fix(thrift): reduce skip decoder mem alloc/copy

normally the ReaderSkipDecoder will be reused,
and it's hard to find out the issue with Benchmark.

In production, GC may happen all the time which make it eaiser to expose
the issue of calling `growSlow`

protocol/thrift/skipdecoder.go

@@ -182,10 +182,15 @@ func (p *ReaderSkipDecoder) Grow(n int) {
 }
 
 func (p *ReaderSkipDecoder) growSlow(n int) {
-	// mcache will take care of the size of newb
-	newb := mcache.Malloc(p.n + n)
-	copy(newb, p.b[:p.n])
-	mcache.Free(p.b)
+	// mcache will take care of the new cap of newb to be power of 2
+	newb := mcache.Malloc((len(p.b) + n) | 255) // at least 255 bytes
+	newb = newb[:cap(newb)]
+	if p.n > 0 {
+		copy(newb, p.b[:p.n])
+	}
+	if p.b != nil {
+		mcache.Free(p.b)
+	}
 	p.b = newb
 }
 

protocol/thrift/skipdecoder_test.go

@@ -263,15 +263,57 @@ func BenchmarkSkipDecoder(b *testing.B) {
 	}
 	sr.Release()
 
-	b.ResetTimer()
-	b.RunParallel(func(pb *testing.PB) {
-		sr := &BytesSkipDecoder{}
-		for pb.Next() {
+	b.Run("BytesSkipDecoder", func(b *testing.B) {
+		sr := NewBytesSkipDecoder(bs)
+		for i := 0; i < b.N; i++ {
+			_, err := sr.Next(STRUCT)
+			if err != nil {
+				b.Fatal(err)
+			}
 			sr.Reset(bs)
+		}
+		sr.Release()
+	})
+
+	b.Run("Parallel_BytesSkipDecoder", func(b *testing.B) {
+		b.RunParallel(func(pb *testing.PB) {
+			for pb.Next() {
+				sr := NewBytesSkipDecoder(bs)
+				_, err := sr.Next(STRUCT)
+				if err != nil {
+					b.Fatal(err)
+				}
+				sr.Release()
+			}
+		})
+	})
+
+	b.Run("ReaderSkipDecoder", func(b *testing.B) {
+		r := bytes.NewReader(bs)
+		sr := NewReaderSkipDecoder(r)
+		for i := 0; i < b.N; i++ {
 			_, err := sr.Next(STRUCT)
 			if err != nil {
 				b.Fatal(err)
 			}
+			r.Reset(bs)
 		}
+		sr.Release()
 	})
+
+	b.Run("Parallel_ReaderSkipDecoder", func(b *testing.B) {
+		b.RunParallel(func(pb *testing.PB) {
+			r := bytes.NewReader(bs)
+			for pb.Next() {
+				sr := NewReaderSkipDecoder(r)
+				_, err := sr.Next(STRUCT)
+				if err != nil {
+					b.Fatal(err)
+				}
+				sr.Release()
+				r.Reset(bs)
+			}
+		})
+	})
+
 }