feat(log): implement custom error log filter to suppress "context can… (#470)

* feat(log): implement custom error log filter to suppress "context canceled" messages

fixes #446

Signed-off-by: Jason Cameron <git@jasoncameron.dev>

* feat(log): suppress 'context canceled' errors in HTTP logs

Signed-off-by: Jason Cameron <git@jasoncameron.dev>

---------

Signed-off-by: Jason Cameron <git@jasoncameron.dev>

internal/log.go

@@ -0,0 +1,59 @@
+package internal
+
+import (
+	"fmt"
+	"log"
+	"log/slog"
+	"net/http"
+	"os"
+	"strings"
+)
+
+func InitSlog(level string) {
+	var programLevel slog.Level
+	if err := (&programLevel).UnmarshalText([]byte(level)); err != nil {
+		fmt.Fprintf(os.Stderr, "invalid log level %s: %v, using info\n", level, err)
+		programLevel = slog.LevelInfo
+	}
+
+	leveler := &slog.LevelVar{}
+	leveler.Set(programLevel)
+
+	h := slog.NewJSONHandler(os.Stderr, &slog.HandlerOptions{
+		AddSource: true,
+		Level:     leveler,
+	})
+	slog.SetDefault(slog.New(h))
+}
+
+func GetRequestLogger(r *http.Request) *slog.Logger {
+	return slog.With(
+		"user_agent", r.UserAgent(),
+		"accept_language", r.Header.Get("Accept-Language"),
+		"priority", r.Header.Get("Priority"),
+		"x-forwarded-for",
+		r.Header.Get("X-Forwarded-For"),
+		"x-real-ip", r.Header.Get("X-Real-Ip"),
+	)
+}
+
+// ErrorLogFilter is used to suppress "context canceled" logs from the http server when a request is canceled (e.g., when a client disconnects).
+type ErrorLogFilter struct {
+	Unwrap *log.Logger
+}
+
+func (elf *ErrorLogFilter) Write(p []byte) (n int, err error) {
+	logMessage := string(p)
+	if strings.Contains(logMessage, "context canceled") {
+		return len(p), nil // Suppress the log by doing nothing
+	}
+	if elf.Unwrap != nil {
+		return elf.Unwrap.Writer().Write(p)
+	}
+	return len(p), nil
+}
+
+func GetFilteredHTTPLogger() *log.Logger {
+	stdErrLogger := log.New(os.Stderr, "", log.LstdFlags) // essentially what the default logger is.
+	return log.New(&ErrorLogFilter{Unwrap: stdErrLogger}, "", 0)
+}