fix(nano): refuse hasJoined requests that declare a body

A GET to hasJoined with a Content-Length and no body held its
connection indefinitely. The handler returned, but net/http tries to
drain an unread body before it writes the answer, and nothing bounds
that wait: ReadHeaderTimeout ends with the headers. One such request
per socket pins a goroutine and a descriptor on nano or on felis-api's
internal face.

Velocity never sends a body, so any request that declares one, including
a chunked one, now gets a 400 with Connection: close, which skips the
drain and releases the connection once the answer is written.

The new subtest writes that request over a raw socket and waits three
seconds for an answer. Before the change it times out with no response
at all; now it reads a 400 marked close.

internal/api/handlers_hasjoined.go

@@ -106,6 +106,15 @@ func HasJoinedHandler(sources []AuthSource, repo Repo) http.Handler {
 // service token. A rejected login is 204 No Content — exactly what Mojang returns for an
 // invalid session, which authlib maps to "failed to verify username".
 func (a *API) handleHasJoined(w http.ResponseWriter, r *http.Request) {
+	// Velocity sends no body. When a request declares one anyway, net/http tries to drain it
+	// before writing any answer, so one that never arrives holds the connection with no
+	// timeout: ReadHeaderTimeout stopped at the headers. Marking the reply as the last one on
+	// this connection skips the drain.
+	if r.ContentLength != 0 {
+		w.Header().Set("Connection", "close")
+		w.WriteHeader(http.StatusBadRequest)
+		return
+	}
 	q := r.URL.Query()
 	username, serverID := q.Get("username"), q.Get("serverId")
 	if username == "" || serverID == "" {

internal/api/handlers_hasjoined_test.go

@@ -1,14 +1,18 @@
 package api
 
 import (
+	"bufio"
 	"encoding/hex"
 	"encoding/json"
+	"io"
+	"net"
 	"net/http"
 	"net/http/httptest"
 	"path"
 	"strings"
 	"sync/atomic"
 	"testing"
+	"time"
 
 	"github.com/google/uuid"
 )
@@ -311,6 +315,29 @@ func TestHasJoined(t *testing.T) {
 		}
 	})
 
+	// A GET that declares a body it never sends must still be answered and lose its
+	// connection; otherwise each such socket stays open for as long as the client likes.
+	t.Run("request declaring a body is refused and closed", func(t *testing.T) {
+		api := newTestAPI(newFakeRepo(), newFakeCluster())
+		srv := httptest.NewServer(api.InternalHandler())
+		t.Cleanup(srv.Close)
+		conn, err := net.Dial("tcp", srv.Listener.Addr().String())
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer conn.Close()
+		_, _ = io.WriteString(conn, "GET /session/minecraft/hasJoined?username=a&serverId=b HTTP/1.1\r\nHost: x\r\nContent-Length: 1000\r\n\r\n")
+		_ = conn.SetReadDeadline(time.Now().Add(3 * time.Second))
+		resp, err := http.ReadResponse(bufio.NewReader(conn), nil)
+		if err != nil {
+			t.Fatalf("no answer while the declared body never arrives: %v", err)
+		}
+		resp.Body.Close()
+		if resp.StatusCode != http.StatusBadRequest || !resp.Close {
+			t.Fatalf("code = %d close = %v, want 400 with Connection: close", resp.StatusCode, resp.Close)
+		}
+	})
+
 	// Missing query fields → 204 without touching any source.
 	t.Run("missing username -> 204", func(t *testing.T) {
 		api := newTestAPI(newFakeRepo(), newFakeCluster())