zebrad/components/sync/
recent_sync_lengths.rs

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1//! A channel which holds a list of recent syncer response lengths.
2
3use tokio::sync::watch;
4
5#[cfg(test)]
6mod tests;
7
8/// A helper type which holds a list of recent syncer response lengths.
9/// These sync lengths can be used to work out if Zebra has reached the end of the chain.
10///
11/// New lengths are added to the front of the list.
12/// Old lengths are dropped if the list is longer than `MAX_RECENT_LENGTHS`.
13//
14// TODO: disable the mempool if obtain or extend tips return errors?
15#[derive(Debug)]
16pub struct RecentSyncLengths {
17    /// A sender for an array of recent sync lengths.
18    sender: watch::Sender<Vec<usize>>,
19
20    /// A local copy of the contents of `sender`.
21    // TODO: Replace with calls to `watch::Sender::borrow` once Tokio is updated to 1.0.0 (#2573)
22    recent_lengths: Vec<usize>,
23}
24
25impl RecentSyncLengths {
26    /// The maximum number of lengths sent by `RecentSyncLengths`.
27    ///
28    /// Older lengths are dropped.
29    ///
30    /// This length was chosen as a tradeoff between:
31    /// * clearing temporary errors and temporary syncs quickly
32    /// * distinguishing between temporary and sustained syncs/errors
33    /// * activating the syncer shortly after reaching the chain tip
34    pub const MAX_RECENT_LENGTHS: usize = 3;
35
36    /// Create a new instance of [`RecentSyncLengths`]
37    /// and a [`watch::Receiver`] endpoint for receiving recent sync lengths.
38    pub fn new() -> (Self, watch::Receiver<Vec<usize>>) {
39        let (sender, receiver) = watch::channel(Vec::new());
40
41        (
42            RecentSyncLengths {
43                sender,
44                recent_lengths: Vec::with_capacity(Self::MAX_RECENT_LENGTHS),
45            },
46            receiver,
47        )
48    }
49
50    // We skip the genesis block download, because it just uses the genesis hash directly,
51    // rather than asking peers for the next blocks in the chain.
52    // (And if genesis downloads kept failing, we could accidentally activate the mempool.)
53
54    /// Insert a sync length from [`ChainSync::obtain_tips`] at the front of the
55    /// list.
56    ///
57    /// [`ChainSync::obtain_tips`]: super::ChainSync::obtain_tips
58    #[instrument(skip(self), fields(self.recent_lengths))]
59    pub fn push_obtain_tips_length(&mut self, sync_length: usize) {
60        // currently, we treat lengths from obtain and extend tips exactly the same,
61        // but we might want to ignore some obtain tips lengths
62        //
63        // See "Response Lengths During Sync -> Details" in:
64        // https://github.com/ZcashFoundation/zebra/issues/2592#issuecomment-897304684
65        self.update(sync_length)
66    }
67
68    /// Insert a sync length from [`ChainSync::extend_tips`] at the front of the
69    /// list.
70    ///
71    /// [`ChainSync::extend_tips`]: super::ChainSync::extend_tips
72    #[instrument(skip(self), fields(self.recent_lengths))]
73    pub fn push_extend_tips_length(&mut self, sync_length: usize) {
74        self.update(sync_length)
75    }
76
77    /// Sends a list update to listeners.
78    ///
79    /// Prunes recent lengths, if the list is longer than `MAX_RECENT_LENGTHS`.
80    fn update(&mut self, sync_length: usize) {
81        self.recent_lengths.insert(0, sync_length);
82
83        self.recent_lengths.truncate(Self::MAX_RECENT_LENGTHS);
84
85        debug!(
86            recent_lengths = ?self.recent_lengths,
87            "sending recent sync lengths"
88        );
89
90        // ignore dropped receivers
91        let _ = self.sender.send(self.recent_lengths.clone());
92    }
93}
zebrad/components/sync/recent_sync_lengths.rs

zebrad/components/sync/
recent_sync_lengths.rs
