Fix connection being made without bridge if there are no bridges

[MarkusPettersson98]

This PR addresses a recent regression where if all bridges would be discarded before random selection, the tunnel parameters would still be generated. We should instead fail loudly and don't attempt to establish a tunnel in that case.

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This change is 

[linear]

DES-770 Connection is made without bridge if bridges array is empty

[dlon]

Reviewed 2 of 3 files at r1, all commit messages.
Reviewable status: 2 of 3 files reviewed, 2 unresolved discussions (waiting on @MarkusPettersson98)

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mullvad-relay-selector/src/error.rs line 31 at r1 (raw file):

    },

    #[error("No candidates remain")]

Maybe this is a bit vague. "No candidate bridge found"? Or just use NoBridge.

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mullvad-relay-selector/src/relay_selector/mod.rs line 934 at r1 (raw file):

                    custom_lists,
                )
                .map(|(settings, relay)| SelectedBridge::Normal { settings, relay });

Maybe Result<Option<_>> would be slightly better?

let bridge = stuff(...)?;
Ok(Some(bridge))

And there's no need to transpose the option above. But this is probably nitpicky.

[dlon]

Reviewed 1 of 3 files at r1.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @MarkusPettersson98)

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mullvad-relay-selector/src/relay_selector/mod.rs line 961 at r1 (raw file):

                .ok_or(Error::NoRelay),
        }?;
        let endpoint = helpers::pick_random_bridge(bridge_data, &bridge).ok_or(Error::NoBridge)?;

This arguably belongs in detailer. Isn't it really just "filling in details" for a bridge we've already picked?

[MarkusPettersson98]

Reviewable status: 0 of 4 files reviewed, 1 unresolved discussion (waiting on @dlon)

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mullvad-relay-selector/src/relay_selector/mod.rs line 934 at r1 (raw file):

Previously, dlon (David Lönnhager) wrote…

Maybe Result<Option<_>> would be slightly better?

let bridge = stuff(...)?;
Ok(Some(bridge))

And there's no need to transpose the option above. But this is probably nitpicky.

Done

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mullvad-relay-selector/src/error.rs line 31 at r1 (raw file):

Previously, dlon (David Lönnhager) wrote…

Maybe this is a bit vague. "No candidate bridge found"? Or just use NoBridge.

Sure, I'll just re-use NoBridge

[dlon]

Reviewed 2 of 2 files at r2, 3 of 3 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved

[dlon]

Reviewed 1 of 1 files at r4, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved