Use a function as the error logger

This commit addresses comment in the code review. Now, instead of
injecting an slog.Logger we can inject any method that can use a more
flexible implementation.

api/log/log.go

@@ -4,26 +4,32 @@ package log
 import (
 	"context"
 	"fmt"
-	"log/slog"
 	"net/http"
 	"os"
 
 	"github.com/pkg/errors"
 )
 
-// ErrorKey is the logging attribute key for error values.
-const ErrorKey = "error"
+type errorLoggerKey struct{}
 
-type loggerKey struct{}
+// ErrorLogger is the function type used to log errors.
+type ErrorLogger func(http.ResponseWriter, *http.Request, error)
 
-// NewContext creates a new context with the given slog.Logger.
-func NewContext(ctx context.Context, logger *slog.Logger) context.Context {
-	return context.WithValue(ctx, loggerKey{}, logger)
+func (fn ErrorLogger) call(w http.ResponseWriter, r *http.Request, err error) {
+	if fn == nil {
+		return
+	}
+	fn(w, r, err)
 }
 
-// FromContext returns the logger from the given context.
-func FromContext(ctx context.Context) (l *slog.Logger, ok bool) {
-	l, ok = ctx.Value(loggerKey{}).(*slog.Logger)
+// WithErrorLogger returns a new context with the given error logger.
+func WithErrorLogger(ctx context.Context, fn ErrorLogger) context.Context {
+	return context.WithValue(ctx, errorLoggerKey{}, fn)
+}
+
+// ErrorLoggerFromContext returns an error logger from the context.
+func ErrorLoggerFromContext(ctx context.Context) (fn ErrorLogger) {
+	fn, _ = ctx.Value(errorLoggerKey{}).(ErrorLogger)
 	return
 }
 
@@ -45,13 +51,10 @@ type fieldCarrier interface {
 // Error adds to the response writer the given error if it implements
 // logging.ResponseLogger. If it does not implement it, then writes the error
 // using the log package.
-func Error(rw http.ResponseWriter, r *http.Request, err error) {
-	ctx := r.Context()
-	if logger, ok := FromContext(ctx); ok && err != nil {
-		logger.ErrorContext(ctx, "request failed", slog.Any(ErrorKey, err))
-	}
+func Error(w http.ResponseWriter, r *http.Request, err error) {
+	ErrorLoggerFromContext(r.Context()).call(w, r, err)
 
-	fc, ok := rw.(fieldCarrier)
+	fc, ok := w.(fieldCarrier)
 	if !ok {
 		return
 	}

api/log/log_test.go

@@ -33,7 +33,11 @@ func TestError(t *testing.T) {
 	var buf bytes.Buffer
 	logger := slog.New(slog.NewJSONHandler(&buf, &slog.HandlerOptions{}))
 	req := httptest.NewRequest("GET", "/test", http.NoBody)
-	reqWithLogger := req.WithContext(NewContext(req.Context(), logger))
+	reqWithLogger := req.WithContext(WithErrorLogger(req.Context(), func(w http.ResponseWriter, r *http.Request, err error) {
+		if err != nil {
+			logger.ErrorContext(r.Context(), "request failed", slog.Any("error", err))
+		}
+	}))
 
 	tests := []struct {
 		name string