QRegularExpression: fix matching over null/empty QString(View)

An empty QString(View) is allowed to have nullptr as its data pointer
(of course, only if its size is 0). This wasn't properly
checked in QRegularExpression, which passed such nullptr to
PCRE, and that resulted in PCRE raising an error (PCRE_ERROR_NULL).
Detect this case and pass a dummy pointer to keep PCRE happy.

Fixing and testing this in turn exposed a problem with QStringView
support in QRegularExpression when used over a null QString: the
code is supposed to use the QStringView(QString) constructor and NOT
qToStringViewIgnoringNull. That's because QRE distinguishes null
and empty subjects; when using qToStringViewIgnoringNull over
a null QString, one gets a non-null QStringView (!). Again, this in
turn exposed a problem with a QRegularExpression autotest that assumed
that a null match could only mean "no match" (instead, it can happen at
position 0 of a null QString(View)).

Change-Id: Ifb3cf14dec42ce76fcdbcb07ea1d80784d52ef65
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit f0d1f50e0294e5a55a0e450993e0810bd4dbf63d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 14 insertions, 2 deletions

diff --git a/src/corelib/text/qregularexpression.cpp b/src/corelib/text/qregularexpression.cpp
index 72079c1996..e055ae064a 100644
--- a/src/corelib/text/qregularexpression.cpp
+++ b/src/corelib/text/qregularexpression.cpp
@@ -1174,7 +1174,19 @@ void QRegularExpressionPrivate::doMatch(QRegularExpressionMatchPrivate *priv,
     pcre2_jit_stack_assign_16(matchContext, &qtPcreCallback, nullptr);
     pcre2_match_data_16 *matchData = pcre2_match_data_create_from_pattern_16(compiledPattern, nullptr);
 
-    const char16_t * const subjectUtf16 = priv->subject.utf16();
+    // PCRE does not accept a null pointer as subject string, even if
+    // its length is zero. We however allow it in input: a QStringView
+    // subject may have data == nullptr. In this case, to keep PCRE
+    // happy, pass a pointer to a dummy character.
+    constexpr char16_t dummySubject = 0;
+    const char16_t * const subjectUtf16 = [&]()
+    {
+        const auto subjectUtf16 = priv->subject.utf16();
+        if (subjectUtf16)
+            return subjectUtf16;
+        Q_ASSERT(subjectLength == 0);
+        return &dummySubject;
+    }();
 
     int result;
 
@@ -1610,7 +1622,7 @@ QRegularExpressionMatch QRegularExpression::match(const QString &subject,
     d.data()->compilePattern();
     auto priv = new QRegularExpressionMatchPrivate(*this,
                                                    subject,
-                                                   qToStringViewIgnoringNull(subject),
+                                                   QStringView(subject),
                                                    matchType,
                                                    matchOptions);
     d->doMatch(priv, offset);