Summary

When SetReader is called (e.g., after enabling encryption), the new reader was not wrapped with fullReader. This caused incomplete reads for large packets because a single Read() call may return fewer bytes than requested.

Problem

The Decoder uses a fullReader wrapper to ensure complete reads via io.ReadFull:

func NewDecoder(r io.Reader, ...) *Decoder {
    return &Decoder{
        rd: &fullReader{r}, // using the fullReader is essential here!
        ...
    }
}

type fullReader struct{ io.Reader }
func (fr *fullReader) Read(p []byte) (int, error) { return io.ReadFull(fr.Reader, p) }

However, SetReader did not wrap the new reader:

func (d *Decoder) SetReader(rd io.Reader) {
    d.mu.Lock()
    d.rd = rd  // Missing fullReader wrapper!
    d.mu.Unlock()
}

When encryption is enabled, SetReader is called with a cipher.StreamReader, losing the fullReader protection. This causes large packets to be read incompletely, resulting in connection drops.

Fix

Wrap the reader with fullReader in SetReader():

func (d *Decoder) SetReader(rd io.Reader) {
    d.mu.Lock()
    d.rd = &fullReader{rd}
    d.mu.Unlock()
}

Testing

Tested with Syncmatica mod which sends large plugin messages (~100KB+) for schematic sharing. Before the fix, players were disconnected immediately after attempting to share schematics. After the fix, schematic sharing works correctly.

Related Issues

Likely related to #587 (large modpack disconnection issues)