Fix underlying connection not being closed on protocol error (#64)

Before this commit, the underlying connection of `Conn` was not being
closed when a protocol error was encountered. This behavior contradicted
with `Conn.DisconnectNotify()` because it reported that the underlying
connection was being closed. Additionally, the underlying connection was
now orphaned because `Conn` was no longer processing any of the
subsequent requests.

With this commit, the underlying connection is now being closed when a
protocol error is encountered, matching what `Conn.DisconnectNotify()`
has already been reporting.

jsonrpc2.go

@@ -366,11 +366,10 @@ type Conn struct {
 
 	h Handler
 
-	mu       sync.Mutex
-	shutdown bool
-	closing  bool
-	seq      uint64
-	pending  map[ID]*call
+	mu      sync.Mutex
+	closed  bool
+	seq     uint64
+	pending map[ID]*call
 
 	sending sync.Mutex
 
@@ -417,13 +416,29 @@ func NewConn(ctx context.Context, stream ObjectStream, h Handler, opts ...ConnOp
 // Close closes the JSON-RPC connection. The connection may not be
 // used after it has been closed.
 func (c *Conn) Close() error {
+	return c.close(nil)
+}
+
+func (c *Conn) close(cause error) error {
+	c.sending.Lock()
 	c.mu.Lock()
-	if c.shutdown || c.closing {
-		c.mu.Unlock()
+	defer c.sending.Unlock()
+	defer c.mu.Unlock()
+
+	if c.closed {
 		return ErrClosed
 	}
-	c.closing = true
-	c.mu.Unlock()
+
+	for _, call := range c.pending {
+		close(call.done)
+	}
+
+	if cause != nil && cause != io.EOF && cause != io.ErrUnexpectedEOF {
+		c.logger.Printf("jsonrpc2: protocol error: %v\n", cause)
+	}
+
+	close(c.disconnect)
+	c.closed = true
 	return c.stream.Close()
 }
 
@@ -436,7 +451,7 @@ func (c *Conn) send(_ context.Context, m *anyMessage, wait bool) (cc *call, err
 	var id ID
 
 	c.mu.Lock()
-	if c.shutdown || c.closing {
+	if c.closed {
 		c.mu.Unlock()
 		return nil, ErrClosed
 	}
@@ -675,28 +690,7 @@ func (c *Conn) readMessages(ctx context.Context) {
 			}
 		}
 	}
-
-	c.sending.Lock()
-	c.mu.Lock()
-	c.shutdown = true
-	closing := c.closing
-	if err == io.EOF {
-		if closing {
-			err = ErrClosed
-		} else {
-			err = io.ErrUnexpectedEOF
-		}
-	}
-	for _, call := range c.pending {
-		call.done <- err
-		close(call.done)
-	}
-	c.mu.Unlock()
-	c.sending.Unlock()
-	if err != io.ErrUnexpectedEOF && !closing {
-		c.logger.Printf("jsonrpc2: protocol error: %v\n", err)
-	}
-	close(c.disconnect)
+	c.close(err)
 }
 
 // call represents a JSON-RPC call over its entire lifecycle.