test(uploads): pin the deployed idle bound and its unavailable warning (#56)

Gatekeeper review of PR #63 (comment 15139):
- the 60 s default is what production runs (cmd/tapir never sets
  UploadIdleTimeout) and no test covered it; 1 ns, 60 ms or 24 h all left
  the suite green. Pinned via export_test.go, plus a socket test with the
  timeout unset that accepts a body with 100 ms gaps (60 ms mutant red).
- the "upload idle bound unavailable" warning, the only signal that the
  control did not apply, is now asserted (removing it goes red).
- the deadline clear on io.EOF was redundant (net/http resets the
  deadline before its background read) and is removed; ADR-033 text
  corrected (the part can end before the body, so "cleared on clean
  EOF" did not hold).

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

DECISIONS.md

@@ -1465,9 +1465,12 @@ an hour of Swedish *was* an unknown here; it is now measured, see decision 8.)
    2026-10-04)**: a `ReadTimeout` would cut legitimate 200 MiB uploads on slow links, so instead
    every read of the upload body re-arms the connection's read deadline 60 s ahead
    (`App.UploadIdleTimeout`). A body that stops delivering bytes fails its read, Accept returns
-   and the slot is released; one that keeps moving is never cut. The deadline is cleared only on
-   a clean end of body and stays armed after an error, because net/http drains an unread
-   remainder before replying. It needs `http.ResponseController` to reach the connection, so the
+   and the slot is released; one that keeps moving is never cut. The handler never clears the
+   deadline: an armed deadline also bounds net/http's drain of an unread remainder before it
+   replies, and net/http resets it itself before its background read. A deadline still armed
+   while the file is probed (the multipart part can end before the body's last bytes arrive)
+   costs at most keep-alive reuse of that connection, not the upload. The 60 s default is the
+   deployed value (cmd/tapir does not set it) and is pinned by a test. It needs `http.ResponseController` to reach the connection, so the
    metrics middleware's writer implements `Unwrap`; where a deadline cannot be set the handler
    logs `upload idle bound unavailable` rather than silently running unbounded. Still not
    bounded: a body that drips one byte per 59 s holds its slot for as long as it keeps that up.

internal/web/export_test.go

@@ -0,0 +1,5 @@
+package web
+
+// DefaultUploadIdleTimeout exposes the production idle bound (#56) to the black-box tests:
+// cmd/tapir never sets App.UploadIdleTimeout, so this constant IS the deployed value.
+const DefaultUploadIdleTimeout = defaultUploadIdleTimeout

internal/web/uploads.go

@@ -172,15 +172,10 @@ type idleDeadlineBody struct {
 
 func (b *idleDeadlineBody) Read(p []byte) (int, error) {
 	_ = b.rc.SetReadDeadline(time.Now().Add(b.idle))
-	n, err := b.ReadCloser.Read(p)
-	if errors.Is(err, io.EOF) {
-		// The body is complete: disarm, so the deadline cannot fire on the server's own
-		// background read while the handler is still probing the file. On any other error
-		// it stays armed: net/http drains an unread remainder before replying, and that
-		// drain must stay bounded too.
-		_ = b.rc.SetReadDeadline(time.Time{})
-	}
-	return n, err
+	// Never cleared here: net/http clears the read deadline itself before its background
+	// read once the handler has the body, and an armed deadline after an error also bounds
+	// net/http's drain of an unread remainder before it replies.
+	return b.ReadCloser.Read(p)
 }
 
 // abandoned reports, logs at INFO and answers 499, because the client has already gone: its request

internal/web/uploads_idle_handler_test.go

@@ -9,6 +9,7 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"log/slog"
 	"mime/multipart"
 	"net"
 	"net/http"
@@ -21,6 +22,7 @@ import (
 
 	"git.d-ma.be/mathias/tapir/internal/metrics"
 	"git.d-ma.be/mathias/tapir/internal/uploads"
+	"git.d-ma.be/mathias/tapir/internal/web"
 )
 
 func multipartBody(t *testing.T, content []byte) (body []byte, contentType string) {
@@ -112,3 +114,39 @@ func TestUploadPost_TheReadDeadlineIsReArmedOnEveryReadSoASlowButMovingBodyIsAcc
 
 	require.Equal(t, http.StatusSeeOther, readStatus(t, c, 3*time.Second))
 }
+
+// cmd/tapir never sets UploadIdleTimeout, so the default is the value production runs.
+// Shorter and legitimate uploads on a slow link are cut; much longer and the bound is moot.
+func TestDefaultUploadIdleTimeoutIsSixtySeconds(t *testing.T) {
+	require.Equal(t, 60*time.Second, web.DefaultUploadIdleTimeout)
+}
+
+// With UploadIdleTimeout unset the default applies: a body with 100 ms gaps between reads is
+// accepted (a default in milliseconds or nanoseconds would cut it).
+func TestUploadPost_AnUnsetIdleTimeoutAppliesTheDefaultAndAcceptsABodyWithSubSecondGaps(t *testing.T) {
+	addr := newIdleBoundServer(t, 0)
+	body, ct := multipartBody(t, bytes.Repeat([]byte("a"), 1000))
+
+	c := rawUpload(t, addr, ct, len(body))
+	const chunks = 5
+	step := len(body)/chunks + 1
+	for i := 0; i < len(body); i += step {
+		time.Sleep(100 * time.Millisecond)
+		_, err := c.Write(body[i:min(i+step, len(body))])
+		require.NoError(t, err)
+	}
+
+	require.Equal(t, http.StatusSeeOther, readStatus(t, c, 3*time.Second))
+}
+
+// A writer that cannot set a read deadline leaves the body unbounded; that must be said in
+// the log, not happen silently. The in-process recorder is such a writer.
+func TestUploadPost_AWriterWithoutReadDeadlineSupportLogsThatTheIdleBoundIsUnavailable(t *testing.T) {
+	app, _ := newUploadApp(t, audioOK, 200<<20)
+	buf := &lockedBuf{}
+	app.Log = slog.New(slog.NewTextHandler(buf, nil))
+
+	do(t, app, uploadReq(t, "t", "a.m4a", []byte("x")))
+
+	require.Contains(t, buf.String(), "upload idle bound unavailable")
+}