Reduce allocations (#17584)

* Reduce allocations

* Add stringsHasPrefixFold which can compare string prefixes, while ignoring case and not allocating.
* Reuse all msgp.Readers
* Reuse metadata buffers when not reading data.

* Make type safe. Make buffer 4K instead of 8.

* Unslice

cmd/erasure-object.go

@@ -527,6 +527,10 @@ func readAllXL(ctx context.Context, disks []StorageAPI, bucket, object string, r
 	metadataArray := make([]*xlMetaV2, len(disks))
 	metaFileInfos := make([]FileInfo, len(metadataArray))
 	metadataShallowVersions := make([][]xlMetaV2ShallowVersion, len(disks))
+	var v2bufs [][]byte
+	if !readData {
+		v2bufs = make([][]byte, len(disks))
+	}
 
 	g := errgroup.WithNErrs(len(disks))
 	// Read `xl.meta` in parallel across disks.
@@ -540,6 +544,10 @@ func readAllXL(ctx context.Context, disks []StorageAPI, bucket, object string, r
 			if err != nil {
 				return err
 			}
+			if !readData {
+				// Save the buffer so we can reuse it.
+				v2bufs[index] = rf.Buf
+			}
 
 			var xl xlMetaV2
 			if err = xl.LoadOrConvert(rf.Buf); err != nil {
@@ -623,6 +631,11 @@ func readAllXL(ctx context.Context, disks []StorageAPI, bucket, object string, r
 		}
 		metaFileInfos[index].DiskMTime = diskMTime
 	}
+	if !readData {
+		for i := range v2bufs {
+			metaDataPoolPut(v2bufs[i])
+		}
+	}
 
 	// Return all the metadata.
 	return metaFileInfos, errs