Give the webhook handler a timeout (closes #38)

The webhook handler's client had no timeout, and slog calls the handler
inside the log call, so a server that accepted the connection and never
answered stopped that log call for good, and every later one. A request
still running after 5 seconds, reading the answer included, now fails
with a timeout error. The handler also reads the answer to the end
before closing it, so the connection is reused for the next record.

Two new tests point the handler at a server that never answers and at
one that sends its status and then stalls the answer, and check that
Handle returns a timeout error within the timeout. The README states
the timeout.

Model: opus-5-5

webhook_handler.go

@@ -6,15 +6,22 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"log/slog"
 	"net/http"
 	"net/url"
+	"time"
 )
 
 // errWebhookStatus is returned when the webhook answers with a status
 // outside 2xx.
 var errWebhookStatus = errors.New("webhook did not accept the record")
 
+// webhookTimeout bounds each webhook request, reading the answer
+// included. Handle runs inside the log call, so a server that never
+// answers would otherwise hold that call up for good.
+const webhookTimeout = 5 * time.Second
+
 // WebhookHandler POSTs each log record as JSON to a configured webhook
 // URL.
 type WebhookHandler struct {
@@ -34,6 +41,7 @@ func NewWebhookHandler(webhookURL string) (*WebhookHandler, error) {
 	return &WebhookHandler{
 		webhookURL: webhookURL,
 		client: &http.Client{
+			Timeout: webhookTimeout,
 			// 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.
@@ -80,7 +88,8 @@ func (w *WebhookHandler) WithGroup(name string) slog.Handler {
 
 // Handle marshals the record, with its attributes, to one JSON object and
 // POSTs it to the webhook URL. It returns an error when the request fails
-// or the server answers with a status outside 2xx.
+// or runs past webhookTimeout, or when the server answers with a status
+// outside 2xx.
 func (w *WebhookHandler) Handle(ctx context.Context, record slog.Record) error {
 	jsonData, err := json.Marshal(recordToMap(record, w.attrs))
 	if err != nil {
@@ -106,6 +115,15 @@ func (w *WebhookHandler) Handle(ctx context.Context, record slog.Record) error {
 
 	defer func() { _ = response.Body.Close() }()
 
+	// The answer is read to the end so the client can reuse the
+	// connection for the next record. The read counts toward
+	// webhookTimeout, so a server that sends its status and then stalls
+	// fails here.
+	_, err = io.Copy(io.Discard, response.Body)
+	if err != nil {
+		return fmt.Errorf("error reading webhook answer: %w", err)
+	}
+
 	if response.StatusCode < http.StatusOK ||
 		response.StatusCode >= http.StatusMultipleChoices {
 		return fmt.Errorf("%w: %s", errWebhookStatus, response.Status)