QBitArray: fix fromBits() and actually test it

When I initially added it, it was ony for QCborValue, but I never added
the tests. Turns out there were two bugs:

[ChangeLog][QtCore][QBitArray] Fixed two bugs that caused QBitArrays
created using fromBits() not to compare equal to the equivalent
QBitArray created using other methods if the size was zero or not a
multiple of 4. If the size modulus 8 was 5, 6, or 7, the data was
actually incorrect.

Fixes: QTBUG-77285
Change-Id: Ife213d861bb14c1787e1fffd15b70573d162042c
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 7 insertions, 5 deletions

diff --git a/src/corelib/tools/qbitarray.cpp b/src/corelib/tools/qbitarray.cpp
index 4e8e3c241e..94aadf7fdf 100644
--- a/src/corelib/tools/qbitarray.cpp
+++ b/src/corelib/tools/qbitarray.cpp
@@ -1,7 +1,7 @@
 /****************************************************************************
 **
 ** Copyright (C) 2016 The Qt Company Ltd.
-** Copyright (C) 2016 Intel Corporation.
+** Copyright (C) 2019 Intel Corporation.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtCore module of the Qt Toolkit.
@@ -132,12 +132,12 @@ QT_BEGIN_NAMESPACE
  * We overallocate the byte array by 1 byte. The first user bit is at
  * d.data()[1]. On the extra first byte, we store the difference between the
  * number of bits in the byte array (including this byte) and the number of
- * bits in the bit array. Therefore, it's always a number between 8 and 15.
+ * bits in the bit array. Therefore, for a non-empty QBitArray, it's always a
+ * number between 8 and 15. For the empty one, d is the an empty QByteArray and
+ * *d.constData() is the QByteArray's terminating NUL (0) byte.
  *
  * This allows for fast calculation of the bit array size:
  *    inline int size() const { return (d.size() << 3) - *d.constData(); }
- *
- * Note: for an array of zero size, *d.constData() is the QByteArray implicit NUL.
  */
 
 /*!
@@ -326,6 +326,8 @@ void QBitArray::fill(bool value, int begin, int end)
 QBitArray QBitArray::fromBits(const char *data, qsizetype size)
 {
     QBitArray result;
+    if (size == 0)
+        return result;
     qsizetype nbytes = (size + 7) / 8;
 
     result.d = QByteArray(nbytes + 1, Qt::Uninitialized);
@@ -334,7 +336,7 @@ QBitArray QBitArray::fromBits(const char *data, qsizetype size)
 
     // clear any unused bits from the last byte
     if (size & 7)
-        bits[nbytes] &= 0xffU >> (size & 7);
+        bits[nbytes] &= 0xffU >> (8 - (size & 7));
 
     *bits = result.d.size() * 8 - size;
     return result;