fix: Don't download pre-message again if it is known already

[Hocuri]

There was a bug in prefetch_should_download() that it made it return true for pre-messages even when they were already downloaded. This meant that pre-messages were downloaded from all relays, rather than just one, wasting internet data.

The fix is in rfc724_mid_download_tried(), which is used by prefetch_should_download() to determine whether a message was already downloaded.

[hpk42]

the docstring is not now mapping the sql statement anymore, if i see it correctly? also half-copying the sql statement is not very helpful IMO. Maybe something like this?

/// Returns `true` if the given `rfc724_mid` has nothing left to fetch from a server,
/// i.e. it was already fetched, sent by us, or received as a pre-message.
pub(crate) async fn rfc724_mid_nothing_to_fetch(context: &Context, rfc724_mid: &str) -> Result<bool> {

[Hocuri]

For pre-messages, this function behaves exactly the same as for normal messages: It returns true iff the message was already fully fetched and is therefore in the database. It is post-messages that are treated differently, because for them there can be some in-between states where they are in the database but were not fetched yet, or it was tried to fetch them but unsuccessfully.

rfc724_mid_nothing_to_fetch() sounds misleading because the function returns true if a post-message was tried to fetch but network was lost halfway through. In this case, we still give the user the possibility to try again.

I tried adding some documentation, is it understandable?

[hpk42]

looks like a good fix overall (see comments for docstrings/naming suggestions).
Still may makes sense if somebody else approves, as i am not on solid ground with the current state of ephemeral/deletion mechanisms, and it's a sensitive area.

[link2xt]

Looks good, agree that the documentation comment for rfc724_mid_download_tried should say something about pre-messages.

[link2xt]

Suggested change

	// because `download_state` is always `Done` and `pre_rfc724_mid is always an empty string.
	// because `download_state` is always `Done` and `pre_rfc724_mid` is always an empty string.