QHttpHeaders: add value(name, fall-back)

The vast majority of header fields appear only once, but there was no
efficient way to get that value: values() returns as a
QList<QByteArray> (allocating) while combinedValue() returns a
QByteArray constructed from values().join() (also allocating).

It follows that the QHttpHeaders API is incomplete (lacks an efficient
basis of operations in EoP terms).

Add a value() function that returns either the value or a
user-provided fall-back as a QByteArrayView. Unlike values() and
combinedValue(), this function can be noexcept, greatly improving
codegen for callers.

Found in API review.

Pick-to: 6.7
Task-number: QTBUG-107042
Change-Id: I2da20815fd46fdd7f150c224f41eee53abed313e
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/network/access/qhttpheaders.cpp

@@ -974,6 +974,29 @@ void QHttpHeaders::removeAt(qsizetype i)
     d->headers.removeAt(i);
 }
 
+/*!
+    Returns the value of the (first) header \a name, or \a defaultValue if it
+    doesn't exist.
+
+    \sa value(QHttpHeaders::WellKnownHeader, QByteArrayView)
+*/
+QByteArrayView QHttpHeaders::value(QAnyStringView name, QByteArrayView defaultValue) const noexcept
+{
+    for (const auto &h : std::as_const(d->headers)) {
+        if (headerNameIs(h, name))
+            return h.value;
+    }
+    return defaultValue;
+}
+
+/*!
+    \overload value(QAnyStringView, QByteArrayView)
+*/
+QByteArrayView QHttpHeaders::value(WellKnownHeader name, QByteArrayView defaultValue) const noexcept
+{
+    return value(headerNames[qToUnderlying(name)], defaultValue);
+}
+
 /*!
     Returns the values of header \a name in a list. Returns an empty
     list if header with \a name doesn't exist.

src/network/access/qhttpheaders.h

@@ -237,6 +237,9 @@ public:
     Q_NETWORK_EXPORT void removeAll(WellKnownHeader name);
     Q_NETWORK_EXPORT void removeAt(qsizetype i);
 
+    Q_NETWORK_EXPORT QByteArrayView value(QAnyStringView name, QByteArrayView defaultValue = {}) const noexcept;
+    Q_NETWORK_EXPORT QByteArrayView value(WellKnownHeader name, QByteArrayView defaultValue = {}) const noexcept;
+
     Q_NETWORK_EXPORT QList<QByteArray> values(QAnyStringView name) const;
     Q_NETWORK_EXPORT QList<QByteArray> values(WellKnownHeader name) const;
 

tests/auto/network/access/qhttpheaders/tst_qhttpheaders.cpp

@@ -168,9 +168,10 @@ void tst_QHttpHeaders::accessors()
     QVERIFY(h1.has(QHttpHeaders::WellKnownHeader::Accept));
     QVERIFY(h1.has("accept"));
 
-    // values()
+    // values()/value()
 #define EXISTS_NOT(H, N) do {                       \
         QVERIFY(!H.has(N));                         \
+        QCOMPARE(H.value(N, "ENOENT"), "ENOENT");   \
         const auto values = H.values(N);            \
         QVERIFY(values.isEmpty());                  \
         QVERIFY(H.combinedValue(N).isNull());       \
@@ -180,6 +181,7 @@ void tst_QHttpHeaders::accessors()
         const std::array expected = { __VA_ARGS__ };                \
         static_assert(std::tuple_size_v<decltype(expected)> == X);  \
         QVERIFY(H.has(N));                                          \
+        QCOMPARE(H.value(N, "ENOENT"), expected.front());           \
         const auto values = H.values(N);                            \
         QCOMPARE(values.size(), X);                                 \
         QCOMPARE(values.front(), expected.front());                 \