Restore support for reading /etc/timezone for system zone name

This restores one of the two mechanisms removed in
commit b0383cbd388336f698ceeac11a4f50cdff931dd9,
transformed to fit in with the new cached system-zone determination.

Fixes: QTBUG-87326
Change-Id: Ic270acb0d958e17dbc74a0ff93a5a1843c939678
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 824d963700a91294ba4a543ebb486aedbd650284)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 15 insertions, 7 deletions

diff --git a/src/corelib/time/qtimezoneprivate_tz.cpp b/src/corelib/time/qtimezoneprivate_tz.cpp
index ace966e15b..a18184b91e 100644
--- a/src/corelib/time/qtimezoneprivate_tz.cpp
+++ b/src/corelib/time/qtimezoneprivate_tz.cpp
@@ -1169,8 +1169,11 @@ public:
          */
         const StatIdent local = identify("/etc/localtime");
         const StatIdent tz = identify("/etc/TZ");
-        if (!m_name.isEmpty() && m_last.isValid() && (m_last == local || m_last == tz))
+        const StatIdent timezone = identify("/etc/timezone");
+        if (!m_name.isEmpty() && m_last.isValid()
+            && (m_last == local || m_last == tz || m_last == timezone)) {
             return m_name;
+        }
 
         m_name = etcLocalTime();
         if (!m_name.isEmpty()) {
@@ -1178,12 +1181,19 @@ public:
             return m_name;
         }
 
-        m_name = etcTZ();
-        m_last = m_name.isEmpty() ? StatIdent() : tz;
+        // Some systems (e.g. uClibc) have a default value for $TZ in /etc/TZ:
+        m_name = etcContent(QStringLiteral("/etc/TZ"));
+        if (!m_name.isEmpty()) {
+            m_last = tz;
+            return m_name;
+        }
+
+        // Gentoo still (2020, QTBUG-87326) uses this:
+        m_name = etcContent(QStringLiteral("/etc/timezone"));
+        m_last = m_name.isEmpty() ? StatIdent() : timezone;
         return m_name;
     }
 
-
 private:
     QByteArray m_name;
     struct StatIdent
@@ -1224,10 +1234,8 @@ private:
         return QByteArray();
     }
 
-    static QByteArray etcTZ()
+    static QByteArray etcContent(const QString &path)
     {
-        // Some systems (e.g. uClibc) have a default value for $TZ in /etc/TZ:
-        const QString path = QStringLiteral("/etc/TZ");
         QFile zone(path);
         if (zone.open(QIODevice::ReadOnly))
             return zone.readAll().trimmed();