Add a benchmark for growing and shrinking.

This should better stress memclr.

internal/buffer/out_message_test.go

@@ -85,3 +85,34 @@ func BenchmarkOutMessageReset(b *testing.B) {
 		b.SetBytes(int64(oms[0].offset))
 	})
 }
+
+func BenchmarkOutMessageGrowShrink(b *testing.B) {
+	// A single buffer, which should fit in some level of CPU cache.
+	b.Run("Single buffer", func(b *testing.B) {
+		var om OutMessage
+		for i := 0; i < b.N; i++ {
+			om.Grow(MaxReadSize)
+			om.ShrinkTo(OutMessageInitialSize)
+		}
+
+		b.SetBytes(int64(MaxReadSize))
+	})
+
+	// Many megabytes worth of buffers, which should defeat the CPU cache.
+	b.Run("Many buffers", func(b *testing.B) {
+		// The number of messages; intentionally a power of two.
+		const numMessages = 128
+
+		var oms [numMessages]OutMessage
+		if s := unsafe.Sizeof(oms); s < 128<<20 {
+			panic(fmt.Sprintf("Array is too small; total size: %d", s))
+		}
+
+		for i := 0; i < b.N; i++ {
+			oms[i%numMessages].Grow(MaxReadSize)
+			oms[i%numMessages].ShrinkTo(OutMessageInitialSize)
+		}
+
+		b.SetBytes(int64(MaxReadSize))
+	})
+}