RE: Random pg_upgrade test failure on drongo

Dear Alexander,

> I agree with your analysis and would like to propose a PoC fix (see
> attached). With this patch applied, 20 iterations succeeded for me.

There are no reviewers so that I will review again. Let's move the PoC
to the concrete patch. Note that I only focused on fixes of random failure,
other parts are out-of-scope.

Basically, code comments can be updated accordingly.

01.

```
/*
* This function might be called for a regular file or for a junction
* point (which we use to emulate symlinks). The latter must be unlinked
* with rmdir() on Windows. Before we worry about any of that, let's see
* if we can unlink directly, since that's expected to be the most common
* case.
*/
snprintf(tmppath, sizeof(tmppath), "%s.tmp", path);
if (pgrename(path, tmppath) == 0)
{
if (unlink(tmppath) == 0)
return 0;
curpath = tmppath;
}
```

You can modify comments atop changes because it is not trivial.
Below is my draft:

```
* XXX: we rename the target file to ".tmp" before calling unlink. The
* removal may fail with STATUS_DELETE_PENDING status on Windows, so
* creating the same file would fail. This assumes that renaming is a
* synchronous operation.
```

02.

```
loops = 0;
while (lstat(curpath, &st) < 0 && lstat_error_was_status_delete_pending())
{
if (++loops > 100) /* time out after 10 sec */
return -1;
pg_usleep(100000); /* us */
}
```

Comments can be added atop the part. Below one is my draft.

```
/*
* Wait until the removal is really finished to avoid ERRORs for creating a
* same file in other functions.
*/
```

Best Regards,
Hayato Kuroda
FUJITSU LIMITED