set mtime for fragmented videos (such as DASH)

[dotysan]

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Description of your pull request and other information

set mtime for fragmented videos (such as DASH)

• it is set to the mtime of the last fragment uploaded

See issue #18384.

[dstftw]

This will set mtime for the final file number-of-fragment times. This is completely useless. mtime must be set one single time.
Another point that appears immediately - why the last fragment's mtime and not the first's or 42th's?

[dotysan]

This will set mtime for the final file number-of-fragment times.

Agreed. Although the mtime of the last fragment is what is effective after the download is complete.

This is completely useless. mtime must be set one single time.

Is the call to os.utime() for each fragment really that expensive? I agree it would be more efficient to do it only once.

Another point that appears immediately - why the last fragment's mtime and not the first's or 42th's?

Because that's the last time the file was modified. I.e. when the last fragment finally uploaded.

[dotysan]

OK @dstftw take another look at this PR. Indeed FileDownloader already calls os.utime() on every fragment, so there's no point in doing it twice on every fragment. I've stuffed it into a variable on the FragmentFD class. And now the syscall is just done once in _finish_frag_download().

[dotysan]

mtime must be set one single time.

@dstftw BTW, it already calls os.utime() on every fragment here:

youtube-dl/youtube_dl/downloader/common.py

Line 222 in 2cd43a0

os.utime(filename, (time.time(), filetime))

This is in FileDownloader.try_utime() called from HttpFD.download(). And in this case of fragmented files it's just as wasteful, since we only need the mtime of the last fragment.

If you are truly concerned with eliminating wasteful syscalls, we should fix this too.

[dstftw]

Nothing conceptually changed: you still request st_mtime unnecessarily n-1 times.

[dstftw]

mtime must be set one single time.

@dstftw BTW, it already calls os.utime() on every fragment here:

youtube-dl/youtube_dl/downloader/common.py

Line 222 in 2cd43a0

os.utime(filename, (time.time(), filetime))

This is in FileDownloader.try_utime() called from HttpFD.download(). And in this case of fragmented files it's just as wasteful, since we only need the mtime of the last fragment.

If you are truly concerned with eliminating wasteful syscalls, we should fix this too.

Calling os.utime on each separate fragment is perfectly legal since fragments are not necessarily removed after the download.