Allow user configuration of which logger to return from Ctx() (#343)

If a user is trying to fetch a logger from their context, they probably
want to log something.  In order to allow returning a useable logger from Ctx()
without breaking backwards compatibilty with the previous behavior, we make it
configurable.

ctx.go

@@ -39,10 +39,13 @@ func (l *Logger) WithContext(ctx context.Context) context.Context {
 }
 
 // Ctx returns the Logger associated with the ctx. If no logger
-// is associated, a disabled logger is returned.
+// is associated, DefaultContextLogger is returned, unless DefaultContextLogger
+// is nil, in which case a disabled logger is returned.
 func Ctx(ctx context.Context) *Logger {
 	if l, ok := ctx.Value(ctxKey{}).(*Logger); ok {
 		return l
+	} else if l = DefaultContextLogger; l != nil {
+		return l
 	}
 	return disabledLogger
 }

ctx_test.go

@@ -27,6 +27,13 @@ func TestCtx(t *testing.T) {
 	if log2 != disabledLogger {
 		t.Error("Ctx did not return the expected logger")
 	}
+
+	DefaultContextLogger = &log
+	t.Cleanup(func() { DefaultContextLogger = nil })
+	log2 = Ctx(context.Background())
+	if log2 != &log {
+		t.Error("Ctx did not return the expected logger")
+	}
 }
 
 func TestCtxDisabled(t *testing.T) {

globals.go

@@ -100,6 +100,10 @@ var (
 	// output. If not set, an error is printed on the stderr. This handler must
 	// be thread safe and non-blocking.
 	ErrorHandler func(err error)
+
+	// DefaultContextLogger is returned from Ctx() if there is no logger associated
+	// with the context.
+	DefaultContextLogger *Logger
 )
 
 var (