Accept multipart checksums with part count (#19680)

Accept multipart uploads where the combined checksum provides the expected part count.

It seems this was added by AWS to make the API more consistent, even if the 
data is entirely superfluous on multiple levels.

Improves AWS S3 compatibility.

cmd/erasure-multipart.go

@@ -1231,7 +1231,7 @@ func (er erasureObjects) CompleteMultipartUpload(ctx context.Context, bucket str
 	}
 
 	if opts.WantChecksum != nil {
-		err := opts.WantChecksum.Matches(checksumCombined)
+		err := opts.WantChecksum.Matches(checksumCombined, len(parts))
 		if err != nil {
 			return oi, err
 		}

internal/hash/checksum.go

@@ -27,6 +27,7 @@ import (
 	"hash"
 	"hash/crc32"
 	"net/http"
+	"strconv"
 	"strings"
 
 	"github.com/minio/minio/internal/hash/sha256"
@@ -71,9 +72,10 @@ const (
 
 // Checksum is a type and base 64 encoded value.
 type Checksum struct {
-	Type    ChecksumType
-	Encoded string
-	Raw     []byte
+	Type      ChecksumType
+	Encoded   string
+	Raw       []byte
+	WantParts int
 }
 
 // Is returns if c is all of t.
@@ -260,13 +262,14 @@ func ReadPartCheckSums(b []byte) (res []map[string]string) {
 		}
 		// Skip main checksum
 		b = b[length:]
-		if !typ.Is(ChecksumIncludesMultipart) {
-			continue
-		}
 		parts, n := binary.Uvarint(b)
 		if n <= 0 {
 			break
 		}
+		if !typ.Is(ChecksumIncludesMultipart) {
+			continue
+		}
+
 		if len(res) == 0 {
 			res = make([]map[string]string, parts)
 		}
@@ -292,11 +295,25 @@ func NewChecksumWithType(alg ChecksumType, value string) *Checksum {
 	if !alg.IsSet() {
 		return nil
 	}
+	wantParts := 0
+	if strings.ContainsRune(value, '-') {
+		valSplit := strings.Split(value, "-")
+		if len(valSplit) != 2 {
+			return nil
+		}
+		value = valSplit[0]
+		nParts, err := strconv.Atoi(valSplit[1])
+		if err != nil {
+			return nil
+		}
+		alg |= ChecksumMultipart
+		wantParts = nParts
+	}
 	bvalue, err := base64.StdEncoding.DecodeString(value)
 	if err != nil {
 		return nil
 	}
-	c := Checksum{Type: alg, Encoded: value, Raw: bvalue}
+	c := Checksum{Type: alg, Encoded: value, Raw: bvalue, WantParts: wantParts}
 	if !c.Valid() {
 		return nil
 	}
@@ -325,12 +342,15 @@ func (c *Checksum) AppendTo(b []byte, parts []byte) []byte {
 	b = append(b, crc...)
 	if c.Type.Is(ChecksumMultipart) {
 		var checksums int
+		if c.WantParts > 0 && !c.Type.Is(ChecksumIncludesMultipart) {
+			checksums = c.WantParts
+		}
 		// Ensure we don't divide by 0:
 		if c.Type.RawByteLen() == 0 || len(parts)%c.Type.RawByteLen() != 0 {
 			hashLogIf(context.Background(), fmt.Errorf("internal error: Unexpected checksum length: %d, each checksum %d", len(parts), c.Type.RawByteLen()))
 			checksums = 0
 			parts = nil
-		} else {
+		} else if len(parts) > 0 {
 			checksums = len(parts) / c.Type.RawByteLen()
 		}
 		if !c.Type.Is(ChecksumIncludesMultipart) {
@@ -358,7 +378,7 @@ func (c Checksum) Valid() bool {
 }
 
 // Matches returns whether given content matches c.
-func (c Checksum) Matches(content []byte) error {
+func (c Checksum) Matches(content []byte, parts int) error {
 	if len(c.Encoded) == 0 {
 		return nil
 	}
@@ -368,6 +388,13 @@ func (c Checksum) Matches(content []byte) error {
 		return err
 	}
 	sum := hasher.Sum(nil)
+	if c.WantParts > 0 && c.WantParts != parts {
+		return ChecksumMismatch{
+			Want: fmt.Sprintf("%s-%d", c.Encoded, c.WantParts),
+			Got:  fmt.Sprintf("%s-%d", base64.StdEncoding.EncodeToString(sum), parts),
+		}
+	}
+
 	if !bytes.Equal(sum, c.Raw) {
 		return ChecksumMismatch{
 			Want: c.Encoded,