bplist: teach the parser about unusual OffsetIntSize values (#78)

This is only a partial fix which works for the value of 3 and for any
value < 8, but won't help in other cases. Supporting values < 16 should
also be possible, but I didn't bother with it. Also this doesn't touch
`bplist_generator` and hence won't make use of non-standard values for
generating plists (while those values could probably give a benefit of
slightly reduced plist size).

We confirmed that this is supported by the CoreFoundation bplist parser.

Closes #77

bplist_parser.go

@@ -137,7 +137,10 @@ func (p *bplistParser) parseSizedInteger(off offset, nbytes int) (lo uint64, hi
 	case 16:
 		lo, hi = binary.BigEndian.Uint64(p.buffer[off+8:]), binary.BigEndian.Uint64(p.buffer[off:])
 	default:
-		panic(errors.New("illegal integer size"))
+		if nbytes > 8 {
+			panic(errors.New("illegal integer size"))
+		}
+		lo, hi = binary.BigEndian.Uint64(p.buffer[off-(8-offset(nbytes)):]) & ((1<<offset(nbytes*8))-1), 0
 	}
 	newOffset = off + offset(nbytes)
 	return

bplist_test.go

@@ -139,3 +139,40 @@ func TestBplistLatin1ToUTF16(t *testing.T) {
 		return
 	}
 }
+
+func TestBplistNonPowerOfTwoOffsetIntSizes(t *testing.T) {
+	bplist := []byte{
+		'b', 'p', 'l', 'i', 's', 't', '0', '0',
+
+		// Array (2 entries)
+		0xA2,
+		0x01, 0x02,
+
+		// 0xFFFFFFFFFFFFFF80 (MinInt8, sign extended)
+		0x13, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x80,
+
+		// 0x7F (MaxInt8)
+		0x10, 0x7f,
+
+		// Offset table (each entry is 3 bytes)
+		0x00, 0x00, 0x08,
+		0x00, 0x00, 0x0b,
+		0x00, 0x00, 0x14,
+
+		// Trailer
+		0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00,
+		0x03,
+		0x01,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x03,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x16,
+	}
+
+	buf := bytes.NewReader(bplist)
+	d := newBplistParser(buf)
+	_, err := d.parseDocument()
+	if err != nil {
+		t.Error("Unexpected error", err)
+	}
+}