter for load balancing
+//   pub cnt: UpstreamCount,
+
+// TODO: カウンタの移動
+#[derive(Debug, Clone, Builder)]
+pub struct LbRoundRobinCount {
+  #[builder(default)]
+  cnt: Arc<AtomicUsize>,
+  #[builder(setter(custom), default)]
+  max_val: usize,
+}
+impl LbRoundRobinCountBuilder {
+  pub fn max_val(&mut self, v: &usize) -> &mut Self {
+    self.max_val = Some(*v);
+    self
+  }
+}
+impl LbRoundRobinCount {
+  /// Get a current count of upstream served
+  fn current_cnt(&self) -> usize {
+    self.cnt.load(Ordering::Relaxed)
+  }
+
+  /// Increment the count of upstream served up to the max value
+  pub fn increment_cnt(&self) -> usize {
+    if self.current_cnt() < self.max_val - 1 {
+      self.cnt.fetch_add(1, Ordering::Relaxed)
+    } else {
+      // Clear the counter
+      self.cnt.fetch_and(0, Ordering::Relaxed)
+    }
+  }
+}
+
 #[derive(Debug, Clone)]
 /// Load Balancing Option
 pub enum LoadBalance {
   /// Fix to the first upstream. Use if only one upstream destination is specified
   FixToFirst,
   /// Simple round robin without session persistance
-  RoundRobin, // TODO: カウンタはここにいれる。randomとかには不要なので
+  RoundRobin(LbRoundRobinCount), // TODO: カウンタはここにいれる。randomとかには不要なので
   /// Randomly chose one upstream server
   Random,
   /// Round robin with session persistance using cookie