From 996d8dcc32b44738a77e4fc5555ce4c82a962a0a Mon Sep 17 00:00:00 2001
From: ffenix113 <razerer@bigmir.net>
Date: Wed, 24 Jun 2020 21:30:38 +0200
Subject: [PATCH]  Fix UpdateContext panic with empty Context

---
 log.go      |  3 +++
 log_test.go | 16 ++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/log.go b/log.go
index cbf68850..d02b15e3 100644
--- a/log.go
+++ b/log.go
@@ -252,6 +252,9 @@ func (l *Logger) UpdateContext(update func(c Context) Context) {
 	if cap(l.context) == 0 {
 		l.context = make([]byte, 0, 500)
 	}
+	if len(l.context) == 0 {
+		l.context = enc.AppendBeginMarker(l.context)
+	}
 	c := update(Context{*l})
 	l.context = c.l.context
 }
diff --git a/log_test.go b/log_test.go
index 2129bb9d..68ffe56a 100644
--- a/log_test.go
+++ b/log_test.go
@@ -772,3 +772,19 @@ func TestErrorHandler(t *testing.T) {
 		t.Errorf("ErrorHandler err = %#v, want %#v", got, want)
 	}
 }
+
+func TestUpdateEmptyContext(t *testing.T) {
+	var buf bytes.Buffer
+	log := New(&buf)
+
+	log.UpdateContext(func(c Context) Context {
+		return c.Str("foo", "bar")
+	})
+	log.Info().Msg("no panic")
+
+	want := `{"level":"info","foo":"bar","message":"no panic"}` + "\n"
+
+	if got := buf.String(); got != want {
+		t.Errorf("invalid log output:\ngot:  %q\nwant: %q", got, want)
+	}
+}