fix: resolve memory leak with gradual buffer shrinking strategy

Implement sophisticated buffer management that only shrinks when buffer is more than half empty, and reduces to half size (not exact size) to provide wiggle room for incoming data.

This prevents both memory leaks from oversized buffers and performance issues from excessive reallocations in high-throughput scenarios.

- Only shrink when buffer utilization < 50%
- When shrinking, reduce to max(half current size, 2x remaining data)
- Gradual reduction allows large buffers to shrink progressively
- Falls back to cursor strategy when buffer utilization is reasonable

Author: wpietrucha

packages/pg-protocol/src/parser.ts

@@ -113,15 +113,25 @@ export class Parser {
       this.bufferOffset = 0
     } else {
       // A partial message remains.
-      // Create a new, smaller buffer and copy only the remaining data into it.
-      // This breaks the reference to the original, potentially huge buffer.
+      // Use a gradual shrinking strategy: only shrink if buffer is at least half empty,
+      // and when shrinking, reduce to half size (not exact size) to provide wiggle room.
       const remainingLength = bufferFullLength - offset
-      const newBuffer = Buffer.allocUnsafe(remainingLength)
-      this.buffer.copy(newBuffer, 0, offset, offset + remainingLength)
-
-      this.buffer = newBuffer
-      this.bufferOffset = 0
-      this.bufferLength = remainingLength
+      const bufferUtilization = remainingLength / this.buffer.byteLength
+      
+      if (bufferUtilization < 0.5) {
+        // Buffer is more than half empty - shrink it to half its current size
+        const newBufferSize = Math.max(this.buffer.byteLength / 2, remainingLength * 2)
+        const newBuffer = Buffer.allocUnsafe(newBufferSize)
+        this.buffer.copy(newBuffer, 0, offset, offset + remainingLength)
+        
+        this.buffer = newBuffer
+        this.bufferOffset = 0
+        this.bufferLength = remainingLength
+      } else {
+        // Buffer utilization is reasonable - use existing cursor strategy
+        this.bufferLength = remainingLength
+        this.bufferOffset = offset
+      }
     }
   }