QNetworkDiskCache: fix expiration calculation heuristic with Last-Modified time

Heuristic with last-modified time in Qt has some problems.
1) Remove redundant expirationDate.isInvalid() check
expirationDate.isInvalid is already checked. So I removed.

2) Add dateHeader.isInvalid() check
The dateHeader is used in expiration calculation.
I add invalid check for the dateHeader.
*. The dateHeader is the origin server's Date

3) Change diff time calculation.
The expirationDate is calculated with time diff.

Previous calculation is
// The lastModified is earlier than the currentDateTime.
// The diff has negative value.
int diff = currentDateTime.secsTo(lastModified);
// The expirationDate is earlier than lastModified
// , currentDateTime and dateHeader.
expirationDate = lastModified.addSecs(diff / 10);
*. currentDateTime: current time
*. lastModified: last modified date in server

It means that files are not cached with the heuristic.

I changed diff calculation.
int diff = lastModified.secsTo(dateHeader);
freshness_lifetime = diff / 10; // RFC 2616 13.2.4

4) httpRequest.headerField setting
If current_age is larger than 1 day, the cache MUST attach Warning 113.
*. The current_age is value of age in header
   or elapsed time from dateHeader in Qt source code.

Previous code does not check current_age is larger than 1 day correctly.
// dt = 1970-01-01T00:00:00 + current_age
dt.setTime_t(current_age);
// currentDateTime is much bigger than 1970-01-01T00:00:00
if (dt.daysTo(currentDateTime) > 1)

Task-number: QTBUG-40836
Change-Id: I4b00c3b287e6fafeea6b02681533fe75a198247e
Reviewed-by: Jung Dong-Heon <dongheon.jung@lge.com>
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>

2 files changed, 13 insertions, 15 deletions

diff --git a/src/network/access/qnetworkreplyhttpimpl.cpp b/src/network/access/qnetworkreplyhttpimpl.cpp
index 920dfdbb38..367c92a460 100644
--- a/src/network/access/qnetworkreplyhttpimpl.cpp
+++ b/src/network/access/qnetworkreplyhttpimpl.cpp
@@ -560,23 +560,21 @@ bool QNetworkReplyHttpImplPrivate::loadFromCacheIfAllowed(QHttpNetworkRequest &h
         int resident_time = now - response_time;
         int current_age   = corrected_initial_age + resident_time;
 
+        int freshness_lifetime = 0;
+
         // RFC 2616 13.2.4 Expiration Calculations
-        if (!expirationDate.isValid()) {
-            if (lastModified.isValid()) {
-                int diff = currentDateTime.secsTo(lastModified);
-                expirationDate = lastModified.addSecs(diff / 10);
-                if (httpRequest.headerField("Warning").isEmpty()) {
-                    QDateTime dt;
-                    dt.setTime_t(current_age);
-                    if (dt.daysTo(currentDateTime) > 1)
-                        httpRequest.setHeaderField("Warning", "113");
-                }
+        if (lastModified.isValid() && dateHeader.isValid()) {
+            int diff = lastModified.secsTo(dateHeader);
+            freshness_lifetime = diff / 10;
+            if (httpRequest.headerField("Warning").isEmpty()) {
+                QDateTime dt = currentDateTime.addSecs(current_age);
+                if (currentDateTime.daysTo(dt) > 1)
+                    httpRequest.setHeaderField("Warning", "113");
             }
         }
 
-        // the cache-saving code below sets the expirationDate with date+max_age
-        // if "max-age" is present, or to Expires otherwise
-        int freshness_lifetime = dateHeader.secsTo(expirationDate);
+        // the cache-saving code below sets the freshness_lifetime with (dateHeader - last_modified) / 10
+        // if "last-modified" is present, or to Expires otherwise
         response_is_fresh = (freshness_lifetime > current_age);
     } else {
         // expiration date was calculated earlier (e.g. when storing object to the cache)
diff --git a/tests/auto/network/access/qabstractnetworkcache/tst_qabstractnetworkcache.cpp b/tests/auto/network/access/qabstractnetworkcache/tst_qabstractnetworkcache.cpp
index da86a2d14d..aca48567eb 100644
--- a/tests/auto/network/access/qabstractnetworkcache/tst_qabstractnetworkcache.cpp
+++ b/tests/auto/network/access/qabstractnetworkcache/tst_qabstractnetworkcache.cpp
@@ -203,9 +203,9 @@ void tst_QAbstractNetworkCache::lastModified_data()
     QTest::newRow("304-3") << QNetworkRequest::PreferCache << "httpcachetest_lastModified304.cgi" << true;
 
     QTest::newRow("200-0") << QNetworkRequest::AlwaysNetwork << "httpcachetest_lastModified200.cgi" << AlwaysFalse;
-    QTest::newRow("200-1") << QNetworkRequest::PreferNetwork << "httpcachetest_lastModified200.cgi" << false;
+    QTest::newRow("200-1") << QNetworkRequest::PreferNetwork << "httpcachetest_lastModified200.cgi" << true;
     QTest::newRow("200-2") << QNetworkRequest::AlwaysCache << "httpcachetest_lastModified200.cgi" << AlwaysTrue;
-    QTest::newRow("200-3") << QNetworkRequest::PreferCache << "httpcachetest_lastModified200.cgi" << false;
+    QTest::newRow("200-3") << QNetworkRequest::PreferCache << "httpcachetest_lastModified200.cgi" << true;
 }
 
 void tst_QAbstractNetworkCache::lastModified()