feat: add a middleware to log requests

doc.go

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+// Package mw provides a collection of middlewares for net/http.
+package mw

go.mod

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+module code.bcarlin.net/go/mw
+
+go 1.20

log_request.go

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+package mw
+
+import (
+	"fmt"
+	"log/slog"
+	"net/http"
+	"time"
+)
+
+const (
+	logRequest  = "request"
+	logResponse = "response"
+	logMethod   = "method"
+	logURI      = "uri"
+	logStatus   = "status"
+	logSize     = "size"
+	logDuration = "duration"
+)
+
+// LoggingMiddleware returns a middleware that logs the request and response,
+// each in a loggroup.
+//
+// The request group contains the request method and URI.
+// The response group contains the response status, size and duration.
+func LoggingMiddleware(l *slog.Logger) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			startTime := time.Now()
+
+			lwr := &loggingResponseWriter{
+				ResponseWriter: w,
+				data:           &responseData{status: http.StatusOK, size: 0},
+			}
+
+			next.ServeHTTP(lwr, r)
+
+			l.InfoContext(r.Context(), "HTTP request answered",
+				slog.Group(
+					logRequest,
+					slog.String(logMethod, r.Method),
+					slog.String(logURI, r.RequestURI),
+				),
+				slog.Group(
+					logResponse,
+					slog.Int(logStatus, lwr.data.status),
+					slog.Int(logSize, lwr.data.size),
+					slog.Duration(logDuration, time.Since(startTime)),
+				),
+			)
+		})
+	}
+}
+
+// Struct for holding response details.
+type responseData struct {
+	status int
+	size   int
+}
+
+// loggingResponseWriter is a wrapper for http.ResponseWriter that records the
+// size of data sent and the status code of the response.
+type loggingResponseWriter struct {
+	http.ResponseWriter // compose original http.ResponseWriter
+	data                *responseData
+}
+
+// Write implements http.ResponseWriter.
+func (r *loggingResponseWriter) Write(b []byte) (int, error) {
+	size, err := r.ResponseWriter.Write(b)
+	r.data.size += size
+
+	if err != nil {
+		return size, fmt.Errorf("cannot forward data: %w", err)
+	}
+
+	return size, nil
+}
+
+// WriteHeader implements http.ResponseWriter.
+func (r *loggingResponseWriter) WriteHeader(statusCode int) {
+	r.ResponseWriter.WriteHeader(statusCode)
+	r.data.status = statusCode
+}