Add support for available ID queries to QTZ chrono backend

We seem to have missed this when adding the backend. Enforce
overriding availableTimeZoneIds(), by = 0'ing it, so that we don't
make that mistake for any future backends. Document this obligation in
QTZ's availableTimeZoneIds(). Fix coding style of isTZIAvailable()'s
signature as a drive-by.

Change-Id: Ibb736fb400644bd25681707b9255415760543ff9
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/time/qtimezone.cpp

@@ -1512,6 +1512,7 @@ static QList<QByteArray> set_union(const QList<QByteArray> &l1, const QList<QByt
 
 QList<QByteArray> QTimeZone::availableTimeZoneIds()
 {
+    // Backends MUST implement availableTimeZoneIds().
     return set_union(QUtcTimeZonePrivate().availableTimeZoneIds(),
                      global_tz->backend->availableTimeZoneIds());
 }

src/corelib/time/qtimezoneprivate.cpp

@@ -609,11 +609,6 @@ bool QTimeZonePrivate::isTimeZoneIdAvailable(const QByteArray& ianaId) const
     return std::binary_search(tzIds.begin(), tzIds.end(), ianaId);
 }
 
-QList<QByteArray> QTimeZonePrivate::availableTimeZoneIds() const
-{
-    return QList<QByteArray>();
-}
-
 static QList<QByteArray> selectAvailable(QList<QByteArrayView> &&desired,
                                          const QList<QByteArray> &all)
 {

src/corelib/time/qtimezoneprivate_chrono.cpp

@@ -4,6 +4,7 @@
 #include "qtimezoneprivate_p.h"
 
 #include <chrono>
+#include <optional>
 
 QT_BEGIN_NAMESPACE
 
@@ -34,6 +35,41 @@ static const std::chrono::time_zone *idToZone(std::string_view id)
     EXCEPTION_CHECKED(return get_tzdb().locate_zone(id), return nullptr);
 }
 
+static const std::vector<std::string_view> getAllZoneIds(std::optional<int> offset = std::nullopt)
+{
+    const tzdb &db = get_tzdb(); // May throw std::runtime_error.
+    std::vector<std::string_view> raw, links, all;
+    auto now = system_clock::now();
+    for (const time_zone &zone : db.zones) {
+        if (offset) {
+            const sys_info info = zone.get_info(now);
+            if (info.offset.count() != *offset)
+                continue;
+        }
+        raw.push_back(zone.name());
+    }
+    for (const time_zone_link &link : db.links) {
+        if (offset) {
+            const std::string_view target = link.target();
+            const time_zone *zone = db.locate_zone(target);
+            const sys_info info = zone->get_info(now);
+            if (info.offset.count() != *offset)
+                continue;
+        }
+        links.push_back(link.name());
+    }
+    // Each is sorted, so we can unite them; and the result shall be sorted.
+    all.reserve(raw.size() + links.size());
+    std::set_union(raw.begin(), raw.end(), links.begin(), links.end(), std::back_inserter(all));
+    return all;
+}
+
+static const std::optional<std::vector<std::string_view>>
+allZoneIds(std::optional<int> offset = std::nullopt)
+{
+    EXCEPTION_CHECKED(return getAllZoneIds(offset), return std::nullopt);
+}
+
 static QChronoTimeZonePrivate::Data
 fromSysInfo(std::chrono::sys_info info, qint64 atMSecsSinceEpoch)
 {
@@ -76,6 +112,35 @@ QByteArray QChronoTimeZonePrivate::systemTimeZoneId() const
     return {};
 }
 
+bool QChronoTimeZonePrivate::isTimeZoneIdAvailable(const QByteArray &ianaId) const
+{
+    return idToZone(std::string_view(ianaId.data(), ianaId.size())) != nullptr;
+}
+
+QList<QByteArray> QChronoTimeZonePrivate::availableTimeZoneIds() const
+{
+    QList<QByteArray> result;
+    if (auto ids = allZoneIds()) {
+        result.reserve(ids->size());
+        for (std::string_view id : *ids)
+            result << QByteArray{id.data(), qsizetype(id.size())};
+        // Order preserved; they were already sorted and unique.
+    }
+    return result;
+}
+
+QList<QByteArray> QChronoTimeZonePrivate::availableTimeZoneIds(int utcOffset) const
+{
+    QList<QByteArray> result;
+    if (auto ids = allZoneIds(utcOffset)) {
+        result.reserve(ids->size());
+        for (std::string_view id : *ids)
+            result << QByteArray{id.data(), qsizetype(id.size())};
+        // Order preserved; they were already sorted and unique.
+    }
+    return result;
+}
+
 QString QChronoTimeZonePrivate::abbreviation(qint64 atMSecsSinceEpoch) const
 {
     if (auto info = infoAtEpochMillis(m_timeZone, atMSecsSinceEpoch))

src/corelib/time/qtimezoneprivate_p.h

@@ -120,7 +120,7 @@ public:
     virtual QByteArray systemTimeZoneId() const;
 
     virtual bool isTimeZoneIdAvailable(const QByteArray &ianaId) const;
-    virtual QList<QByteArray> availableTimeZoneIds() const;
+    virtual QList<QByteArray> availableTimeZoneIds() const = 0;
     virtual QList<QByteArray> availableTimeZoneIds(QLocale::Territory territory) const;
     virtual QList<QByteArray> availableTimeZoneIds(int utcOffset) const;
 
@@ -256,6 +256,10 @@ public:
     int standardTimeOffset(qint64 atMSecsSinceEpoch) const override;
     int daylightTimeOffset(qint64 atMSecsSinceEpoch) const override;
 
+    bool isTimeZoneIdAvailable(const QByteArray &ianaId) const override;
+    QList<QByteArray> availableTimeZoneIds() const override;
+    QList<QByteArray> availableTimeZoneIds(int utcOffset) const override;
+
     bool hasDaylightTime() const override;
     bool isDaylightTime(qint64 atMSecsSinceEpoch) const override;