This PR fixes a race condition where `ServerSession.keepaliveCancel` was
accessed in `Close` concurrently with its initialization in
`ServerSession.startKeepalive`. It also removes a now redundant test.
The fix moves keepalive initialization into `ServerSession.Connect`
similar to the implementation in `ClientSession.Connect`. This is safe
because the MCP spec allows keepalive pings before the initialization
messages. From the MCP
[spec](https://modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle)
(emphasis added):
> The client SHOULD NOT send requests **other than pings** before the
server has responded to the initialize request.
The PR includes a reproducer test case, which fails with the following
error without this fix:
```
> go test -count=10 -race -run ^TestStreamableStatelessKeepaliveRace$ github.com/modelcontextprotocol/go-sdk/mcp 8s
==================
WARNING: DATA RACE
Write at 0x00c00007fbf0 by goroutine 1095:
github.com/modelcontextprotocol/go-sdk/mcp.startKeepalive()
/Users/benjamin/Documents/go-sdk/mcp/shared.go:590 +0x58
github.com/modelcontextprotocol/go-sdk/mcp.(*ServerSession).startKeepalive()
/Users/benjamin/Documents/go-sdk/mcp/server.go:1526 +0x130
github.com/modelcontextprotocol/go-sdk/mcp.(*ServerSession).initialized()
/Users/benjamin/Documents/go-sdk/mcp/server.go:1059 +0xf8
github.com/modelcontextprotocol/go-sdk/mcp.init.serverSessionMethod[go.shape.*uint8,go.shape.interface { GetMeta() map[string]interface {}; SetMeta(map[string]interface {}); github.com/modelcontextprotocol/go-sdk/mcp.isResult() }].func27()
/Users/benjamin/Documents/go-sdk/mcp/shared.go:332 +0x98
github.com/modelcontextprotocol/go-sdk/mcp.newServerMethodInfo[go.shape.*github.com/modelcontextprotocol/go-sdk/mcp.InitializedParams,go.shape.interface { GetMeta() map[string]interface {}; SetMeta(map[string]interface {}); github.com/modelcontextprotocol/go-sdk/mcp.isResult() },go.shape.struct { github.com/modelcontextprotocol/go-sdk/mcp.Meta "json:\"_meta,omitempty\"" }].func2()
/Users/benjamin/Documents/go-sdk/mcp/shared.go:272 +0x84
github.com/modelcontextprotocol/go-sdk/mcp.defaultReceivingMethodHandler[go.shape.*uint8]()
/Users/benjamin/Documents/go-sdk/mcp/shared.go:152 +0xe8
github.com/modelcontextprotocol/go-sdk/mcp.defaultReceivingMethodHandler[*github.com/modelcontextprotocol/go-sdk/mcp.ServerSession]()
/Users/benjamin/Documents/go-sdk/mcp/shared.go:146 +0x60
github.com/modelcontextprotocol/go-sdk/mcp.handleReceive[go.shape.*uint8]()
/Users/benjamin/Documents/go-sdk/mcp/shared.go:169 +0x228
github.com/modelcontextprotocol/go-sdk/mcp.(*ServerSession).handle()
/Users/benjamin/Documents/go-sdk/mcp/server.go:1445 +0x370
github.com/modelcontextprotocol/go-sdk/mcp.connect[go.shape.*uint8,go.shape.*uint8].func1.1()
/Users/benjamin/Documents/go-sdk/mcp/transport.go:170 +0x5c
github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.HandlerFunc.Handle()
/Users/benjamin/Documents/go-sdk/internal/jsonrpc2/jsonrpc2.go:84 +0x48
github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.(*Connection).handleAsync.func3()
/Users/benjamin/Documents/go-sdk/internal/jsonrpc2/conn.go:717 +0xd8
Previous read at 0x00c00007fbf0 by goroutine 1089:
github.com/modelcontextprotocol/go-sdk/mcp.(*ServerSession).Close()
/Users/benjamin/Documents/go-sdk/mcp/server.go:1502 +0x34
github.com/modelcontextprotocol/go-sdk/mcp.(*StreamableHTTPHandler).ServeHTTP.deferwrap1()
/Users/benjamin/Documents/go-sdk/mcp/streamable.go:519 +0x34
runtime.deferreturn()
/opt/homebrew/Cellar/go/1.25.4/libexec/src/runtime/panic.go:589 +0x5c
github.com/modelcontextprotocol/go-sdk/mcp.TestStreamableStatelessKeepaliveRace.mustNotPanic.func2()
/Users/benjamin/Documents/go-sdk/mcp/streamable_test.go:2156 +0x94
net/http.HandlerFunc.ServeHTTP()
/opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:2322 +0x48
net/http.serverHandler.ServeHTTP()
/opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:3340 +0x270
net/http.(*conn).serve()
/opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:2109 +0x9b0
net/http.(*Server).Serve.gowrap3()
/opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:3493 +0x48
Goroutine 1095 (running) created at:
github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.(*Connection).handleAsync()
/Users/benjamin/Documents/go-sdk/internal/jsonrpc2/conn.go:715 +0x334
github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.(*Connection).acceptRequest.func2.gowrap1()
/Users/benjamin/Documents/go-sdk/internal/jsonrpc2/conn.go:676 +0x34
Goroutine 1089 (running) created at:
net/http.(*Server).Serve()
/opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:3493 +0x5dc
net/http/httptest.(*Server).goServe.func1()
/opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/httptest/server.go:311 +0x98
==================
--- FAIL: TestStreamableStatelessKeepaliveRace (0.14s)
testing.go:1617: race detected during execution of test
FAIL
FAIL github.com/modelcontextprotocol/go-sdk/mcp 1.727s
FAIL
```
---------
Co-authored-by: Maciej Kisiel <mkisiel@google.com>
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