Content appearing and disappearing when refreshing a channel

[devos50]

I noticed that when going to a channel in Tribler and refreshing it by clicking on the channel name in the breadcrumb navigator, sometimes content appears and disappears. While this is expected (since Tribler can discover new content), sometimes the amount of content is changing significantly. Note that it requires like 20+ or 30+ refreshes sometimes to trigger this behavior.

Below are two screenshots that were taken within the same channel. Refreshing it occasionally gives different content. Note that sometimes, item A in the first screenshot is not present in screenshot B, hinting that this might be a bug somewhere.

[ichorid]

Reproduced it locally. #6415 does not fix the problem completely, but greatly reduces the probability of triggering it.

The problem is threefold:

1. There is no integrity/versioning control for preview channel entries neither in the local channels DB nor when querying for channels remotely
2. The local list of remote peers serving the channel does not account for channel versions
3. The local list of remote peers is cycled based on what peer was seen last

As a result, after 10-30 refreshes we may connect to a peer serving an outdated (or even broken!) version of a channel. Or even get a timeout and show the local cache, which could contain duplicates and deleted folders, etc.

There is no real way to fix it in Channels 2.0 architecture, but we can still alleviate the problem by adding better methods for selecting remote peers, for instance:

• filtering based on channel version - only serve from peers that got the version that is up to date with our version
• note the service time for each request and prioritize peers that have better latency (comparing the latency for the same query)

[devos50]

I have the impression that #6415 even makes it easier to reproduce. Anyway, in like 20 refreshes of one of the most popular channels in Tribler, I get random content in like 5 refreshes.

[ichorid]

I had to click it about 20-30 times to get to see the thing 🤷 . I guess it is highly dependent on network and PC performance.

[ichorid]

This is a result of remote hosts providing an older version of the channel through the remote preview system. Currently, the remote preview system does not filter remote preview sources by version. We are facing a dilemma here: if we add such filtering we may make reduce channel previews availability, because peers with older versions of a channel will not be considered as legitimate sources for channel preview.
Example:
Local channel version: 100 (not subscribed)
Host A channel version: 99
Host B channel version: 100

Currently, both A and B can be selected as the preview source.

If we enable filtering of preview sources by channel version, only host B will be considered as a legitimate source.

Should we do so?