encoding/json: don't mangle strings in an edge case when decoding

The added comment contains some context. The original optimization
assumed that each call to unquoteBytes (or unquote) followed its
corresponding call to rescanLiteral. Otherwise, unquoting a literal
might use d.safeUnquote from another re-scanned literal.

Unfortunately, this assumption is wrong. When decoding {"foo": "bar"}
into a map[T]string where T implements TextUnmarshaler, the sequence of
calls would be as follows:

	1) rescanLiteral "foo"
	2) unquoteBytes "foo"
	3) rescanLiteral "bar"
	4) unquoteBytes "foo" (for UnmarshalText)
	5) unquoteBytes "bar"

Note that the call to UnmarshalText happens in literalStore, which
repeats the work to unquote the input string literal. But, since that
happens after we've re-scanned "bar", we're using the wrong safeUnquote
field value.

In the added test case, the second string had a non-zero number of safe
bytes, and the first string had none since it was all non-ASCII. Thus,
"safely" unquoting a number of the first string's bytes could cut a rune
in half, and thus mangle the runes.

A rather simple fix, without a full revert, is to only allow one use of
safeUnquote per call to unquoteBytes. Each call to rescanLiteral when
we have a string is soon followed by a call to unquoteBytes, so it's no
longer possible for us to use the wrong index.

Also add a test case from #38126, which is the same underlying bug, but
affecting the ",string" option.

Before the fix, the test would fail, just like in the original two issues:

	--- FAIL: TestUnmarshalRescanLiteralMangledUnquote (0.00s)
	    decode_test.go:2443: Key "开源" does not exist in map: map[开���:12345开源]
	    decode_test.go:2458: Unmarshal unexpected error: json: invalid use of ,string struct tag, trying to unmarshal "\"aaa\tbbb\"" into string

Fixes #38105.
For #38126.

Change-Id: I761e54924e9a971a4f9eaa70bbf72014bb1476e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/226218
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

decode.go

@@ -1221,6 +1221,11 @@ func (d *decodeState) unquoteBytes(s []byte) (t []byte, ok bool) {
 	if r == -1 {
 		return s, true
 	}
+	// Only perform up to one safe unquote for each re-scanned string
+	// literal. In some edge cases, the decoder unquotes a literal a second
+	// time, even after another literal has been re-scanned. Thus, only the
+	// first unquote can safely use safeUnquote.
+	d.safeUnquote = 0
 
 	b := make([]byte, len(s)+2*utf8.UTFMax)
 	w := copy(b, s[0:r])

decode_test.go

@@ -2432,7 +2432,7 @@ func (m *textUnmarshalerString) UnmarshalText(text []byte) error {
 	return nil
 }
 
-// Test unmarshal to a map, with map key is a user defined type.
+// Test unmarshal to a map, where the map key is a user defined type.
 // See golang.org/issues/34437.
 func TestUnmarshalMapWithTextUnmarshalerStringKey(t *testing.T) {
 	var p map[textUnmarshalerString]string
@@ -2441,7 +2441,36 @@ func TestUnmarshalMapWithTextUnmarshalerStringKey(t *testing.T) {
 	}
 
 	if _, ok := p["foo"]; !ok {
-		t.Errorf(`Key "foo" is not existed in map: %v`, p)
+		t.Errorf(`Key "foo" does not exist in map: %v`, p)
+	}
+}
+
+func TestUnmarshalRescanLiteralMangledUnquote(t *testing.T) {
+	// See golang.org/issues/38105.
+	var p map[textUnmarshalerString]string
+	if err := Unmarshal([]byte(`{"开源":"12345开源"}`), &p); err != nil {
+		t.Fatalf("Unmarshal unexpected error: %v", err)
+	}
+	if _, ok := p["开源"]; !ok {
+		t.Errorf(`Key "开源" does not exist in map: %v`, p)
+	}
+
+	// See golang.org/issues/38126.
+	type T struct {
+		F1 string `json:"F1,string"`
+	}
+	t1 := T{"aaa\tbbb"}
+
+	b, err := Marshal(t1)
+	if err != nil {
+		t.Fatalf("Marshal unexpected error: %v", err)
+	}
+	var t2 T
+	if err := Unmarshal(b, &t2); err != nil {
+		t.Fatalf("Unmarshal unexpected error: %v", err)
+	}
+	if t1 != t2 {
+		t.Errorf("Marshal and Unmarshal roundtrip mismatch: want %q got %q", t1, t2)
 	}
 }