please add an option to File.writeAsBytes to avoid overwriting.

[stephane-archer]

var outputFile = File("a.jpg");
if (!outputFile.exist()) {
    // another program creates a.jpg
    await outputFile.writeAsBytes(jpegData); // boom overwrite!!!
}

please add an option to File.writeAsBytes to avoid overwriting.

maybe something like await outputFile.writeAsBytes(jpegData, exclusive: true);

[dart-github-bot]

Summary: User requests File.writeAsBytes option to prevent overwriting existing files, avoiding data loss in concurrent scenarios. Proposed solution: exclusive flag.

[julemand101]

What behavior do you expect in case the file already exists? Throw exception?

[stephane-archer]

@julemand101 yes

[aam]

Have you looked at https://api.dart.dev/dart-io/File/create.html which allows you to specify exclusive: true to ensure new file is created, get PathExistsException if file already exists?

[stephane-archer]

var file = await File("a.txt").create(recursive : false,
exclusive: true,
});
// someone write to a.txt
file.white(data); // overwrite!

From my understanding create guarantee you to create the file, but from my understanding File is not a file descriptor so now that the file is created overwrite can still happens

[mraleph]

@stephane-archer AFAIK there is no way to prevent that in POSIX though - if the other program does not take precautions (e.g. does not use O_EXCL with O_CREAT) then you can't stop it from writing something into the same file you have opened.

[stephane-archer]

@mraleph there is no way to ask for exclusive access until the file is closed on POSIX?

[mraleph]

@stephane-archer not to the best of my knowledge. there are file locks but they are advisory (so writers should all cooperate and use them - write operations themselves don't check locks). Mandatory locks API kinda sorta exists - but not really, see man fcntl, section "Mandatory locking".

[stephane-archer]

Thanks for your answers

So the best we have right know in Dart to write data without overwrite is:

var file = await File("a.txt").create(recursive : false,
exclusive: true,
})
file.writeAsBytes(data);

I think it would be still nice to have a short version for this:

await File("a.txt").writeAsBytes(data, exclusive: true);
// or 

File("a.txt", exclusive: true).writeAsBytes(data);