Optimise QFile::seek that doesn't move the file pointer

If the seek position matches the current file position, then
avoid doing an implicit flush that could be expensive.

Change-Id: I019858ffb36fe832b9aee8da0a7803cafc8f7f75
Reviewed-by: Jaakko Helanti <ext-jaakko.helanti@nokia.com>
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>
(cherry picked from commit a3a2ff59284840f1980197e3f6d06d99fcc0f512)
Reviewed-by: Pasi Pentikäinen <ext-pasi.a.pentikainen@nokia.com>

3 files changed, 162 insertions, 0 deletions

diff --git a/src/corelib/io/qfile.cpp b/src/corelib/io/qfile.cpp
index 09d0c589f9..f993580e87 100644
--- a/src/corelib/io/qfile.cpp
+++ b/src/corelib/io/qfile.cpp
@@ -1692,6 +1692,9 @@ bool QFile::seek(qint64 off)
         return false;
     }
 
+    if (off == d->pos && off == d->devicePos)
+        return true; //avoid expensive flush for NOP seek to current position
+
     if (!d->ensureFlushed())
         return false;
 
diff --git a/tests/auto/qfile/tst_qfile.cpp b/tests/auto/qfile/tst_qfile.cpp
index b7a33731ee..5c8bc2b17d 100644
--- a/tests/auto/qfile/tst_qfile.cpp
+++ b/tests/auto/qfile/tst_qfile.cpp
@@ -128,6 +128,7 @@ private slots:
     void seek();
     void setSize();
     void setSizeSeek();
+    void seekToSamePosition();
     void atEnd();
     void readLine();
     void readLine2();
@@ -458,6 +459,7 @@ void tst_QFile::cleanupTestCase()
     QFile::remove("readAllBuffer.txt");
     QFile::remove("qt_file.tmp");
     QFile::remove("File.txt");
+    QFile::remove("seekToSamePosition.txt");
 }
 
 //------------------------------------------
@@ -756,6 +758,59 @@ void tst_QFile::setSizeSeek()
     QCOMPARE(f.pos(), qint64(2));
 }
 
+void tst_QFile::seekToSamePosition()
+{
+    QFile in("testfile.txt");
+    QFile out("seekToSamePosition.txt");
+    QVERIFY(in.open(QFile::ReadOnly));
+    QVERIFY(out.open(QFile::WriteOnly));
+    QByteArray plusses;
+    plusses.fill('+', 58);
+
+    qint64 cursor = 0;
+    QVERIFY(in.seek(cursor));
+    QVERIFY(out.seek(cursor));
+
+    QVERIFY(out.write(plusses));
+
+    cursor += 58;
+    QVERIFY(in.seek(cursor));
+    QVERIFY(out.seek(cursor));
+
+    QByteArray copy = in.read(60);
+    QVERIFY(out.write(copy));
+
+    cursor += 60;
+    QVERIFY(in.seek(cursor));
+    QVERIFY(out.seek(cursor));
+
+    QVERIFY(out.write(plusses));
+
+    cursor += 58;
+    QVERIFY(in.seek(cursor));
+    QVERIFY(out.seek(cursor));
+
+    copy = in.readAll();
+    QVERIFY(out.write(copy));
+
+    //compare
+    out.close();
+    QVERIFY(out.open(QFile::ReadOnly));
+    QVERIFY(in.seek(0));
+
+    QByteArray clean = in.readAll();
+    QByteArray dirty = out.readAll();
+    out.close();
+
+    QVERIFY(clean.size() == dirty.size());
+    for (int i=0;i<clean.size();i++) {
+        if (clean[i] == '-')
+            QVERIFY2(dirty[i] == '+', qPrintable(QString("no + at pos %1").arg(i)));
+        else
+            QVERIFY2(dirty[i] == clean[i], qPrintable(QString("char at pos %1 mismatched, %2 vs %3").arg(i).arg(clean[i]).arg(dirty[i])));
+    }
+}
+
 void tst_QFile::atEnd()
 {
     QFile f( SRCDIR "testfile.txt" );
diff --git a/tests/benchmarks/corelib/io/qfile/main.cpp b/tests/benchmarks/corelib/io/qfile/main.cpp
index e22637ae7b..e7d260945c 100644
--- a/tests/benchmarks/corelib/io/qfile/main.cpp
+++ b/tests/benchmarks/corelib/io/qfile/main.cpp
@@ -113,6 +113,13 @@ private slots:
     void readBigFile_posix();
     void readBigFile_Win32();
 
+    void writeFileSequential_data();
+    void writeFileSequential();
+    void writeFileBackwards_data();
+    void writeFileBackwards();
+    void writeFileSequentialWithSeeks_data();
+    void writeFileSequentialWithSeeks();
+
 private:
     void readBigFile_data(BenchmarkType type, QIODevice::OpenModeFlag t, QIODevice::OpenModeFlag b);
     void readBigFile();
@@ -678,6 +685,103 @@ void tst_qfile::readSmallFiles()
     delete[] buffer;
 }
 
+void tst_qfile::writeFileSequential_data()
+{
+    QTest::addColumn<int>("blockSize");
+    QTest::addColumn<QString>("path");
+
+    QTest::newRow("internal 16b") << 16 << QDir::tempPath();
+    QTest::newRow("internal 512b") << 512 << QDir::tempPath();
+    QTest::newRow("internal 4k") << 4096 << QDir::tempPath();
+    QTest::newRow("internal 16k") << 16384 << QDir::tempPath();
+    QTest::newRow("internal 64k") << 65536 << QDir::tempPath();
+
+    //slow media (e.g. SD card)
+    QString externalPath;
+#ifdef Q_OS_SYMBIAN
+    externalPath = "E:/";
+#endif
+    if (!externalPath.isEmpty()) {
+        QTest::newRow("external 16b") << 16 << externalPath;
+        QTest::newRow("external 512b") << 512 << externalPath;
+        QTest::newRow("external 4k") << 4096 << externalPath;
+        QTest::newRow("external 16k") << 16384 << externalPath;
+        QTest::newRow("external 64k") << 65536 << externalPath;
+    }
+}
+
+void tst_qfile::writeFileSequential()
+{
+    const qint64 limit = 1024 * 1024;
+    QFETCH(int, blockSize);
+    QFETCH(QString, path);
+    QTemporaryFile f;
+    f.setFileTemplate(path);
+    QByteArray block;
+    block.fill('@', blockSize);
+    QBENCHMARK {
+        QVERIFY(f.open());
+        for (qint64 pos = 0; pos < limit; pos += blockSize) {
+            QVERIFY(f.write(block));
+        }
+        QVERIFY(f.flush());
+        QCOMPARE(f.size(), limit);
+        f.close();
+    }
+}
+
+void tst_qfile::writeFileBackwards_data()
+{
+    writeFileSequential_data();
+}
+
+void tst_qfile::writeFileBackwards()
+{
+    const qint64 limit = 1024 * 1024;
+    QFETCH(int, blockSize);
+    QFETCH(QString, path);
+    QTemporaryFile f;
+    f.setFileTemplate(path);
+    QByteArray block;
+    block.fill('@', blockSize);
+    QBENCHMARK {
+        QVERIFY(f.open());
+        for (qint64 pos = limit - blockSize; pos >= 0; pos -= blockSize) {
+            QVERIFY(f.seek(pos));
+            QVERIFY(f.write(block));
+        }
+        QVERIFY(f.flush());
+        QCOMPARE(f.size(), limit);
+        f.close();
+    }
+}
+
+void tst_qfile::writeFileSequentialWithSeeks_data()
+{
+    writeFileSequential_data();
+}
+
+void tst_qfile::writeFileSequentialWithSeeks()
+{
+    const qint64 limit = 1024 * 1024;
+    QFETCH(int, blockSize);
+    QFETCH(QString, path);
+    QTemporaryFile f;
+    f.setFileTemplate(path);
+    QByteArray block;
+    block.fill('@', blockSize);
+    QBENCHMARK {
+        QVERIFY(f.open());
+        for (qint64 pos = 0; pos < limit; pos += blockSize) {
+            QVERIFY(f.seek(pos));
+            QVERIFY(f.write(block));
+        }
+        QVERIFY(f.flush());
+        QCOMPARE(f.size(), limit);
+        f.close();
+    }
+}
+
 QTEST_MAIN(tst_qfile)
 
 #include "main.moc"