Set default ETag value if vendor returns empty md5 string (#4409)

The ETag is constructed from md5 atttribute of object attributes
returned by the vendor's Composer. The md5 attribute comes back
as nil for large uploads. Instead the CRC32C should be used.

Refer to https://cloud.google.com/storage/docs/hashes-etags

Fixes #4397

cmd/gateway-gcs.go

@@ -17,7 +17,6 @@
 package cmd
 
 import (
-	"bytes"
 	"context"
 	"crypto/sha256"
 	"encoding/base64"
@@ -356,7 +355,7 @@ func (l *gcsGateway) ListObjects(bucket string, prefix string, marker string, de
 			Bucket:          attrs.Bucket,
 			ModTime:         attrs.Updated,
 			Size:            attrs.Size,
-			ETag:            hex.EncodeToString(attrs.MD5),
+			ETag:            fmt.Sprintf("%d", attrs.CRC32C),
 			UserDefined:     attrs.Metadata,
 			ContentType:     attrs.ContentType,
 			ContentEncoding: attrs.ContentEncoding,
@@ -401,7 +400,7 @@ func (l *gcsGateway) ListObjectsV2(bucket, prefix, continuationToken string, fet
 			continue
 		}
 
-		objects = append(objects, fromGCSObjectInfo(attrs))
+		objects = append(objects, fromGCSAttrsToObjectInfo(attrs))
 	}
 
 	return ListObjectsV2Info{
@@ -441,18 +440,21 @@ func (l *gcsGateway) GetObject(bucket string, key string, startOffset int64, len
 	return nil
 }
 
-// fromGCSObjectInfo converts GCS BucketAttrs to gateway ObjectInfo
-func fromGCSObjectInfo(attrs *storage.ObjectAttrs) ObjectInfo {
+// fromGCSAttrsToObjectInfo converts GCS BucketAttrs to gateway ObjectInfo
+func fromGCSAttrsToObjectInfo(attrs *storage.ObjectAttrs) ObjectInfo {
+	// All google cloud storage objects have a CRC32c hash, whereas composite objects may not have a MD5 hash
+	// Refer https://cloud.google.com/storage/docs/hashes-etags. Use CRC32C for ETag
 	return ObjectInfo{
 		Name:            attrs.Name,
 		Bucket:          attrs.Bucket,
 		ModTime:         attrs.Updated,
 		Size:            attrs.Size,
-		ETag:            hex.EncodeToString(attrs.MD5),
+		ETag:            fmt.Sprintf("%d", attrs.CRC32C),
 		UserDefined:     attrs.Metadata,
 		ContentType:     attrs.ContentType,
 		ContentEncoding: attrs.ContentEncoding,
 	}
+
 }
 
 // GetObjectInfo - reads object info and replies back ObjectInfo
@@ -464,11 +466,14 @@ func (l *gcsGateway) GetObjectInfo(bucket string, object string) (ObjectInfo, er
 	}
 
 	attrs, err := l.client.Bucket(bucket).Object(object).Attrs(l.ctx)
+
 	if err != nil {
 		return ObjectInfo{}, gcsToObjectError(traceError(err), bucket, object)
 	}
+	objInfo := fromGCSAttrsToObjectInfo(attrs)
+	objInfo.ETag = fmt.Sprintf("%d", attrs.CRC32C)
 
-	return fromGCSObjectInfo(attrs), nil
+	return objInfo, nil
 }
 
 // PutObject - Create a new object with the incoming data,
@@ -499,6 +504,13 @@ func (l *gcsGateway) PutObject(bucket string, key string, size int64, data io.Re
 
 	w.ContentType = metadata["content-type"]
 	w.ContentEncoding = metadata["content-encoding"]
+	if md5sum == "" {
+	} else if md5, err := hex.DecodeString(md5sum); err != nil {
+		return ObjectInfo{}, gcsToObjectError(traceError(err), bucket, key)
+	} else {
+		w.MD5 = md5
+	}
+
 	w.Metadata = metadata
 
 	_, err := io.Copy(w, teeReader)
@@ -522,14 +534,7 @@ func (l *gcsGateway) PutObject(bucket string, key string, size int64, data io.Re
 		return ObjectInfo{}, traceError(SHA256Mismatch{})
 	}
 
-	if md5sum == "" {
-	} else if b, err := hex.DecodeString(md5sum); err != nil {
-	} else if bytes.Compare(b, attrs.MD5) != 0 {
-		object.Delete(l.ctx)
-		return ObjectInfo{}, traceError(SignatureDoesNotMatch{})
-	}
-
-	return fromGCSObjectInfo(attrs), nil
+	return fromGCSAttrsToObjectInfo(attrs), nil
 }
 
 // CopyObject - Copies a blob from source container to destination container.
@@ -542,7 +547,7 @@ func (l *gcsGateway) CopyObject(srcBucket string, srcObject string, destBucket s
 		return ObjectInfo{}, gcsToObjectError(traceError(err), destBucket, destObject)
 	}
 
-	return fromGCSObjectInfo(attrs), nil
+	return fromGCSAttrsToObjectInfo(attrs), nil
 }
 
 // DeleteObject - Deletes a blob in bucket
@@ -747,7 +752,7 @@ func (l *gcsGateway) ListObjectParts(bucket string, key string, uploadID string,
 		parts = append(parts, PartInfo{
 			PartNumber:   partID,
 			LastModified: attrs.Updated,
-			ETag:         hex.EncodeToString(attrs.MD5),
+			ETag:         fmt.Sprintf("%d", attrs.CRC32C),
 			Size:         attrs.Size,
 		})
 	}
@@ -805,11 +810,12 @@ func (l *gcsGateway) CompleteMultipartUpload(bucket string, key string, uploadID
 	parts := make([]*storage.ObjectHandle, len(uploadedParts))
 	for i, uploadedPart := range uploadedParts {
 		object := l.client.Bucket(bucket).Object(toGCSMultipartKey(key, uploadID, uploadedPart.PartNumber))
-
-		if etag, partErr := hex.DecodeString(uploadedPart.ETag); partErr != nil {
-		} else if attrs, partErr := object.Attrs(l.ctx); partErr != nil {
+		attrs, partErr := object.Attrs(l.ctx)
+		if partErr != nil {
 			return ObjectInfo{}, gcsToObjectError(traceError(partErr), bucket, key)
-		} else if bytes.Compare(attrs.MD5, etag) != 0 {
+		}
+		crc32cStr := fmt.Sprintf("%d", attrs.CRC32C)
+		if crc32cStr != uploadedPart.ETag {
 			return ObjectInfo{}, gcsToObjectError(traceError(InvalidPart{}), bucket, key)
 		}
 
@@ -832,7 +838,6 @@ func (l *gcsGateway) CompleteMultipartUpload(bucket string, key string, uploadID
 	composer.Metadata = partZeroAttrs.Metadata
 
 	attrs, err := composer.Run(l.ctx)
-
 	// cleanup, delete all parts
 	for _, uploadedPart := range uploadedParts {
 		l.client.Bucket(bucket).Object(toGCSMultipartKey(key, uploadID, uploadedPart.PartNumber)).Delete(l.ctx)
@@ -840,7 +845,7 @@ func (l *gcsGateway) CompleteMultipartUpload(bucket string, key string, uploadID
 
 	partZero.Delete(l.ctx)
 
-	return fromGCSObjectInfo(attrs), gcsToObjectError(traceError(err), bucket, key)
+	return fromGCSAttrsToObjectInfo(attrs), gcsToObjectError(traceError(err), bucket, key)
 }
 
 // SetBucketPolicies - Set policy on bucket