fix: align multipart completion checksum errors

Return AWS-compatible errors for CompleteMultipartUpload checksum failures without changing the global streaming checksum mapping. Compare explicit multipart checksum types symmetrically, distinguish missing composite part checksums, and preserve the CRC64NVME canonicalization pending a direct AWS probe.

Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/object-api-errors.go

@@ -22,6 +22,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"strings"
 )
 
 // Converts underlying storage error. Convenience function written to
@@ -653,6 +654,41 @@ func (e InvalidPart) Error() string {
 		e.PartNumber, e.ExpETag, e.GotETag)
 }
 
+var (
+	errCompleteMultipartChecksumMismatch     = errors.New("complete multipart checksum mismatch")
+	errCompleteMultipartChecksumTypeMismatch = errors.New("complete multipart checksum type mismatch")
+	errMissingPartChecksum                   = errors.New("missing multipart part checksum")
+)
+
+// completeMultipartChecksumMismatch reports an object checksum mismatch
+// detected while completing a multipart upload. It is distinct from
+// hash.ChecksumMismatch because AWS maps completion failures to BadDigest,
+// while streaming PutObject and UploadPart failures keep using
+// XAmzContentChecksumMismatch.
+func completeMultipartChecksumMismatch(algorithm string) error {
+	description := "The checksum you specified did not match the calculated checksum."
+	if algorithm != "" {
+		description = fmt.Sprintf("The %s checksum you specified did not match the calculated checksum.", algorithm)
+	}
+	return fmt.Errorf("%w: %s", errCompleteMultipartChecksumMismatch, description)
+}
+
+// completeMultipartChecksumTypeMismatch reports an explicit checksum type that
+// differs from the type selected when the multipart upload was initiated.
+func completeMultipartChecksumTypeMismatch(providedType, expectedType string) error {
+	description := fmt.Sprintf("The checksum type %s does not match the multipart upload checksum type %s.",
+		providedType, expectedType)
+	return fmt.Errorf("%w: %s", errCompleteMultipartChecksumTypeMismatch, description)
+}
+
+// missingPartChecksum reports a part whose checksum is absent from a
+// composite CompleteMultipartUpload request.
+func missingPartChecksum(algorithm string, partNumber int) error {
+	description := fmt.Sprintf("The upload was created using a %s checksum. The complete request must include the checksum for each part. It was missing for part %d in the request.",
+		strings.ToLower(algorithm), partNumber)
+	return fmt.Errorf("%w: %s", errMissingPartChecksum, description)
+}
+
 // PartTooSmall - error if part size is less than 5MB.
 type PartTooSmall struct {
 	PartSize   int64