s3: Add MD5 metadata to objects uploaded with SSE-AWS/SSE-C

Before this change, small objects uploaded with SSE-AWS/SSE-C would
not have MD5 sums.

This change adds metadata for these objects in the same way that the
metadata is stored for multipart uploaded objects.

See: #1824 #2827

backend/s3/s3.go

@@ -3118,6 +3118,7 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 	// read the md5sum if available
 	// - for non multipart
 	//    - so we can add a ContentMD5
+	//    - so we can add the md5sum in the metadata as metaMD5Hash if using SSE/SSE-C
 	// - for multipart provided checksums aren't disabled
 	//    - so we can add the md5sum in the metadata as metaMD5Hash
 	var md5sum string
@@ -3127,7 +3128,11 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 			hashBytes, err := hex.DecodeString(hash)
 			if err == nil {
 				md5sum = base64.StdEncoding.EncodeToString(hashBytes)
-				if multipart {
+				if (multipart || o.fs.etagIsNotMD5) && !o.fs.opt.DisableChecksum {
+					// Set the md5sum as metadata on the object if
+					// - a multipart upload
+					// - the Etag is not an MD5, eg when using SSE/SSE-C
+					// provided checksums aren't disabled
 					metadata[metaMD5Hash] = &md5sum
 				}
 			}

docs/content/s3.md

@@ -277,6 +277,28 @@ side copy to update the modification if the object can be copied in a single par
 In the case the object is larger than 5Gb or is in Glacier or Glacier Deep Archive
 storage the object will be uploaded rather than copied.
 
+Note that reading this from the object takes an additional `HEAD`
+request as the metadata isn't returned in object listings.
+
+### Hashes ###
+
+For small objects which weren't uploaded as multipart uploads (objects
+sized below `--s3-upload-cutoff` if uploaded with rclone) rclone uses
+the `ETag:` header as an MD5 checksum.
+
+However for objects which were uploaded as multipart uploads or with
+server side encryption (SSE-AWS or SSE-C) the `ETag` header is no
+longer the MD5 sum of the data, so rclone adds an additional piece of
+metadata `X-Amz-Meta-Md5chksum` which is a base64 encoded MD5 hash (in
+the same format as is required for `Content-MD5`).
+
+For large objects, calculating this hash can take some time so the
+addition of this hash can be disabled with `--s3-disable-checksum`.
+This will mean that these objects do not have an MD5 checksum.
+
+Note that reading this from the object takes an additional `HEAD`
+request as the metadata isn't returned in object listings.
+
 ### Cleanup ###
 
 If you run `rclone cleanup s3:bucket` then it will remove all pending