feat: report delivery failures through OnError

README.md

@@ -107,6 +107,10 @@ type Option struct {
   // default: 10s
   Timeout time.Duration
 
+  // optional: called when a record cannot be delivered
+  // (default: errors are ignored)
+  OnError func(err error)
+
   // optional: customize record builder
   Converter Converter
   // optional: custom marshaler
@@ -120,6 +124,15 @@ type Option struct {
 }
 ```
 
+Delivery failures (marshaling error, network error, non-2xx response) are not returned by the logger. They are passed to `OnError`:
+
+```go
+slogbetterstack.Option{
+  Token:   "xxxxx",
+  OnError: func(err error) { fmt.Fprintf(os.Stderr, "slog-betterstack: %v\n", err) },
+}.NewBetterstackHandler()
+```
+
 Other global parameters:
 
 ```go

handler.go

@@ -4,6 +4,8 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"fmt"
+	"io"
 	"net/http"
 	"time"
 
@@ -14,17 +16,26 @@ import (
 
 const BetterstackEndpoint = "https://in.logs.betterstack.com/"
 
+// maxErrorBodyBytes caps how much of a rejected response is quoted in an error,
+// so a large HTML error page cannot flood the error handler.
+const maxErrorBodyBytes = 512
+
 type Option struct {
 	// log level (default: debug)
 	Level slog.Leveler
 
 	// token
-	Token   string
+	Token string
 	// optional: endpoint
 	Endpoint string
 	// default: 10s
 	Timeout time.Duration
 
+	// optional: called when a record cannot be delivered (marshaling failure,
+	// network error, non-2xx response).
+	// default: errors are ignored
+	OnError func(err error)
+
 	// optional: customize record builder
 	Converter Converter
 	// optional: custom marshaler
@@ -54,6 +65,10 @@ func (o Option) NewBetterstackHandler() slog.Handler {
 		o.Timeout = 10 * time.Second
 	}
 
+	if o.OnError == nil {
+		o.OnError = defaultOnError
+	}
+
 	if o.Converter == nil {
 		o.Converter = DefaultConverter
 	}
@@ -92,7 +107,10 @@ func (h *BetterstackHandler) Handle(ctx context.Context, record slog.Record) err
 	// non-blocking
 	go func() {
 		// @TODO: batching ?
-		_ = send(h.option.Endpoint, h.option.Token, h.option.Timeout, h.option.Marshaler, []map[string]any{payload})
+		err := send(h.option.Endpoint, h.option.Token, h.option.Timeout, h.option.Marshaler, []map[string]any{payload})
+		if err != nil {
+			h.option.OnError(err)
+		}
 	}()
 
 	return nil
@@ -151,5 +169,14 @@ func send(endpoint string, token string, timeout time.Duration, marshaler func(v
 
 	defer resp.Body.Close() //nolint:errcheck
 
+	if resp.StatusCode < 200 || resp.StatusCode > 299 {
+		// best effort: the status code alone is already actionable
+		snippet, _ := io.ReadAll(io.LimitReader(resp.Body, maxErrorBodyBytes))
+		return fmt.Errorf("betterstack rejected logs: status %d: %s", resp.StatusCode, bytes.TrimSpace(snippet))
+	}
+
 	return nil
 }
+
+// defaultOnError ignores errors: a logging library must not write to stderr unasked.
+func defaultOnError(error) {}

handler_test.go

@@ -0,0 +1,146 @@
+package slogbetterstack
+
+import (
+	"errors"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"strings"
+	"testing"
+	"time"
+)
+
+const testWait = 2 * time.Second // generous: delivery runs in a goroutine
+
+func newTestHandler(t *testing.T, status int, body string, onError func(error)) slog.Handler {
+	t.Helper()
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(status)
+		_, _ = w.Write([]byte(body))
+	}))
+	t.Cleanup(srv.Close)
+
+	return Option{Token: "t", Endpoint: srv.URL, OnError: onError}.NewBetterstackHandler()
+}
+
+func TestOnError_RejectedStatus(t *testing.T) {
+	for _, status := range []int{http.StatusUnauthorized, http.StatusInternalServerError, http.StatusMultipleChoices} {
+		errs := make(chan error, 1)
+		logger := slog.New(newTestHandler(t, status, "nope", func(err error) { errs <- err }))
+
+		logger.Info("hello")
+
+		select {
+		case err := <-errs:
+			if !strings.Contains(err.Error(), "nope") {
+				t.Fatalf("status %d: unexpected error %q", status, err)
+			}
+		case <-time.After(testWait):
+			t.Fatalf("status %d: OnError not called", status)
+		}
+	}
+}
+
+func TestOnError_BodyIsTruncated(t *testing.T) {
+	errs := make(chan error, 1)
+	logger := slog.New(newTestHandler(t, http.StatusBadRequest, strings.Repeat("x", 10*maxErrorBodyBytes), func(err error) { errs <- err }))
+
+	logger.Info("hello")
+
+	select {
+	case err := <-errs:
+		if got := strings.Count(err.Error(), "x"); got != maxErrorBodyBytes {
+			t.Fatalf("expected %d quoted bytes, got %d", maxErrorBodyBytes, got)
+		}
+	case <-time.After(testWait):
+		t.Fatal("OnError not called")
+	}
+}
+
+func TestOnError_NotCalledOnSuccess(t *testing.T) {
+	for _, status := range []int{http.StatusOK, http.StatusAccepted, 299} {
+		called := make(chan struct{}, 1)
+		done := make(chan struct{})
+		srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(status)
+			close(done)
+		}))
+
+		logger := slog.New(Option{Token: "t", Endpoint: srv.URL, OnError: func(error) { called <- struct{}{} }}.NewBetterstackHandler())
+		logger.Info("hello")
+
+		select {
+		case <-done:
+		case <-time.After(testWait):
+			t.Fatalf("status %d: request not received", status)
+		}
+		time.Sleep(50 * time.Millisecond) // let the sender goroutine return
+		srv.Close()
+
+		select {
+		case <-called:
+			t.Fatalf("status %d: OnError called on success", status)
+		default:
+		}
+	}
+}
+
+func TestOnError_NetworkAndMarshalFailure(t *testing.T) {
+	errs := make(chan error, 2)
+
+	srv := httptest.NewServer(http.NotFoundHandler())
+	endpoint := srv.URL
+	srv.Close() // connection refused from here on
+
+	slog.New(Option{Token: "t", Endpoint: endpoint, OnError: func(err error) { errs <- err }}.NewBetterstackHandler()).Info("hello")
+
+	marshalErr := errors.New("boom")
+	slog.New(Option{
+		Token:     "t",
+		Endpoint:  endpoint,
+		Marshaler: func(any) ([]byte, error) { return nil, marshalErr },
+		OnError:   func(err error) { errs <- err },
+	}.NewBetterstackHandler()).Info("hello")
+
+	sawMarshal := false
+	for i := 0; i < 2; i++ {
+		select {
+		case err := <-errs:
+			sawMarshal = sawMarshal || errors.Is(err, marshalErr)
+		case <-time.After(testWait):
+			t.Fatal("OnError not called")
+		}
+	}
+	if !sawMarshal {
+		t.Fatal("marshal error not reported")
+	}
+}
+
+func TestOnError_DefaultWritesNothing(t *testing.T) {
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	stderr := os.Stderr
+	os.Stderr = w
+	defer func() { os.Stderr = stderr }()
+
+	srv := httptest.NewServer(http.HandlerFunc(func(rw http.ResponseWriter, _ *http.Request) {
+		rw.WriteHeader(http.StatusUnauthorized)
+	}))
+	defer srv.Close()
+
+	h := Option{Token: "t", Endpoint: srv.URL}.NewBetterstackHandler()
+	slog.New(h).Info("hello")
+	time.Sleep(100 * time.Millisecond) // the failing send runs in a goroutine
+
+	_ = w.Close()
+	out, _ := io.ReadAll(r)
+	if len(out) != 0 {
+		t.Fatalf("expected no output by default, got %q", out)
+	}
+}