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client: allow re-using connections in more cases

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I am seeing a bug where connections with a Streamable HTTP client are
not reused at all. This makes performance terrible, since each request
establishes a new TCP connection.

I was able to root cause this down to behavior of the `processStream`
function. This sets up a bufio.NewScanner, and waits to get data. Once
we get a message, we abort and *close the connection*.

At this point, there are two paths:
* The bufio reader read the *full* response body to EOF, and then we
  closed. This case is fine; we can re-use the idle connection.
* The bufio reader *did not* real the full response body. It may have
  read the entire actual data that *would be sent*, but since its
streaming happened to not yet be delivered. In this case, we call
`body.Close()` before an EOF, which causes the HTTP client to not
attempt to re-use the connection.

In my case, the server I am using reliable sends the response over 2 TCP
packets (one with the event, and the other with the closure of the
`chunked` encoding (`0`)). This causes the body to never be fully read.

This PR fixes that case by unconditionally reading the full body before
we close the connection. With this change, I am able to reliably re-use
connections for future requests.
This commit is contained in:
John Howard 2025-12-10 13:45:36 -08:00 • committed by Robert Findley
commit b012d0824e
1 changed files with 5 additions and 1 deletions
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6
mcp/streamable.go
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@ -1835,7 +1835,11 @@ func (c *streamableClientConn) checkResponse(requestSummary string, resp *http.R
// indicating if the connection was closed by the client. If resp is nil, it
// returns "", false.
func (c *streamableClientConn) processStream(ctx context.Context, requestSummary string, resp *http.Response, forCall *jsonrpc.Request) (lastEventID string, reconnectDelay time.Duration, clientClosed bool) {
defer resp.Body.Close()
defer func() {
// Drain any remaining unprocessed body. This allows the connection to be re-used after closing.
io.Copy(io.Discard, resp.Body)
resp.Body.Close()
}()
for evt, err := range scanEvents(resp.Body) {
if err != nil {
if ctx.Err() != nil {