fix: multipart replication with single part objects (#20895)

x-amz-checksum-algorithm is not set, causing all multipart single-part objects
to fail to replicate going via sftp/FTP uploads.

internal/hash/checksum.go

@@ -203,6 +203,24 @@ func (c ChecksumType) String() string {
 	return "invalid"
 }
 
+// StringFull returns the type and all flags as a string.
+func (c ChecksumType) StringFull() string {
+	out := []string{c.String()}
+	if c.Is(ChecksumMultipart) {
+		out = append(out, "MULTIPART")
+	}
+	if c.Is(ChecksumIncludesMultipart) {
+		out = append(out, "INCLUDESMP")
+	}
+	if c.Is(ChecksumTrailing) {
+		out = append(out, "TRAILING")
+	}
+	if c.Is(ChecksumFullObject) {
+		out = append(out, "FULLOBJ")
+	}
+	return strings.Join(out, "|")
+}
+
 // FullObjectRequested will return if the checksum type indicates full object checksum was requested.
 func (c ChecksumType) FullObjectRequested() bool {
 	return c&(ChecksumFullObject) == ChecksumFullObject || c.Is(ChecksumCRC64NVME)
@@ -263,7 +281,8 @@ func NewChecksumFromData(t ChecksumType, data []byte) *Checksum {
 }
 
 // ReadCheckSums will read checksums from b and return them.
-func ReadCheckSums(b []byte, part int) map[string]string {
+// Returns whether this is (part of) a multipart checksum.
+func ReadCheckSums(b []byte, part int) (cs map[string]string, isMP bool) {
 	res := make(map[string]string, 1)
 	for len(b) > 0 {
 		t, n := binary.Uvarint(b)
@@ -277,9 +296,11 @@ func ReadCheckSums(b []byte, part int) map[string]string {
 		if length == 0 || len(b) < length {
 			break
 		}
+
 		cs := base64.StdEncoding.EncodeToString(b[:length])
 		b = b[length:]
 		if typ.Is(ChecksumMultipart) {
+			isMP = true
 			t, n := binary.Uvarint(b)
 			if n < 0 {
 				break
@@ -317,7 +338,7 @@ func ReadCheckSums(b []byte, part int) map[string]string {
 	if len(res) == 0 {
 		res = nil
 	}
-	return res
+	return res, isMP
 }
 
 // ReadPartCheckSums will read all part checksums from b and return them.