QDataStream::readBytes(): make the growth of the buffer geometric

The algorithm tries to allocate the memory using 1 Mb blocks.
When the input data is large, this results in a lot of reallocations,
which is slow and inefficient.

This patch modifies the algorithm in such way that the allocation size
increases at each step, resulting in geometric growth of the buffer.

As all the variables in the algorithm are quint32, and the blockSize
is int, we need to make sure that it does not become negative,
because readBlock() is not designed to handle negative length.

Note that we cannot simply "fix" the problem by converting blockSize
to quint32, because that will just postpone quint32 -> int conversion
until the function call. So, add an extra check to make sure that
the value does not exceed INT_MAX.

Change-Id: I071df68d51ba1dbd8b2eb5f94eb078a33223505f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit a1bfac287ee5d3719646d68dc91dc8e8e4cec04e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 44dc38ebc9fa407565ec1699bdbb1f97ffd13f1c)
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/serialization/qdatastream.cpp

@@ -1056,13 +1056,15 @@ QDataStream &QDataStream::readBytes(char *&s, uint &l)
     if (len == 0)
         return *this;
 
-    const quint32 Step = 1024 * 1024;
+    quint32 step = 1024 * 1024;
     quint32 allocated = 0;
     char *prevBuf = nullptr;
     char *curBuf = nullptr;
 
+    constexpr quint32 MaxBlockSize = std::numeric_limits<int>::max();
     do {
-        int blockSize = qMin(Step, len - allocated);
+        const quint32 sz = qMin(step, len - allocated);
+        int blockSize = qMin(sz, MaxBlockSize);
         prevBuf = curBuf;
         curBuf = new char[allocated + blockSize + 1];
         if (prevBuf) {
@@ -1074,6 +1076,7 @@ QDataStream &QDataStream::readBytes(char *&s, uint &l)
             return *this;
         }
         allocated += blockSize;
+        step *= 2;
     } while (allocated < len);
 
     s = curBuf;