Files names are not sanitized under Android

[alervd]

Please provide the following information

libtorrent version (or branch): 1.2.19

platform/architecture: Android

compiler and compiler version: clang

please describe what symptom you see, what you would expect to see instead and
how to reproduce it.

Try to download the following torrent under Android:

magnet:?xt=urn:btih:ED05708B7CFB452E6042C54EEDDED94943FD05AD&dn=Various+Artists+-+Top+Country+Bangers+%282024%29+Mp3+320kbps+%5BPMEDIA%5D+%E2%AD%90%EF%B8%8F&tr=udp%3A%2F%2Ftracker.opentrackr.org%3A1337%2Fannounce&tr=udp%3A%2F%2Fopen.stealth.si%3A80%2Fannounce&tr=udp%3A%2F%2Fopentracker.i2p.rocks%3A6969%2Fannounce&tr=udp%3A%2F%2Ftracker.openbittorrent.com%3A6969%2Fannounce&tr=udp%3A%2F%2Fopen.demonii.com%3A1337&tr=udp%3A%2F%2Faarsen.me%3A6969%2Fannounce&tr=udp%3A%2F%2Fexodus.desync.com%3A6969%2Fannounce&tr=udp%3A%2F%2Ftracker.torrent.eu.org%3A451%2Fannounce&tr=udp%3A%2F%2Fexplodie.org%3A6969%2Fannounce&tr=udp%3A%2F%2Fuploads.gamecoast.net%3A6969%2Fannounce&tr=http%3A%2F%2Ftracker.files.fm%3A6969%2Fannounce&tr=udp%3A%2F%2Ftracker.tiny-vps.com%3A6969%2Fannounce&tr=udp%3A%2F%2Fp4p.arenabg.com%3A1337%2Fannounce&tr=udp%3A%2F%2Ftracker.dler.com%3A6969%2Fannounce&tr=udp%3A%2F%2Ftracker.opentrackr.org%3A1337%2Fannounce&tr=http%3A%2F%2Ftracker.openbittorrent.com%3A80%2Fannounce&tr=udp%3A%2F%2Fopentracker.i2p.rocks%3A6969%2Fannounce&tr=udp%3A%2F%2Ftracker.internetwarriors.net%3A1337%2Fannounce&tr=udp%3A%2F%2Ftracker.leechers-paradise.org%3A6969%2Fannounce&tr=udp%3A%2F%2Fcoppersurfer.tk%3A6969%2Fannounce&tr=udp%3A%2F%2Ftracker.zer0day.to%3A1337%2Fannounce

In meta data received signal you'll get a torrent with non-sanitized files names. This is not the case under Windows. For example, the second file of this torrent ("02. Orville Peck - How Far Will We Take It?.mp3"). Under Windows it's name in torrent object provided by Libtorrent is 02. Orville Peck - How Far Will We Take It_.mp3 ("?" replaced with "_"). However, under Android, this is not the case. And because of this, we get file creation failure.

[arvidn]

filenames (path elements) are sanitized in this function: https://github.com/arvidn/libtorrent/blob/RC_2_0/src/torrent_info.cpp#L158

As you can see, it differs between windows and other operating systems. For example, to determine whether a character is valid in a filename or not, this function is used:

https://github.com/arvidn/libtorrent/blob/RC_2_0/src/torrent_info.cpp#L88

On linux, filenames and directory names are allowed to contain "?". Is that not the case on Android?

[alervd]

Yes, it seems that '?' is not allowed under Android. At least on FAT file system which is used for SD cards.
https://stackoverflow.com/a/64021421/3765267
Samsung My Files app does not allow me to use '?' in file names on both internal storage and SD card.

[OvercookedBeef]

I've tried the same on LineageOS and using Material Files (from f-droid) and same thing here, so not specific to Samsung or SD card (I tried with internal storage)

[arvidn]

On windows, I believe the operating system restricts some characters, but I suspect that on Android it's the file system that limits valid characters. Perhaps FAT32 is common there. What I'm doing in that PR is an approximation.

[arvidn]

how does this look? #7828

[alervd]

how does this look? #7828

Thanks, it became better than ever. :)
However, I would recommend to use raw string literals.

Would it be possible to merge the fix into 1.2.x?

[arvidn]

That is a string literal. I don't know what would make it "raw".

[alervd]

@arvidn I meant this: https://en.cppreference.com/w/cpp/language/string_literal.
E.g. static const char invalid_chars[] = R"("*:<>?|";`)".

[arvidn]

back-port: #7832