mcp: protect ioConn.protocolVersion with a mutex (#832)

This is done to prevent concurrent reads and writes, which violate Go's
memory model. The concurrent write may be triggered e.g. by setting the
log level on
the client side.

Fixes #828

mcp/transport.go

@@ -344,7 +344,11 @@ func (r rwc) Close() error {
 //
 // See [msgBatch] for more discussion of message batching.
 type ioConn struct {
-	protocolVersion string // negotiated version, set during session initialization.
+	// protocolVersion is the negotiated version of the protocol,
+	// set during session initialization.
+	// Since writes may be concurrent to reads, we need to guard this with a mutex.
+	sessionMu       sync.Mutex
+	protocolVersion string
 
 	writeMu sync.Mutex         // guards Write, which must be concurrency safe.
 	rwc     io.ReadWriteCloser // the underlying stream
@@ -430,9 +434,15 @@ func (c *ioConn) sessionUpdated(state ServerSessionState) {
 		protocolVersion = state.InitializeParams.ProtocolVersion
 	}
 	if protocolVersion == "" {
+		// 2025-03-26 is used, because it's the last spec version
+		// where specifying the protocol version in the HTTP header
+		// was not required.
 		protocolVersion = protocolVersion20250326
 	}
-	c.protocolVersion = negotiatedVersion(protocolVersion)
+	protocolVersion = negotiatedVersion(protocolVersion)
+	c.sessionMu.Lock()
+	c.protocolVersion = protocolVersion
+	c.sessionMu.Unlock()
 }
 
 // addBatch records a msgBatch for an incoming batch payload.
@@ -533,8 +543,12 @@ func (t *ioConn) Read(ctx context.Context) (jsonrpc.Message, error) {
 	if err != nil {
 		return nil, err
 	}
-	if batch && t.protocolVersion >= protocolVersion20250618 {
-		return nil, fmt.Errorf("JSON-RPC batching is not supported in %s and later (request version: %s)", protocolVersion20250618, t.protocolVersion)
+	var protocolVersion string
+	t.sessionMu.Lock()
+	protocolVersion = t.protocolVersion
+	t.sessionMu.Unlock()
+	if batch && protocolVersion >= protocolVersion20250618 {
+		return nil, fmt.Errorf("JSON-RPC batching is not supported in %s and later (request version: %s)", protocolVersion20250618, protocolVersion)
 	}
 
 	t.queue = msgs[1:]