Implement a TODO: cache the list of MS TZ IDs

The MS TZ-backend's lookup of available Windows IDs included a TODO
advocating caching the result, since it is unlikely to change at
runtime. Given that MS's time_t functions don't believe in times
before 1970 and two-digit year parsing (such as the ASN.1 parser does,
apparently many times, during SSL handshakes) assumes the 1900s (and
certs in use today have mostly been issued this century), SSL involves
DTP falling back repeatedly to lookups of local time's offsets from
UTC (before it discovers it shouldn't have been using local time
anyway) via the system timezone object, this cacheable list can be
accessed many times. (A user has reported thousands per SSL socket
created.) So let's implement that TODO and trust that updates to the
registry's set of available zones aren't a common occurrence at
runtime.

Pick-to: 6.6 6.5
Change-Id: Iefe235c71da1bf5c3372c52dba89ad0657e06c0b
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 16 insertions, 14 deletions

diff --git a/src/corelib/time/qtimezoneprivate_win.cpp b/src/corelib/time/qtimezoneprivate_win.cpp
index b1abcb7a97..82666407d2 100644
--- a/src/corelib/time/qtimezoneprivate_win.cpp
+++ b/src/corelib/time/qtimezoneprivate_win.cpp
@@ -186,22 +186,24 @@ bool isSameRule(const QWinTimeZonePrivate::QWinTransitionRule &last,
 
 QList<QByteArray> availableWindowsIds()
 {
-    // TODO Consider caching results in a global static, very unlikely to change.
-    QList<QByteArray> list;
-    QWinRegistryKey key(HKEY_LOCAL_MACHINE, tzRegPath);
-    if (key.isValid()) {
-        DWORD idCount = 0;
-        if (RegQueryInfoKey(key, 0, 0, 0, &idCount, 0, 0, 0, 0, 0, 0, 0) == ERROR_SUCCESS
-            && idCount > 0) {
-            for (DWORD i = 0; i < idCount; ++i) {
-                DWORD maxLen = MAX_KEY_LENGTH;
-                TCHAR buffer[MAX_KEY_LENGTH];
-                if (RegEnumKeyEx(key, i, buffer, &maxLen, 0, 0, 0, 0) == ERROR_SUCCESS)
-                    list.append(QString::fromWCharArray(buffer).toUtf8());
+    static const QList<QByteArray> cache = [] {
+        QList<QByteArray> list;
+        QWinRegistryKey key(HKEY_LOCAL_MACHINE, tzRegPath);
+        if (key.isValid()) {
+            DWORD idCount = 0;
+            if (RegQueryInfoKey(key, 0, 0, 0, &idCount, 0, 0, 0, 0, 0, 0, 0) == ERROR_SUCCESS
+                && idCount > 0) {
+                for (DWORD i = 0; i < idCount; ++i) {
+                    DWORD maxLen = MAX_KEY_LENGTH;
+                    TCHAR buffer[MAX_KEY_LENGTH];
+                    if (RegEnumKeyEx(key, i, buffer, &maxLen, 0, 0, 0, 0) == ERROR_SUCCESS)
+                        list.append(QString::fromWCharArray(buffer).toUtf8());
+                }
             }
         }
-    }
-    return list;
+        return list;
+    }();
+    return cache;
 }
 
 QByteArray windowsSystemZoneId()