api: unexport Stream fields, add Id() accessor

Make Stream.Id and Stream.Conn unexported to enforce encapsulation
and prevent unsafe direct mutation. The stream identifier is now
exposed via a Stream.Id() method; the underlying connection is no
longer reachable from outside the package — callers should hold
their own *Connection reference if they need it.

- Rename Stream.Id to Stream.id, add Stream.Id() accessor.
- Rename Stream.Conn to Stream.conn, drop public access.
- Add SetStreamIdForTesting helper in export_test.go so external
  tests can still exercise wire-encoding for arbitrary stream ids.
- Document the breaking change in CHANGELOG.md and MIGRATION.md.

Closes #471

Author: bigbes

CHANGELOG.md

@@ -93,6 +93,9 @@ Versioning](http://semver.org/spec/v2.0.0.html) except to the first release.
   Use `Opts.Logger *slog.Logger` instead. Pool `Opts.Logger *slog.Logger`
   replaces direct `log.Printf` calls that were not customizable.
   By default, logs are discarded (silent). See MIGRATION.md for details.
+* `Stream` struct fields `Id` and `Conn` are now unexported. Use the new
+  `Stream.Id()` accessor for the stream identifier; the underlying
+  connection is no longer reachable from outside the package (#471).
 
 ### Removed
 

MIGRATION.md

@@ -166,6 +166,22 @@ TODO
   `tarantool.pool` group. When a pool logger is set and a connection
   does not have its own logger, the pool passes its logger to each
   connection, which then applies its own `WithGroup("tarantool")`.
+* `Stream` struct fields are unexported. The exported `Stream.Id` field
+  is now an `Id()` method, and `Stream.Conn` is no longer accessible —
+  the underlying connection is an internal detail and callers should
+  hold their own `*Connection` reference if they need it.
+
+  Before:
+  ```go
+  stream, _ := conn.NewStream()
+  log.Printf("opened stream %d on %v", stream.Id, stream.Conn)
+  ```
+
+  After:
+  ```go
+  stream, _ := conn.NewStream()
+  log.Printf("opened stream %d on %v", stream.Id(), conn)
+  ```
 
 ## Migration from v1.x.x to v2.x.x
 

connection.go

@@ -1296,8 +1296,8 @@ func (conn *Connection) NewPrepared(expr string) (*Prepared, error) {
 func (conn *Connection) NewStream() (*Stream, error) {
 	next := conn.lastStreamId.Add(1)
 	return &Stream{
-		Id:   next,
-		Conn: conn,
+		id:   next,
+		conn: conn,
 	}, nil
 }
 

export_test.go

@@ -0,0 +1,7 @@
+package tarantool
+
+// SetStreamIdForTesting overrides a Stream's identifier for tests that need
+// to exercise wire-encoding of arbitrary stream id values.
+func SetStreamIdForTesting(s *Stream, id uint64) {
+	s.id = id
+}

stream.go

@@ -31,8 +31,13 @@ var (
 )
 
 type Stream struct {
-	Id   uint64
-	Conn *Connection
+	id   uint64
+	conn *Connection
+}
+
+// Id returns the stream identifier assigned by the connection.
+func (s *Stream) Id() uint64 {
+	return s.id
 }
 
 // BeginRequest helps you to create a begin request object for execution
@@ -241,9 +246,9 @@ func (req *RollbackRequest) Context(ctx context.Context) *RollbackRequest {
 // create the future.
 func (s *Stream) Do(req Request) Future {
 	if connectedReq, ok := req.(ConnectedRequest); ok {
-		if connectedReq.Conn() != s.Conn {
+		if connectedReq.Conn() != s.conn {
 			return NewFutureWithErr(req, errUnknownStreamRequest)
 		}
 	}
-	return s.Conn.send(req, s.Id)
+	return s.conn.send(req, s.id)
 }

tarantool_test.go

@@ -2112,7 +2112,7 @@ func TestStream_IdValues(t *testing.T) {
 
 	for _, id := range cases {
 		t.Run(fmt.Sprintf("%d", id), func(t *testing.T) {
-			stream.Id = id
+			SetStreamIdForTesting(stream, id)
 			_, err := stream.Do(req).Get()
 			require.NoError(t, err, "Failed to Ping")
 		})