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mcp: systematically improve streamable client errors

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The streamable client connection can break for a variety of reasons,
asynchronously to the client's request. Decorate these failures with
additional context to clarify why they occurred.

Add a test for the failure message of #393.

Fixes #393
This commit is contained in:
Rob Findley 2025-09-04 17:16:14 +00:00 • committed by Robert Findley
commit 2c40bdc4d7
1 changed files with 114 additions and 21 deletions
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43
mcp/streamable.go
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@ -1130,7 +1130,7 @@ func (c *streamableClientConn) sessionUpdated(state clientSessionState) {
// § 2.5: A server using the Streamable HTTP transport MAY assign a session
// ID at initialization time, by including it in an Mcp-Session-Id header
// on the HTTP response containing the InitializeResult.
go c.handleSSE(nil, true, nil)
go c.handleSSE("hanging GET", nil, true, nil)
}
// fail handles an asynchronous error while reading.
@ -1224,24 +1224,27 @@ func (c *streamableClientConn) Write(ctx context.Context, msg jsonrpc.Message) e
return nil
}
var requestSummary string
switch msg := msg.(type) {
case *jsonrpc.Request:
requestSummary = fmt.Sprintf("sending %q", msg.Method)
case *jsonrpc.Response:
requestSummary = fmt.Sprintf("sending jsonrpc response #%d", msg.ID)
default:
panic("unreachable")
}
switch ct := resp.Header.Get("Content-Type"); ct {
case "application/json":
go c.handleJSON(resp)
go c.handleJSON(requestSummary, resp)
case "text/event-stream":
jsonReq, _ := msg.(*jsonrpc.Request)
go c.handleSSE(resp, false, jsonReq)
go c.handleSSE(requestSummary, resp, false, jsonReq)
default:
resp.Body.Close()
switch msg := msg.(type) {
case *jsonrpc.Request:
return fmt.Errorf("unsupported content type %q when sending %q (status: %d)", ct, msg.Method, resp.StatusCode)
case *jsonrpc.Response:
return fmt.Errorf("unsupported content type %q when sending jsonrpc response #%d (status: %d)", ct, msg.ID, resp.StatusCode)
default:
panic("unreachable")
}
return fmt.Errorf("%s: unsupported content type %q", requestSummary, ct)
}
return nil
}
@ -1265,16 +1268,16 @@ func (c *streamableClientConn) setMCPHeaders(req *http.Request) {
}
}
func (c *streamableClientConn) handleJSON(resp *http.Response) {
func (c *streamableClientConn) handleJSON(requestSummary string, resp *http.Response) {
body, err := io.ReadAll(resp.Body)
resp.Body.Close()
if err != nil {
c.fail(err)
c.fail(fmt.Errorf("%s: failed to read body: %v", requestSummary, err))
return
}
msg, err := jsonrpc.DecodeMessage(body)
if err != nil {
c.fail(fmt.Errorf("failed to decode response: %v", err))
c.fail(fmt.Errorf("%s: failed to decode response: %v", requestSummary, err))
return
}
select {
@ -1289,12 +1292,12 @@ func (c *streamableClientConn) handleJSON(resp *http.Response) {
//
// If forReq is set, it is the request that initiated the stream, and the
// stream is complete when we receive its response.
func (c *streamableClientConn) handleSSE(initialResp *http.Response, persistent bool, forReq *jsonrpc2.Request) {
func (c *streamableClientConn) handleSSE(requestSummary string, initialResp *http.Response, persistent bool, forReq *jsonrpc2.Request) {
resp := initialResp
var lastEventID string
for {
if resp != nil {
eventID, clientClosed := c.processStream(resp, forReq)
eventID, clientClosed := c.processStream(requestSummary, resp, forReq)
lastEventID = eventID
// If the connection was closed by the client, we're done.
@ -1312,7 +1315,7 @@ func (c *streamableClientConn) handleSSE(initialResp *http.Response, persistent
newResp, err := c.reconnect(lastEventID)
if err != nil {
// All reconnection attempts failed: fail the connection.
c.fail(err)
c.fail(fmt.Errorf("%s: failed to reconnect: %v", requestSummary, err))
return
}
resp = newResp
@ -1323,7 +1326,7 @@ func (c *streamableClientConn) handleSSE(initialResp *http.Response, persistent
}
if resp.StatusCode < 200 || resp.StatusCode >= 300 {
resp.Body.Close()
c.fail(fmt.Errorf("failed to reconnect: %v", http.StatusText(resp.StatusCode)))
c.fail(fmt.Errorf("%s: failed to reconnect: %v", requestSummary, http.StatusText(resp.StatusCode)))
return
}
// Reconnection was successful. Continue the loop with the new response.
@ -1334,7 +1337,7 @@ func (c *streamableClientConn) handleSSE(initialResp *http.Response, persistent
// incoming channel. It returns the ID of the last processed event and a flag
// indicating if the connection was closed by the client. If resp is nil, it
// returns "", false.
func (c *streamableClientConn) processStream(resp *http.Response, forReq *jsonrpc.Request) (lastEventID string, clientClosed bool) {
func (c *streamableClientConn) processStream(requestSummary string, resp *http.Response, forReq *jsonrpc.Request) (lastEventID string, clientClosed bool) {
defer resp.Body.Close()
for evt, err := range scanEvents(resp.Body) {
if err != nil {
@ -1347,7 +1350,7 @@ func (c *streamableClientConn) processStream(resp *http.Response, forReq *jsonrp
msg, err := jsonrpc.DecodeMessage(evt.Data)
if err != nil {
c.fail(fmt.Errorf("failed to decode event: %v", err))
c.fail(fmt.Errorf("%s: failed to decode event: %v", requestSummary, err))
return "", true
}