create subcomposed objects if total parts are > 32 (#4593)

cmd/gateway-gcs.go

@@ -25,6 +25,7 @@ import (
 	"fmt"
 	"hash"
 	"io"
+	"math"
 	"regexp"
 	"strings"
 	"time"
@@ -56,6 +57,13 @@ const (
 	// token prefixed with GCS returned marker to differentiate
 	// from user supplied marker.
 	gcsTokenPrefix = "##minio"
+
+	// maxComponents - maximum component object count to create a composite object.
+	// Refer https://cloud.google.com/storage/docs/composite-objects
+	maxComponents = 32
+
+	// maxPartCount - maximum multipart parts GCS supports which is 32 x 32 = 1024.
+	maxPartCount = 1024
 )
 
 // Stored in gcs.json - Contents of this file is not used anywhere. It can be
@@ -899,14 +907,39 @@ func (l *gcsGateway) CompleteMultipartUpload(bucket string, key string, uploadID
 		parts[i] = l.client.Bucket(bucket).Object(gcsMultipartDataName(uploadID, uploadedPart.ETag))
 	}
 
-	if len(parts) > 32 {
-		// we need to split up the compose of more than 32 parts
-		// into subcomposes. This means that the first 32 parts will
-		// compose to a composed-object-0, next parts to composed-object-1,
-		// the final compose will compose composed-object* to 1.
+	if len(parts) > maxPartCount {
 		return ObjectInfo{}, traceError(NotSupported{})
 	}
 
+	composeCount := int(math.Ceil(float64(len(parts)) / float64(maxComponents)))
+	if composeCount > 1 {
+		// Create composes of every 32 parts.
+		composeParts := make([]*storage.ObjectHandle, composeCount)
+		for i := 0; i < composeCount; i++ {
+			// Create 'composed-object-N' using next 32 parts.
+			composeName := fmt.Sprintf("composed-object-%d", i)
+			composeParts[i] = l.client.Bucket(bucket).Object(gcsMultipartDataName(uploadID, composeName))
+
+			start := i * maxComponents
+			end := start + maxComponents
+			if end > len(parts) {
+				end = len(parts)
+			}
+
+			composer := composeParts[i].ComposerFrom(parts[start:end]...)
+			composer.ContentType = partZeroAttrs.ContentType
+			composer.Metadata = partZeroAttrs.Metadata
+
+			_, err = composer.Run(l.ctx)
+			if err != nil {
+				return ObjectInfo{}, gcsToObjectError(traceError(err), bucket, key)
+			}
+		}
+
+		// As composes are successfully created, final object needs to be created using composes.
+		parts = composeParts
+	}
+
 	dst := l.client.Bucket(bucket).Object(key)
 
 	composer := dst.ComposerFrom(parts...)