Port #243 to the Prost branch (#259)

Signed-off-by: Nick Cameron <nrc@ncameron.org>

proto/proto/eraftpb.proto

@@ -71,6 +71,7 @@ message Message {
     repeated Entry entries = 7;
     uint64 commit = 8;
     Snapshot snapshot = 9;
+    uint64 request_snapshot = 13;
     bool reject = 10;
     uint64 reject_hint = 11;
     bytes context = 12;

proto/src/prost/eraftpb.rs

@@ -61,6 +61,8 @@ pub struct Message {
     pub commit: u64,
     #[prost(message, optional, tag = "9")]
     pub snapshot: ::std::option::Option<Snapshot>,
+    #[prost(uint64, tag = "13")]
+    pub request_snapshot: u64,
     #[prost(bool, tag = "10")]
     pub reject: bool,
     #[prost(uint64, tag = "11")]

proto/src/prost/wrapper_eraftpb.rs

@@ -364,6 +364,18 @@ impl Message {
         self.snapshot.take().unwrap_or_else(Snapshot::default)
     }
     #[inline]
+    pub fn clear_request_snapshot(&mut self) {
+        self.request_snapshot = 0
+    }
+    #[inline]
+    pub fn set_request_snapshot(&mut self, v: u64) {
+        self.request_snapshot = v;
+    }
+    #[inline]
+    pub fn get_request_snapshot(&self) -> u64 {
+        self.request_snapshot
+    }
+    #[inline]
     pub fn clear_reject(&mut self) {
         self.reject = false
     }

src/errors.rs

@@ -69,6 +69,10 @@ quick_error! {
         NotExists(id: u64, set: &'static str) {
             display("The node {} is not in the {} set.", id, set)
         }
+        /// The request snapshot is dropped.
+        RequestSnapshotDropped {
+            description("raft: request snapshot dropped")
+        }
     }
 }
 
@@ -82,6 +86,7 @@ impl cmp::PartialEq for Error {
             (&Error::Io(ref e1), &Error::Io(ref e2)) => e1.kind() == e2.kind(),
             (&Error::StepLocalMsg, &Error::StepLocalMsg) => true,
             (&Error::ConfigInvalid(ref e1), &Error::ConfigInvalid(ref e2)) => e1 == e2,
+            (&Error::RequestSnapshotDropped, &Error::RequestSnapshotDropped) => true,
             _ => false,
         }
     }

src/progress.rs

@@ -26,6 +26,7 @@
 // limitations under the License.
 
 use crate::errors::Error;
+use crate::raft::INVALID_INDEX;
 use fxhash::FxHashMap;
 use std::cmp;
 use std::collections::hash_map::HashMap;
@@ -189,6 +190,10 @@ pub struct Progress {
     /// this Progress will be paused. raft will not resend snapshot until the pending one
     /// is reported to be failed.
     pub pending_snapshot: u64,
+    /// This field is used in request snapshot.
+    /// If there is a pending request snapshot, this will be set to the request
+    /// index of the snapshot.
+    pub pending_request_snapshot: u64,
 
     /// This is true if the progress is recently active. Receiving any messages
     /// from the corresponding follower indicates the progress is active.
@@ -248,8 +253,8 @@ impl Progress {
         self.pending_snapshot = 0;
     }
 
-    /// Unsets pendingSnapshot if Match is equal or higher than
-    /// the pendingSnapshot
+    /// Unsets pending_snapshot if matched is equal or higher than
+    /// the pending_snapshot and the snapshot is not requested.
     pub fn maybe_snapshot_abort(&self) -> bool {
         self.state == ProgressState::Snapshot && self.matched >= self.pending_snapshot
     }
@@ -278,25 +283,41 @@ impl Progress {
     /// Returns false if the given index comes from an out of order message.
     /// Otherwise it decreases the progress next index to min(rejected, last)
     /// and returns true.
-    pub fn maybe_decr_to(&mut self, rejected: u64, last: u64) -> bool {
+    pub fn maybe_decr_to(&mut self, rejected: u64, last: u64, request_snapshot: u64) -> bool {
         if self.state == ProgressState::Replicate {
             // the rejection must be stale if the progress has matched and "rejected"
             // is smaller than "match".
-            if rejected <= self.matched {
+            // Or rejected equals to matched and request_snapshot is the INVALID_INDEX.
+            if rejected < self.matched
+                || (rejected == self.matched && request_snapshot == INVALID_INDEX)
+            {
                 return false;
             }
-            self.next_idx = self.matched + 1;
+            if request_snapshot == INVALID_INDEX {
+                self.next_idx = self.matched + 1;
+            } else {
+                self.pending_request_snapshot = request_snapshot;
+            }
             return true;
         }
 
-        // the rejection must be stale if "rejected" does not match next - 1
-        if self.next_idx == 0 || self.next_idx - 1 != rejected {
+        // The rejection must be stale if "rejected" does not match next - 1.
+        // Do not consider it stale if it is a request snapshot message.
+        if (self.next_idx == 0 || self.next_idx - 1 != rejected)
+            && request_snapshot == INVALID_INDEX
+        {
             return false;
         }
 
-        self.next_idx = cmp::min(rejected, last + 1);
-        if self.next_idx < 1 {
-            self.next_idx = 1;
+        // Do not decrease next index if it's requesting snapshot.
+        if request_snapshot == INVALID_INDEX {
+            self.next_idx = cmp::min(rejected, last + 1);
+            if self.next_idx < 1 {
+                self.next_idx = 1;
+            }
+        } else if self.pending_request_snapshot == INVALID_INDEX {
+            // Allow requesting snapshot even if it's not Replicate.
+            self.pending_request_snapshot = request_snapshot;
         }
         self.resume();
         true

src/raft.rs

@@ -106,6 +106,10 @@ pub struct Raft<T: Storage> {
     /// The maximum length (in bytes) of all the entries.
     pub max_msg_size: u64,
 
+    /// The peer is requesting snapshot, it is the index that the follower
+    /// needs it to be included in a snapshot.
+    pub pending_request_snapshot: u64,
+
     prs: Option<ProgressSet>,
 
     /// The current role of this node.
@@ -243,6 +247,7 @@ impl<T: Storage> Raft<T> {
             raft_log,
             max_inflight: c.max_inflight_msgs,
             max_msg_size: c.max_size_per_msg,
+            pending_request_snapshot: INVALID_INDEX,
             prs: Some(ProgressSet::new(peers.len(), learners.len())),
             state: StateRole::Follower,
             is_learner: false,
@@ -447,7 +452,7 @@ impl<T: Storage> Raft<T> {
         }
 
         m.set_msg_type(MessageType::MsgSnapshot);
-        let snapshot_r = self.raft_log.snapshot();
+        let snapshot_r = self.raft_log.snapshot(pr.pending_request_snapshot);
         if let Err(e) = snapshot_r {
             if e == Error::Store(StorageError::SnapshotTemporarilyUnavailable) {
                 debug!(
@@ -520,17 +525,24 @@ impl<T: Storage> Raft<T> {
         if pr.is_paused() {
             return;
         }
-        let term = self.raft_log.term(pr.next_idx - 1);
-        let ents = self.raft_log.entries(pr.next_idx, self.max_msg_size);
         let mut m = Message::default();
         m.set_to(to);
-        if term.is_err() || ents.is_err() {
-            // send snapshot if we failed to get term or entries
+        if pr.pending_request_snapshot != INVALID_INDEX {
+            // Check pending request snapshot first to avoid unnecessary loading entries.
             if !self.prepare_send_snapshot(&mut m, pr, to) {
                 return;
             }
         } else {
-            self.prepare_send_entries(&mut m, pr, term.unwrap(), ents.unwrap());
+            let term = self.raft_log.term(pr.next_idx - 1);
+            let ents = self.raft_log.entries(pr.next_idx, self.max_msg_size);
+            if term.is_err() || ents.is_err() {
+                // send snapshot if we failed to get term or entries.
+                if !self.prepare_send_snapshot(&mut m, pr, to) {
+                    return;
+                }
+            } else {
+                self.prepare_send_entries(&mut m, pr, term.unwrap(), ents.unwrap());
+            }
         }
         self.send(m);
     }
@@ -625,6 +637,7 @@ impl<T: Storage> Raft<T> {
 
         self.pending_conf_index = 0;
         self.read_only = ReadOnly::new(self.read_only.option);
+        self.pending_request_snapshot = INVALID_INDEX;
 
         let (last_index, max_inflight) = (self.raft_log.last_index(), self.max_inflight);
         let self_id = self.id;
@@ -716,9 +729,11 @@ impl<T: Storage> Raft<T> {
 
     /// Converts this node to a follower.
     pub fn become_follower(&mut self, term: u64, leader_id: u64) {
+        let pending_request_snapshot = self.pending_request_snapshot;
         self.reset(term);
         self.leader_id = leader_id;
         self.state = StateRole::Follower;
+        self.pending_request_snapshot = pending_request_snapshot;
         info!("{} became follower at term {}", self.tag, self.term);
     }
 
@@ -1155,7 +1170,7 @@ impl<T: Storage> Raft<T> {
                 m.get_index()
             );
 
-            if pr.maybe_decr_to(m.get_index(), m.get_reject_hint()) {
+            if pr.maybe_decr_to(m.get_index(), m.get_reject_hint(), m.get_request_snapshot()) {
                 debug!(
                     "{} decreased progress of {} to [{:?}]",
                     self.tag,
@@ -1224,7 +1239,8 @@ impl<T: Storage> Raft<T> {
         if pr.state == ProgressState::Replicate && pr.ins.full() {
             pr.ins.free_first_one();
         }
-        if pr.matched < self.raft_log.last_index() {
+        // Does it request snapshot?
+        if pr.matched < self.raft_log.last_index() || pr.pending_request_snapshot != INVALID_INDEX {
             *send_append = true;
         }
 
@@ -1332,6 +1348,7 @@ impl<T: Storage> Raft<T> {
         // out the next msgAppend.
         // If snapshot failure, wait for a heartbeat interval before next try
         pr.pause();
+        pr.pending_request_snapshot = INVALID_INDEX;
     }
 
     /// Check message's progress to decide which action should be taken.
@@ -1706,9 +1723,43 @@ impl<T: Storage> Raft<T> {
         Ok(())
     }
 
+    /// Request a snapshot from a leader.
+    pub fn request_snapshot(&mut self, request_index: u64) -> Result<()> {
+        if self.state == StateRole::Leader {
+            info!(
+                "{} can not request snapshot on leader; dropping request snapshot",
+                self.tag
+            );
+        } else if self.leader_id == INVALID_ID {
+            info!(
+                "{} no leader at term {}; dropping request snapshot",
+                self.tag, self.term
+            );
+        } else if self.get_snap().is_some() {
+            info!(
+                "{} there is a pending snapshot; dropping request snapshot",
+                self.tag
+            );
+        } else if self.pending_request_snapshot != INVALID_INDEX {
+            info!(
+                "{} there is a pending snapshot; dropping request snapshot",
+                self.tag
+            );
+        } else {
+            self.pending_request_snapshot = request_index;
+            self.send_request_snapshot();
+            return Ok(());
+        }
+        Err(Error::RequestSnapshotDropped)
+    }
+
     // TODO: revoke pub when there is a better way to test.
     /// For a given message, append the entries to the log.
     pub fn handle_append_entries(&mut self, m: &Message) {
+        if self.pending_request_snapshot != INVALID_INDEX {
+            self.send_request_snapshot();
+            return;
+        }
         if m.get_index() < self.raft_log.committed {
             let mut to_send = Message::default();
             to_send.set_to(m.get_from());
@@ -1753,6 +1804,10 @@ impl<T: Storage> Raft<T> {
     /// For a message, commit and send out heartbeat.
     pub fn handle_heartbeat(&mut self, mut m: Message) {
         self.raft_log.commit_to(m.get_commit());
+        if self.pending_request_snapshot != INVALID_INDEX {
+            self.send_request_snapshot();
+            return;
+        }
         let mut to_send = Message::default();
         to_send.set_to(m.get_from());
         to_send.set_msg_type(MessageType::MsgHeartbeatResponse);
@@ -1790,7 +1845,10 @@ impl<T: Storage> Raft<T> {
 
     fn restore_raft(&mut self, snap: &Snapshot) -> Option<bool> {
         let meta = snap.get_metadata();
-        if self.raft_log.match_term(meta.get_index(), meta.get_term()) {
+        // Do not fast-forward commit if we are requesting snapshot.
+        if self.pending_request_snapshot == INVALID_INDEX
+            && self.raft_log.match_term(meta.get_index(), meta.get_term())
+        {
             info!(
                 "{} [commit: {}, lastindex: {}, lastterm: {}] fast-forwarded commit to \
                  snapshot [index: {}, term: {}]",
@@ -1833,6 +1891,7 @@ impl<T: Storage> Raft<T> {
             meta.get_term()
         );
 
+        self.pending_request_snapshot = INVALID_INDEX;
         let nodes = meta.get_conf_state().get_nodes();
         let learners = meta.get_conf_state().get_learners();
         self.prs = Some(ProgressSet::new(nodes.len(), learners.len()));
@@ -2054,4 +2113,15 @@ impl<T: Storage> Raft<T> {
     pub fn abort_leader_transfer(&mut self) {
         self.lead_transferee = None;
     }
+
+    fn send_request_snapshot(&mut self) {
+        let mut m = Message::default();
+        m.set_msg_type(MessageType::MsgAppendResponse);
+        m.set_index(self.raft_log.committed);
+        m.set_reject(true);
+        m.set_reject_hint(self.raft_log.last_index());
+        m.set_to(self.leader_id);
+        m.set_request_snapshot(self.pending_request_snapshot);
+        self.send(m);
+    }
 }

src/raft_log.rs

@@ -384,11 +384,13 @@ impl<T: Storage> RaftLog<T> {
     }
 
     /// Returns the current snapshot
-    pub fn snapshot(&self) -> Result<Snapshot> {
-        self.unstable
-            .snapshot
-            .clone()
-            .map_or_else(|| self.store.snapshot(), Ok)
+    pub fn snapshot(&self, request_index: u64) -> Result<Snapshot> {
+        if let Some(snap) = self.unstable.snapshot.as_ref() {
+            if snap.get_metadata().get_index() >= request_index {
+                return Ok(snap.clone());
+            }
+        }
+        self.store.snapshot(request_index)
     }
 
     fn must_check_outofbounds(&self, low: u64, high: u64) -> Option<Error> {

src/raw_node.rs

@@ -458,6 +458,12 @@ impl<T: Storage> RawNode<T> {
         let _ = self.raft.step(m);
     }
 
+    /// Request a snapshot from a leader.
+    /// The snapshot's index must be greater or equal to the request_index.
+    pub fn request_snapshot(&mut self, request_index: u64) -> Result<()> {
+        self.raft.request_snapshot(request_index)
+    }
+
     /// TransferLeader tries to transfer leadership to the given transferee.
     pub fn transfer_leader(&mut self, transferee: u64) {
         let mut m = Message::default();