Return sink write errors from every handler (closes #22)

The console and JSON handlers threw away the error from their write to
stdout, so a lost log line looked delivered. They now return it,
wrapped. The webhook handler also returns an error for a status outside
2xx, and no longer follows redirects, which could resend the request
without the record. The multiplex handler passes the record to every
handler and returns their errors joined with errors.Join instead of
stopping at the first.

Both stdout handlers gain an unexported writer, nil meaning os.Stdout at
write time, so tests can supply a sink that fails. The README says what
each handler returns and that the slog.Logger methods discard a
handler's error.

Model: opus-5-5

README.md

@@ -115,7 +115,8 @@ the record:
 -   `ConsoleHandler` and `JSONHandler` return the error from their write to
     stdout, wrapped, so `errors.Is` still matches the original
 -   `WebhookHandler` returns an error when the request fails, and also when
-    the server answers with a status outside 2xx
+    the server answers with a status outside 2xx, a redirect included,
+    since it does not follow redirects
 -   `MultiplexHandler`, which simplelog installs as the default, passes the
     record to every handler it holds even after one of them fails, then
     returns all their errors joined with `errors.Join` (nil if none failed)

TODO.md

@@ -24,7 +24,7 @@ files it depends on: .golangci.yml, REPO_POLICIES.md, .editorconfig,
 
 # Completed Steps
 
-* 2026-10-06: every handler now returns a failed delivery from Handle
+* 2026-10-06: every handler now returns a failed delivery from `Handle`
     instead of discarding it: console and JSON return the stdout write
     error, the webhook also fails on a non-2xx answer, and the multiplex
     delivers to every handler and returns their errors joined

handler_errors_internal_test.go

@@ -134,3 +134,29 @@ func TestWebhookHandlerReturnsErrorOnServerError(t *testing.T) {
 		t.Fatalf("Handle returned %v, want an error for the 500 answer", err)
 	}
 }
+
+// Following the redirect would resend the request as a GET without the
+// record, and the GET is answered with 200, so only an error for the
+// redirect itself tells the caller the record was lost.
+func TestWebhookHandlerReturnsErrorOnRedirect(t *testing.T) {
+	t.Parallel()
+
+	server := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			if r.URL.Path != "/moved" {
+				http.Redirect(w, r, "/moved", http.StatusFound)
+			}
+		},
+	))
+	defer server.Close()
+
+	handler, err := NewWebhookHandler(server.URL)
+	if err != nil {
+		t.Fatalf("NewWebhookHandler: %v", err)
+	}
+
+	err = handler.Handle(context.Background(), errorTestRecord())
+	if !errors.Is(err, errWebhookStatus) {
+		t.Fatalf("Handle returned %v, want an error for the redirect", err)
+	}
+}

webhook_handler.go

@@ -19,6 +19,7 @@ var errWebhookStatus = errors.New("webhook did not accept the record")
 // URL.
 type WebhookHandler struct {
 	webhookURL string
+	client     *http.Client
 	attrs      handlerAttrs
 }
 
@@ -30,7 +31,17 @@ func NewWebhookHandler(webhookURL string) (*WebhookHandler, error) {
 		return nil, fmt.Errorf("invalid webhook URL: %w", err)
 	}
 
-	return &WebhookHandler{webhookURL: webhookURL}, nil
+	return &WebhookHandler{
+		webhookURL: webhookURL,
+		client: &http.Client{
+			// Following a redirect can resend the request as a GET
+			// without the record, so Handle gets the redirect answer
+			// itself and returns it as an error.
+			CheckRedirect: func(*http.Request, []*http.Request) error {
+				return http.ErrUseLastResponse
+			},
+		},
+	}, nil
 }
 
 // Enabled reports whether the handler processes records at the given
@@ -48,6 +59,7 @@ func (w *WebhookHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
 
 	return &WebhookHandler{
 		webhookURL: w.webhookURL,
+		client:     w.client,
 		attrs:      w.attrs.withAttrs(attrs),
 	}
 }
@@ -61,6 +73,7 @@ func (w *WebhookHandler) WithGroup(name string) slog.Handler {
 
 	return &WebhookHandler{
 		webhookURL: w.webhookURL,
+		client:     w.client,
 		attrs:      w.attrs.withGroup(name),
 	}
 }
@@ -86,7 +99,7 @@ func (w *WebhookHandler) Handle(ctx context.Context, record slog.Record) error {
 
 	request.Header.Set("Content-Type", "application/json")
 
-	response, err := http.DefaultClient.Do(request)
+	response, err := w.client.Do(request)
 	if err != nil {
 		return err
 	}