fix: resolve memory leak in parser buffer management with threshold-based shrinking

Implement smart buffer shrinking that only reallocates when there's significant waste:
- Only shrink if waste > max(4x remaining data, 64KB)
- When shrinking, allocate 2x remaining data for growth room
- Fall back to cursor strategy for normal cases

This prevents memory leaks from large buffers while avoiding excessive allocations in high-throughput scenarios, addressing both memory efficiency and performance concerns.

Author: wpietrucha

packages/pg-protocol/src/parser.ts

@@ -112,9 +112,25 @@ export class Parser {
       this.bufferLength = 0
       this.bufferOffset = 0
     } else {
-      // Adjust the cursors of remainingBuffer
-      this.bufferLength = bufferFullLength - offset
-      this.bufferOffset = offset
+      // A partial message remains.
+      // Only shrink the buffer if it's significantly larger than needed to prevent memory leaks
+      // while avoiding excessive allocations in high-throughput scenarios.
+      const remainingLength = bufferFullLength - offset
+      const bufferWaste = this.buffer.byteLength - remainingLength
+      const wasteThreshold = Math.max(remainingLength * 4, 1024 * 64) // 4x remaining data or 64KB, whichever is larger
+      
+      if (bufferWaste > wasteThreshold) {
+        // Significant waste detected - create a new appropriately-sized buffer
+        const newBuffer = Buffer.allocUnsafe(remainingLength * 2) // Give some room for growth
+        this.buffer.copy(newBuffer, 0, offset, offset + remainingLength)
+        this.buffer = newBuffer
+        this.bufferOffset = 0
+        this.bufferLength = remainingLength
+      } else {
+        // Not enough waste to justify reallocation - use existing cursor strategy
+        this.bufferLength = remainingLength
+        this.bufferOffset = offset
+      }
     }
   }