fix: re-encrypt a key rotation the object layer has to rewrite

A key rotation rewraps the object key held in metadata; it never re-encrypts
the stored bytes. CopyObjectHandler took that shortcut whenever the request
looked like a same-object SSE-C rotation, on the assumption that the object
layer would then leave the stored bytes alone. That is the same assumption
copyRewritesObjectData() was added to stop making.

When the source is a null version on a bucket that gained versioning after the
object was written, the object layer cannot reference that version and falls
back to PutObject. The reader at that point holds plaintext decrypted with the
old key and no EncryptFn is set, so the destination ends up storing plaintext
under metadata that claims the object is SSE-C encrypted. A subsequent GET
failed with "sio: unsupported version".

Gate the rotation shortcut on the same prediction the compression metadata
already uses. When the object layer stores new object data the rotation falls
through to the regular re-encrypting copy, which decrypts with the old key and
re-encrypts with the new one. The source version selection moves next to the
gate because both decisions need it.

That fallback authenticates the source key through the source decryptor, which
GetObjectNInfo does not build for a zero byte object. Check the key explicitly
before the destination is written, so the gate cannot turn a rotation that the
shortcut rejected with AccessDenied into one that succeeds. The re-encrypting
copy regenerates the encrypted ETag, unlike an in-place rotation; the test
records that difference.

The other three object layer CopyObject callers that set metadataOnly -
PostRestoreObjectHandler, updateRestoreMetadata and batchKeyRotate - address
the same version on both sides and never set Versioned, so they only reach the
two in-place cases already covered by the copyRewritesObjectData table.

Signed-off-by: Feng Ruohang <rh@vonng.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Fk3PAD7DHCYzcyegYWqAmt

cmd/erasure-server-pool.go

@@ -1328,6 +1328,8 @@ func (z *erasureServerPools) CopyObject(ctx context.Context, srcBucket, srcObjec
 		return objInfo, err
 	}
 
+	// CopyObjectHandler predicts the outcome of this decision in
+	// copyRewritesObjectData(); keep the two in sync.
 	if cpSrcDstSame && srcInfo.metadataOnly {
 		// Version ID is set for the destination and source == destination version ID.
 		if dstOpts.VersionID != "" && srcOpts.VersionID == dstOpts.VersionID {