From fc9a26ea3a0b76de8a437d6417af334a7a9dfde5 Mon Sep 17 00:00:00 2001
From: Edward Welbourne <edward.welbourne@qt.io>
Date: Wed, 9 Apr 2025 17:00:15 +0200
Subject: [PATCH] Include "is known to ICU" check in
 QTimeZonePrivate::localeName()

As long noted in the QIcuTimeZonePrivate constructor, we can't trust
ucal_open()'s non-null return to mean it actually recognised the ID we
passed it. So break out the "is available" check to a function in the
QtTimeZoneLocale namespace so the check can be shared with
localeName() when compiled with ICU but some other backend. We were
previously getting GMT as display name for zones unknown to ICU.

Pick-to: 6.9 6.8
Change-Id: I57d57f94d8db7df76f24193a8ef1b5c71b08b0fc
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
---
 src/corelib/time/qtimezonelocale.cpp      | 23 +++++++++++++++++++++++
 src/corelib/time/qtimezonelocale_p.h      |  2 ++
 src/corelib/time/qtimezoneprivate_icu.cpp | 16 +---------------
 3 files changed, 26 insertions(+), 15 deletions(-)

diff --git a/src/corelib/time/qtimezonelocale.cpp b/src/corelib/time/qtimezonelocale.cpp
index 1ea1f40b701..8995dc87a91 100644
--- a/src/corelib/time/qtimezonelocale.cpp
+++ b/src/corelib/time/qtimezonelocale.cpp
@@ -94,6 +94,24 @@ QString ucalTimeZoneDisplayName(UCalendar *ucal,
     return result;
 }
 
+bool ucalKnownTimeZoneId(const QString &ianaStr)
+{
+    const UChar *const name = reinterpret_cast<const UChar *>(ianaStr.constData());
+    // We are not interested in the value, but we have to pass something.
+    // No known IANA zone name is (up to 2023) longer than 30 characters.
+    constexpr size_t size = 64;
+    UChar buffer[size];
+
+    // TODO: convert to ucal_getIanaTimeZoneID(), new draft in ICU 74, once we
+    // can rely on its availability, assuming it works the same once not draft.
+    UErrorCode status = U_ZERO_ERROR;
+    UBool isSys = false;
+    // Returns the length of the IANA zone name (but we don't care):
+    ucal_getCanonicalTimeZoneID(name, ianaStr.size(), buffer, size, &isSys, &status);
+    // We're only interested if the result is a "system" (i.e. IANA) ID:
+    return isSys;
+}
+
 } // QtTimeZoneLocale
 
 // Used by TZ backends when ICU is available:
@@ -110,6 +128,11 @@ QString QTimeZonePrivate::localeName(qint64 atMSecsSinceEpoch, int offsetFromUtc
         return isoOffsetFormat(offsetFromUtc);
 
     const QString id = QString::fromUtf8(m_id);
+    // Need to check id is known to ICU, since ucal_open() will return a
+    // misleading "valid" GMT ucal when it doesn't recognise id.
+    if (!QtTimeZoneLocale::ucalKnownTimeZoneId(id))
+        return QString();
+
     const QByteArray loc = locale.name().toUtf8();
     UErrorCode status = U_ZERO_ERROR;
     // TODO: QTBUG-124271 can we cache any of this ?
diff --git a/src/corelib/time/qtimezonelocale_p.h b/src/corelib/time/qtimezonelocale_p.h
index 5b716da5229..586207345b6 100644
--- a/src/corelib/time/qtimezonelocale_p.h
+++ b/src/corelib/time/qtimezonelocale_p.h
@@ -36,6 +36,8 @@ namespace QtTimeZoneLocale {
 QString ucalTimeZoneDisplayName(UCalendar *ucal, QTimeZone::TimeType timeType,
                                 QTimeZone::NameType nameType,
                                 const QByteArray &localeCode);
+
+bool ucalKnownTimeZoneId(const QString &id);
 #else
 
 QList<QByteArrayView> ianaIdsForTerritory(QLocale::Territory territory);
diff --git a/src/corelib/time/qtimezoneprivate_icu.cpp b/src/corelib/time/qtimezoneprivate_icu.cpp
index ca456ee117e..9698143cd30 100644
--- a/src/corelib/time/qtimezoneprivate_icu.cpp
+++ b/src/corelib/time/qtimezoneprivate_icu.cpp
@@ -387,21 +387,7 @@ QByteArray QIcuTimeZonePrivate::systemTimeZoneId() const
 
 bool QIcuTimeZonePrivate::isTimeZoneIdAvailable(const QByteArray &ianaId) const
 {
-    const QString ianaStr = QString::fromUtf8(ianaId);
-    const UChar *const name = reinterpret_cast<const UChar *>(ianaStr.constData());
-    // We are not interested in the value, but we have to pass something.
-    // No known IANA zone name is (up to 2023) longer than 30 characters.
-    constexpr size_t size = 64;
-    UChar buffer[size];
-
-    // TODO: convert to ucal_getIanaTimeZoneID(), new draft in ICU 74, once we
-    // can rely on its availability, assuming it works the same once not draft.
-    UErrorCode status = U_ZERO_ERROR;
-    UBool isSys = false;
-    // Returns the length of the IANA zone name (but we don't care):
-    ucal_getCanonicalTimeZoneID(name, ianaStr.size(), buffer, size, &isSys, &status);
-    // We're only interested if the result is a "system" (i.e. IANA) ID:
-    return isSys;
+    return QtTimeZoneLocale::ucalKnownTimeZoneId(QString::fromUtf8(ianaId));
 }
 
 QList<QByteArray> QIcuTimeZonePrivate::availableTimeZoneIds() const