From 0ace442800fffae2e946cc4bc4fc3451085b50a8 Mon Sep 17 00:00:00 2001
From: William Poussier <william.poussier@gmail.com>
Date: Sat, 17 Aug 2019 21:41:48 +0200
Subject: [PATCH] encoding/json: fix panic for nil instances of TextMarshaler
 in map keys

This change adds a a check in the encodeWithString.resolve method
to ensure that a reflect.Value with kind Ptr is not nil before
the type assertion to TextMarshaler.

If the value is nil, the method returns a nil error, and the map key
encodes to an empty string.

Fixes #33675
---
 src/encoding/json/encode.go      |  3 +++
 src/encoding/json/encode_test.go | 14 ++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index 67412763d64009..5c54e410f55996 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -872,6 +872,9 @@ func (w *reflectWithString) resolve() error {
 		return nil
 	}
 	if tm, ok := w.v.Interface().(encoding.TextMarshaler); ok {
+		if w.v.Kind() == reflect.Ptr && w.v.IsNil() {
+			return nil
+		}
 		buf, err := tm.MarshalText()
 		w.s = string(buf)
 		return err
diff --git a/src/encoding/json/encode_test.go b/src/encoding/json/encode_test.go
index bdf2a9f0792393..04ddb6f419d218 100644
--- a/src/encoding/json/encode_test.go
+++ b/src/encoding/json/encode_test.go
@@ -793,6 +793,20 @@ func TestTextMarshalerMapKeysAreSorted(t *testing.T) {
 	}
 }
 
+func TestIssue33675(t *testing.T) {
+	b, err := Marshal(map[*unmarshalerText]int{
+		(*unmarshalerText)(nil):    1,
+		&unmarshalerText{"A", "B"}: 2,
+	})
+	if err != nil {
+		t.Fatalf("Failed to Marshal *text.Marshaler: %v", err)
+	}
+	const want = `{"":1,"A:B":2}`
+	if string(b) != want {
+		t.Errorf("Marshal map with *text.Marshaler keys: got %#q, want %#q", b, want)
+	}
+}
+
 var re = regexp.MustCompile
 
 // syntactic checks on form of marshaled floating point numbers.