QMessageAuthenticationCode: make finalizeUnchecked() properly noexcept

Replace the QVarLengthArray with HashBlock, an instantiation of
QSmallByteArray. Unlike QVLA, which may allocate memory when
capacity() exceeds Prealloc (not the case here, but could become an
issue with future hash algorithms, we're at 144 now, up from the
traditional 64), QSmallByteArray never throws, and the few Q_ASSERT()s
it contains don't matter, because we use the class' functions
in-contract here.

Requires to add an indexing operator to QSmallByteArray.

Pick-to: 6.5
Change-Id: Ica3656adc190e0141e065287fadc38e0cebce0f4
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qcryptographichash.cpp

@@ -131,6 +131,11 @@ public:
     // all SMFs are ok!
     quint8 *data() noexcept { return m_data.data(); }
     qsizetype size() const noexcept { return qsizetype{m_size}; }
+    quint8 &operator[](qsizetype n)
+    {
+        Q_ASSERT(n < size());
+        return data()[n];
+    }
     bool isEmpty() const noexcept { return size() == 0; }
     void clear() noexcept { m_size = 0; }
     void resizeForOverwrite(qsizetype s)
@@ -1003,6 +1008,17 @@ static constexpr int qt_hash_block_size(QCryptographicHash::Algorithm method)
     return 0;
 }
 
+constexpr int maxHashBlockSize()
+{
+    int result = 0;
+    using A = QCryptographicHash::Algorithm;
+    for (int i = 0; i < A::NumAlgorithms ; ++i)
+        result = std::max(result, qt_hash_block_size(A(i)));
+    return result;
+}
+
+using HashBlock = QSmallByteArray<maxHashBlockSize()>;
+
 class QMessageAuthenticationCodePrivate
 {
 public:
@@ -1022,7 +1038,7 @@ public:
 
     // when not called from the static hash() function, this function needs to be
     // called with finalizeMutex held:
-    void finalizeUnchecked();
+    void finalizeUnchecked() noexcept;
     // END functions that need to be called with finalizeMutex held
 };
 
@@ -1201,21 +1217,22 @@ void QMessageAuthenticationCodePrivate::finalize()
     finalizeUnchecked();
 }
 
-void QMessageAuthenticationCodePrivate::finalizeUnchecked()
+void QMessageAuthenticationCodePrivate::finalizeUnchecked() noexcept
 {
     const int blockSize = qt_hash_block_size(method);
 
     messageHash.finalizeUnchecked();
     const HashResult hashedMessage = messageHash.result;
 
-    QVarLengthArray<char> oKeyPad(blockSize);
+    HashBlock oKeyPad;
+    oKeyPad.resizeForOverwrite(blockSize);
     const char * const keyData = key.constData();
 
     for (int i = 0; i < blockSize; ++i)
         oKeyPad[i] = keyData[i] ^ 0x5c;
 
     messageHash.reset();
-    messageHash.addData(oKeyPad);
+    messageHash.addData(oKeyPad.toByteArrayView());
     messageHash.addData(hashedMessage.toByteArrayView());
     messageHash.finalizeUnchecked();