Allow app to register a download provider

[artonge]

This PR allows applications to respond to file download requests by registering providers.

Another way of doing it would be to get the content from our DAV API, but I am not sure that I can get the content of a file without doing an HTTP request.

• Apps can register download providers like so:

// AppInfo/Application.php
class Application extends App implements IBootstrap {
	// ...
	public function register(IRegistrationContext $context): void {
		$context->registerFileDownloadProvider(MyFileDownloadProvider::class);
	}
	// ...
}

// Provider/MyFileDownloadProvider
use OCP\Files\IFileDownloadProvider;

class MyFileDownloadProvider implements IFileDownloadProvider {
	public function getNode(string $davPath): ?Node {
		return $theFileContent;
	}
}

• The new DownloadController will then query all the providers looking for a content to put in a zip file.
• As an example, I add a download provider for /files/$userId/... URLs in FileDownloadProvider.php
• Other implementation example for photos: Register a download provider photos#1450
• The frontend can the request for an array of files by running a GET on https://server.example/apps/files/api/v1/download?files=${stringified_json_array}

Notes:

• The DownloadController is accessible from a logged-out user. (@PublicPage)
• The endpoint support heterogeneous DAV paths. Meaning, I can do ?files=['files/alice/hello.txt','photos/sharedalbums/bob/img.png']

Todo:

• Port the downloadStartSecret logic from ajax/download.php

[nickvergessen]

The DownloadController is accessible from a logged-out user. (@publicpage)

Please make use of the rate limit then, so guests can not DDoS the server:

server/lib/private/AppFramework/Middleware/Security/RateLimitingMiddleware.php

Lines 42 to 43 in 6312c0d

	* @UserRateThrottle(limit=5, period=100)
	* @AnonRateThrottle(limit=1, period=100)

Should also add brute force protection and log attempts for guessing share tokens

server/lib/private/AppFramework/Middleware/Security/BruteForceMiddleware.php

Line 44 in 215aef3

* that are annotated with @BruteForceProtection(action=$action) whereas $action

[artonge]

Shouldn't there be a way to indicate failed guess attempts for brute force protection ? Like in:

https://github.com/nextcloud/photos/blob/05c3c2f2271b5ee58c1a0b9a7b3f55573230d20b/lib/Sabre/PublicAlbumAuthBackend.php#L53-L60

[nickvergessen]

Yes for brute force protection is $response->throttle();

For rate limiting this is not needed as rate limiting always kicks in.

[nickvergessen]

Should we have an upper limit as well? Server can easily run into a timeout anyway.

[artonge]

Difficult to predict from the number of files. Also, what to do if we reach a timeout ? Because this prevent users from downloading their files. We can maybe create a background job that will make the file available at a specific URL.

Would checking for the execTime = currentTime - startTime in the loop make sense? If execTime is bigger than timeoutTime, we can return with an message saying, "Download will be available later at url".

Same thing with the size. Not sure how the zip is created, but if this is in memory, then this will lead to OOM. So when the size reaches 2Gb, we can also return the same message.

[nickvergessen]

What is the goal of this function? It looks very dangerous and I don't think it does what you think it does?

?files=['photos/sharedalbums/bob/img.png','photos/sharedalbums/bar/img.png'] has commonPrefix of photos/sharedalbums/b and then try to find notes like ob/img.png and ar/img.png?!

[artonge]

It's to prevent having a zip file with this kind of hierarchy:

files/
├─ userId/
│  ├─ folderName/
│  │  ├─ theFileIWant.txt

But only

theFileIWant.txt

Note that $commonPrefix is not used for searching, but only for building the zip.

However, your concern is still valid. The best way would be to be able to split the path and then compare the parts. But I am not sure that we have a proper way to do that in PHP. Any tips ?

[nickvergessen]

explode('/', $path) and then compare the segments?