erasure: simplify XL backend operations (#4649) (#4758)

This change provides new implementations of the XL backend operations:
 - create file
 - read   file
 - heal   file
Further this change adds table based tests for all three operations.

This affects also the bitrot algorithm integration. Algorithms are now
integrated in an idiomatic way (like crypto.Hash).
Fixes #4696
Fixes #4649
Fixes #4359

cmd/xl-v1-healing-common.go

@@ -17,11 +17,23 @@
 package cmd
 
 import (
-	"encoding/hex"
+	"crypto/subtle"
 	"path/filepath"
+	"sync"
 	"time"
+
+	"io"
 )
 
+// healBufferPool is a pool of reusable buffers used to verify a stream
+// while healing.
+var healBufferPool = sync.Pool{
+	New: func() interface{} {
+		b := make([]byte, readSizeV1)
+		return &b
+	},
+}
+
 // commonTime returns a maximally occurring time from a list of time.
 func commonTime(modTimes []time.Time) (modTime time.Time, count int) {
 	var maxima int // Counter for remembering max occurrence of elements.
@@ -252,36 +264,32 @@ func xlHealStat(xl xlObjects, partsMetadata []xlMetaV1, errs []error) HealObject
 // calculating blake2b checksum.
 func disksWithAllParts(onlineDisks []StorageAPI, partsMetadata []xlMetaV1, errs []error, bucket, object string) ([]StorageAPI, []error, error) {
 	availableDisks := make([]StorageAPI, len(onlineDisks))
+	buffer := healBufferPool.Get().(*[]byte)
+	defer healBufferPool.Put(buffer)
+
 	for diskIndex, onlineDisk := range onlineDisks {
-		if onlineDisk == nil {
+		if onlineDisk == OfflineDisk {
 			continue
 		}
 		// disk has a valid xl.json but may not have all the
 		// parts. This is considered an outdated disk, since
 		// it needs healing too.
 		for _, part := range partsMetadata[diskIndex].Parts {
-			// compute blake2b sum of part.
 			partPath := filepath.Join(object, part.Name)
-			checkSumInfo := partsMetadata[diskIndex].Erasure.GetCheckSumInfo(part.Name)
-			hash := newHash(checkSumInfo.Algorithm)
-			blakeBytes, hErr := hashSum(onlineDisk, bucket, partPath, hash)
+			checkSumInfo := partsMetadata[diskIndex].Erasure.GetChecksumInfo(part.Name)
+			hash := checkSumInfo.Algorithm.New()
+			_, hErr := io.CopyBuffer(hash, StorageReader(onlineDisk, bucket, partPath, 0), *buffer)
 			if hErr == errFileNotFound {
 				errs[diskIndex] = errFileNotFound
-				availableDisks[diskIndex] = nil
+				availableDisks[diskIndex] = OfflineDisk
 				break
 			}
-
 			if hErr != nil && hErr != errFileNotFound {
 				return nil, nil, traceError(hErr)
 			}
-
-			blakeSum := hex.EncodeToString(blakeBytes)
-			// if blake2b sum doesn't match for a part
-			// then this disk is outdated and needs
-			// healing.
-			if blakeSum != checkSumInfo.Hash {
+			if subtle.ConstantTimeCompare(hash.Sum(nil), checkSumInfo.Hash) != 1 {
 				errs[diskIndex] = errFileNotFound
-				availableDisks[diskIndex] = nil
+				availableDisks[diskIndex] = OfflineDisk
 				break
 			}
 			availableDisks[diskIndex] = onlineDisk