Deleted file still reported as existing (Windows only)

Question

(Note that this is not primarily a Qt question)

It seems to me that the return value of QFile::exists() is sometimes incorrect.

Consider the following two unit-test-like snippets (each of which I have executed a few thousand times in a loop)

// create file
QFile file("test.tmp");
QVERIFY(file.open(QIODevice::WriteOnly));
QVERIFY(file.write("some data") != -1);
file.close();

// delete file
QVERIFY(file.remove());

// assert file is gone
QVERIFY(!file.exists());  // <-- 5..10 % chance of failure

and

// create file
QFile file("test.tmp");
QVERIFY(file.open(QIODevice::WriteOnly));
QVERIFY(file.write("some data") != -1);
file.close();

// delete file
QVERIFY(file.remove());

// retry until file is gone (or until timeout)
for (auto i = 0; i < 10; i++)
{
  if (!file.exists())  // <-- note that only the check is retried, not the actual delete
    return;

  QThread::yieldCurrentThread();
}

QFAIL("file is still reported as existing");  // <-- never reached in my tests

The first unit test fails about 8 out of 100 times. Always on the last line of code (indicating that the file still exists). The second unit test never fails.

This behavior was observed on a Windows 10 system using NTFS (with Qt 5.2.1). It could not be reproduced using ubuntu 16.04 LTS using ext4 on a VM (with Qt 5.8.0)

Not sure if this helps:

• Process Monitor (when it succeeds)
• Process Monitor (when it fails)

So my questions are:

• what is happening?
• what are implications that I might be interested in?

---

update:

For clarification: I am hoping for an answer like "this is caused by the NTFS feature 'bills-fancy-caching-magic'". From there I would like to find out, whether Qt does look over this feature intentionally.

Answer 1

According to the Windows API documentation, it is defined behaviour:

The DeleteFile function marks a file for deletion on close. Therefore, the file deletion does not occur until the last handle to the file is closed. Subsequent calls to CreateFile to open the file fail with ERROR_ACCESS_DENIED.

It seems to be a property of the Windows kernel and therefore not to be limited to NTFS.

The behaviour seems to be unpredictable, as other services (think virus scanners) might open the file in question.