From 7474cfac08491d8486dad312655d2d645baf8dde Mon Sep 17 00:00:00 2001
From: Arseny Sher <sher-ars@yandex.ru>
Date: Fri, 27 Aug 2021 14:55:21 +0300
Subject: [PATCH] Rename VCL to epochStartLsn and restart_lsn to truncate_lsn.

epochStartLsn is the LSN since which new proposer writes its WAL in its epoch,
let's be more explicit here.

truncate_lsn is LSN still needed by the most lagging safekeeper. restart_lsn is
terminology from pg_replicaton_slots, but here we don't really have 'restart';
hopefully truncate word makes it clearer.
---
 walkeeper/src/safekeeper.rs | 34 ++++++++++++++++++----------------
 1 file changed, 18 insertions(+), 16 deletions(-)

diff --git a/walkeeper/src/safekeeper.rs b/walkeeper/src/safekeeper.rs
index b9fcae80f3..d85a901696 100644
--- a/walkeeper/src/safekeeper.rs
+++ b/walkeeper/src/safekeeper.rs
@@ -67,10 +67,12 @@ pub struct SafeKeeperState {
     /// Unique id of the last *elected* proposer we dealed with. Not needed
     /// correctness, exists for monitoring purposes.
     pub proposer_uuid: PgUuid,
-    /// part of WAL acknowledged by quorum
+    /// part of WAL acknowledged by quorum (note that we might not have wal to
+    /// up this point locally)
     pub commit_lsn: Lsn,
-    /// minimal LSN which may be needed for recovery of some safekeeper: min(commit_lsn) for all safekeepers
-    pub restart_lsn: Lsn,
+    /// minimal LSN which may be needed for recovery of some safekeeper (end lsn
+    /// + 1 of last record streamed to everyone)
+    pub truncate_lsn: Lsn,
 }
 
 impl SafeKeeperState {
@@ -87,8 +89,8 @@ impl SafeKeeperState {
                 wal_seg_size: 0,
             },
             proposer_uuid: [0; 16],
-            commit_lsn: Lsn(0),  /* part of WAL acknowledged by quorum */
-            restart_lsn: Lsn(0), /* minimal LSN which may be needed for recovery of some safekeeper */
+            commit_lsn: Lsn(0),   /* part of WAL acknowledged by quorum */
+            truncate_lsn: Lsn(0), /* minimal LSN which may be needed for recovery of some safekeeper */
         }
     }
 }
@@ -136,7 +138,7 @@ pub struct VoteResponse {
 }
 
 /// Request with WAL message sent from proposer to safekeeper. Along the way it
-/// announces 1) successful election (with VCL); 2) commit_lsn.
+/// announces 1) successful election (with epoch_start_lsn); 2) commit_lsn.
 #[derive(Debug, Serialize, Deserialize)]
 pub struct AppendRequest {
     h: AppendRequestHeader,
@@ -145,8 +147,8 @@ pub struct AppendRequest {
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct AppendRequestHeader {
     term: Term,
-    /// volume commit LSN
-    vcl: Lsn,
+    // LSN since the proposer appends WAL; determines epoch switch point.
+    epoch_start_lsn: Lsn,
     /// start position of message in WAL
     begin_lsn: Lsn,
     /// end position of message in WAL
@@ -272,7 +274,7 @@ pub struct SafeKeeper<ST: Storage> {
     /// reading wal.
     pub flush_lsn: Lsn,
     pub tli: u32,
-    pub flushed_restart_lsn: Lsn,
+    pub flushed_truncate_lsn: Lsn,
     pub storage: ST,
     pub s: SafeKeeperState,          // persistent part
     pub elected_proposer_term: Term, // for monitoring/debugging
@@ -287,7 +289,7 @@ where
         SafeKeeper {
             flush_lsn,
             tli,
-            flushed_restart_lsn: Lsn(0),
+            flushed_truncate_lsn: Lsn(0),
             storage,
             s: state,
             elected_proposer_term: 0,
@@ -369,7 +371,7 @@ where
             resp.vote_given = true as u64;
             resp.epoch = self.s.acceptor_state.epoch;
             resp.flush_lsn = self.flush_lsn;
-            resp.restart_lsn = self.s.restart_lsn;
+            resp.restart_lsn = self.s.truncate_lsn;
         }
         info!("processed VoteRequest for term {}: {:?}", msg.term, &resp);
         Ok(AcceptorProposerMessage::VoteResponse(resp))
@@ -414,7 +416,7 @@ where
          * XXX: this is wrong, we must actively truncate not matching part of log.
          */
         if self.s.acceptor_state.epoch < msg.h.term
-            && msg.h.end_lsn > max(self.flush_lsn, msg.h.vcl)
+            && msg.h.end_lsn > max(self.flush_lsn, msg.h.epoch_start_lsn)
         {
             info!("switched to new epoch {}", msg.h.term);
             self.s.acceptor_state.epoch = msg.h.term; /* bump epoch */
@@ -426,7 +428,7 @@ where
 
         self.s.proposer_uuid = msg.h.proposer_uuid;
         self.s.commit_lsn = msg.h.commit_lsn;
-        self.s.restart_lsn = msg.h.restart_lsn;
+        self.s.truncate_lsn = msg.h.restart_lsn;
 
         /*
          * Update restart LSN in control file.
@@ -434,10 +436,10 @@ where
          * when restart_lsn delta exceeds WAL segment size.
          */
         sync_control_file |=
-            self.flushed_restart_lsn + (self.s.server.wal_seg_size as u64) < self.s.restart_lsn;
+            self.flushed_truncate_lsn + (self.s.server.wal_seg_size as u64) < self.s.truncate_lsn;
         self.storage.persist(&self.s, sync_control_file)?;
         if sync_control_file {
-            self.flushed_restart_lsn = self.s.restart_lsn;
+            self.flushed_truncate_lsn = self.s.truncate_lsn;
         }
 
         let resp = AppendResponse {
@@ -517,7 +519,7 @@ mod tests {
 
         let mut ar_hdr = AppendRequestHeader {
             term: 1,
-            vcl: Lsn(2),
+            epoch_start_lsn: Lsn(2),
             begin_lsn: Lsn(1),
             end_lsn: Lsn(2),
             commit_lsn: Lsn(0),