perf: avoid redundant scans in BalancedPool dispatcher selection (#5146)

Author: trivikr

lib/dispatcher/balanced-pool.js

@@ -164,35 +164,14 @@ class BalancedPool extends PoolBase {
       throw new BalancedPoolMissingUpstreamError()
     }
 
-    const dispatcher = this[kClients].find(dispatcher => (
-      !dispatcher[kNeedDrain] &&
-      dispatcher.closed !== true &&
-      dispatcher.destroyed !== true
-    ))
-
-    if (!dispatcher) {
-      return
-    }
-
-    const allClientsBusy = this[kClients].map(pool => pool[kNeedDrain]).reduce((a, b) => a && b, true)
-
-    if (allClientsBusy) {
-      return
-    }
-
     let counter = 0
 
-    let maxWeightIndex = this[kClients].findIndex(pool => !pool[kNeedDrain])
+    let maxWeightIndex = -1
 
     while (counter++ < this[kClients].length) {
       this[kIndex] = (this[kIndex] + 1) % this[kClients].length
       const pool = this[kClients][this[kIndex]]
 
-      // find pool index with the largest weight
-      if (pool[kWeight] > this[kClients][maxWeightIndex][kWeight] && !pool[kNeedDrain]) {
-        maxWeightIndex = this[kIndex]
-      }
-
       // decrease the current weight every `this[kClients].length`.
       if (this[kIndex] === 0) {
         // Set the current weight to the next lower weight.
@@ -202,11 +181,30 @@ class BalancedPool extends PoolBase {
           this[kCurrentWeight] = this[kMaxWeightPerServer]
         }
       }
-      if (pool[kWeight] >= this[kCurrentWeight] && (!pool[kNeedDrain])) {
+
+      // Skip unavailable pools after updating the current weight for this cycle.
+      if (
+        pool[kNeedDrain] ||
+        pool.closed === true ||
+        pool.destroyed === true
+      ) {
+        continue
+      }
+
+      // Track the best fallback if no pool matches the current weight.
+      if (maxWeightIndex === -1 || pool[kWeight] > this[kClients][maxWeightIndex][kWeight]) {
+        maxWeightIndex = this[kIndex]
+      }
+
+      if (pool[kWeight] >= this[kCurrentWeight]) {
         return pool
       }
     }
 
+    if (maxWeightIndex === -1) {
+      return
+    }
+
     this[kCurrentWeight] = this[kClients][maxWeightIndex][kWeight]
     this[kIndex] = maxWeightIndex
     return this[kClients][maxWeightIndex]