fix(uploads): bound the silence in an upload body with a re-armed read deadline

A body that stopped arriving held one of the receive slots (#47) and a
spool file indefinitely. Every read of the upload body now pushes the
connection read deadline UploadIdleTimeout (default 60 s) ahead, so a
stalled body fails its read, Accept returns and the slot is released,
while a slow but moving body is never cut (ReadTimeout would cut it).

Two things found by running it over a socket:
- metrics.statusWriter had no Unwrap, so http.ResponseController could
  not reach the connection and the bound silently did not apply in the
  production handler stack. It now unwraps; a writer that still cannot
  set a deadline logs "upload idle bound unavailable".
- clearing the deadline on any read error left net/http's pre-reply
  drain of the unread remainder unbounded, so the connection hung. The
  deadline is now cleared only on a clean io.EOF.

ADR-033 limitation amended. A 1-byte-per-59-s drip is still unbounded.

Closes #56

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/uploads_idle_handler_test.go

@@ -0,0 +1,114 @@
+package web_test
+
+// #56: a request body that stops arriving must not hold a receive slot forever. The bound is a
+// read deadline on the connection, re-armed on every read, so it only exercises over a real
+// socket and through the production handler stack (metrics.HTTPMiddleware wraps the writer).
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"fmt"
+	"mime/multipart"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	"git.d-ma.be/mathias/tapir/internal/metrics"
+	"git.d-ma.be/mathias/tapir/internal/uploads"
+)
+
+func multipartBody(t *testing.T, content []byte) (body []byte, contentType string) {
+	t.Helper()
+	var buf bytes.Buffer
+	mw := multipart.NewWriter(&buf)
+	fw, err := mw.CreateFormFile("file", "a.m4a")
+	require.NoError(t, err)
+	_, err = fw.Write(content)
+	require.NoError(t, err)
+	require.NoError(t, mw.Close())
+	return buf.Bytes(), mw.FormDataContentType()
+}
+
+// rawUpload opens a connection and sends the request headers; the caller feeds the body.
+func rawUpload(t *testing.T, addr, contentType string, length int) net.Conn {
+	t.Helper()
+	c, err := net.Dial("tcp", addr)
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = c.Close() })
+	_, err = fmt.Fprintf(c, "POST /uploads HTTP/1.1\r\nHost: x\r\nContent-Type: %s\r\nContent-Length: %d\r\n\r\n", contentType, length)
+	require.NoError(t, err)
+	return c
+}
+
+// serverEnded reports whether the server ended the request (answered it or closed the
+// connection) before wait elapsed, rather than leaving the client waiting.
+func serverEnded(t *testing.T, c net.Conn, wait time.Duration) bool {
+	t.Helper()
+	require.NoError(t, c.SetReadDeadline(time.Now().Add(wait)))
+	_, err := http.ReadResponse(bufio.NewReader(c), nil)
+	var ne net.Error
+	return !errors.As(err, &ne) || !ne.Timeout()
+}
+
+// readStatus returns the response status code, or 0 if none arrived before wait elapsed.
+func readStatus(t *testing.T, c net.Conn, wait time.Duration) int {
+	t.Helper()
+	require.NoError(t, c.SetReadDeadline(time.Now().Add(wait)))
+	resp, err := http.ReadResponse(bufio.NewReader(c), nil)
+	if err != nil {
+		return 0
+	}
+	_ = resp.Body.Close()
+	return resp.StatusCode
+}
+
+func newIdleBoundServer(t *testing.T, idle time.Duration) (addr string) {
+	t.Helper()
+	app, _ := newUploadApp(t, audioOK, 200<<20)
+	app.UploadIdleTimeout = idle
+	app.Uploads.(*uploads.Intake).MaxReceivesPerUser = 1
+	srv := httptest.NewServer(metrics.HTTPMiddleware(app.Router()))
+	t.Cleanup(srv.Close)
+	return strings.TrimPrefix(srv.URL, "http://")
+}
+
+func TestUploadPost_AConnectionReadDeadlineEndsABodyThatStallsAndReleasesItsReceiveSlot(t *testing.T) {
+	addr := newIdleBoundServer(t, 200*time.Millisecond)
+	body, ct := multipartBody(t, bytes.Repeat([]byte("a"), 8000))
+
+	stalled := rawUpload(t, addr, ct, len(body))
+	_, err := stalled.Write(body[:4000]) // half the body, then nothing more
+	require.NoError(t, err)
+
+	require.True(t, serverEnded(t, stalled, 3*time.Second),
+		"the server never ended the stalled request: it still holds its slot")
+
+	next := rawUpload(t, addr, ct, len(body))
+	_, err = next.Write(body)
+	require.NoError(t, err)
+	require.Equal(t, http.StatusSeeOther, readStatus(t, next, 3*time.Second),
+		"the same user's next upload gets the slot the stalled one held")
+}
+
+func TestUploadPost_TheReadDeadlineIsReArmedOnEveryReadSoASlowButMovingBodyIsAccepted(t *testing.T) {
+	addr := newIdleBoundServer(t, 300*time.Millisecond)
+	body, ct := multipartBody(t, bytes.Repeat([]byte("a"), 2000))
+
+	c := rawUpload(t, addr, ct, len(body))
+	// ~1.2 s in total, four times the idle bound, but never 300 ms without a byte.
+	const chunks = 20
+	step := len(body)/chunks + 1
+	for i := 0; i < len(body); i += step {
+		_, err := c.Write(body[i:min(i+step, len(body))])
+		require.NoError(t, err)
+		time.Sleep(60 * time.Millisecond)
+	}
+
+	require.Equal(t, http.StatusSeeOther, readStatus(t, c, 3*time.Second))
+}