fix: prevent stack overflow on concurrent transport close

When multiple MCP transport sessions close simultaneously (server
restart, network partition, mass disconnect), the onclose callbacks
can trigger recursive close operations that overflow the call stack.
The process stays alive but becomes completely unresponsive.

Added a _closing guard flag that prevents re-entrant close() calls.
Once close() is entered, subsequent calls return immediately.

Fixes #1699

Author: Charlie Tonneslan

.changeset/prevent-recursive-close.md

@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/server": patch
+---
+
+Prevent stack overflow when multiple transports close simultaneously by guarding against re-entrant close() calls

packages/server/src/server/streamableHttp.ts

@@ -238,6 +238,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
     private _allowedOrigins?: string[];
     private _enableDnsRebindingProtection: boolean;
     private _retryInterval?: number;
+    private _closing: boolean = false;
     private _supportedProtocolVersions: string[];
 
     sessionId?: string;
@@ -887,6 +888,14 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
     }
 
     async close(): Promise<void> {
+        // Guard against re-entrant close to prevent recursive stack overflow
+        // when multiple transports close simultaneously and onclose triggers
+        // additional close operations.
+        if (this._closing) {
+            return;
+        }
+        this._closing = true;
+
         // Close all SSE connections
         for (const { cleanup } of this._streamMapping.values()) {
             cleanup();