tracker: maintain Match < Next invariant

This commit documents the meaning of Match and Next field in the
Progress struct. It also ensures that the invariant is maintained in all
code places that initialize or modify these fields.

Signed-off-by: Pavel Kalinnikov <[email protected]>

confchange/confchange.go

@@ -259,8 +259,8 @@ func (c Changer) initProgress(cfg *tracker.Config, trk tracker.ProgressMap, id u
 		// at all (and will thus likely need a snapshot), though the app may
 		// have applied a snapshot out of band before adding the replica (thus
 		// making the first index the better choice).
-		Next:      c.LastIndex,
 		Match:     0,
+		Next:      max(c.LastIndex, 1), // invariant: Match < Next
 		Inflights: tracker.NewInflights(c.Tracker.MaxInflight, c.Tracker.MaxInflightBytes),
 		IsLearner: isLearner,
 		// When a node is first added, we should mark it as recently active.

confchange/testdata/joint_autoleave.txt

@@ -5,14 +5,14 @@ simple
 v1
 ----
 voters=(1)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 
 # Autoleave is reflected in the config.
 enter-joint autoleave=true
 v2 v3
 ----
 voters=(1 2 3)&&(1) autoleave
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 2: StateProbe match=0 next=1
 3: StateProbe match=0 next=1
 
@@ -24,6 +24,6 @@ config is already joint
 leave-joint
 ----
 voters=(1 2 3)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 2: StateProbe match=0 next=1
 3: StateProbe match=0 next=1

confchange/testdata/joint_idempotency.txt

@@ -5,19 +5,19 @@ simple
 v1
 ----
 voters=(1)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 
 enter-joint
 r1 r2 r9 v2 v3 v4 v2 v3 v4 l2 l2 r4 r4 l1 l1
 ----
 voters=(3)&&(1) learners=(2) learners_next=(1)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 2: StateProbe match=0 next=1 learner
 3: StateProbe match=0 next=1
 
 leave-joint
 ----
 voters=(3) learners=(1 2)
-1: StateProbe match=0 next=0 learner
+1: StateProbe match=0 next=1 learner
 2: StateProbe match=0 next=1 learner
 3: StateProbe match=0 next=1

confchange/testdata/joint_learners_next.txt

@@ -8,17 +8,17 @@ simple
 v1
 ----
 voters=(1)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 
 enter-joint
 v2 l1
 ----
 voters=(2)&&(1) learners_next=(1)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 2: StateProbe match=0 next=1
 
 leave-joint
 ----
 voters=(2) learners=(1)
-1: StateProbe match=0 next=0 learner
+1: StateProbe match=0 next=1 learner
 2: StateProbe match=0 next=1

confchange/testdata/simple_idempotency.txt

@@ -2,33 +2,33 @@ simple
 v1
 ----
 voters=(1)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 
 simple
 v1
 ----
 voters=(1)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 
 simple
 v2
 ----
 voters=(1 2)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 2: StateProbe match=0 next=2
 
 simple
 l1
 ----
 voters=(2) learners=(1)
-1: StateProbe match=0 next=0 learner
+1: StateProbe match=0 next=1 learner
 2: StateProbe match=0 next=2
 
 simple
 l1
 ----
 voters=(2) learners=(1)
-1: StateProbe match=0 next=0 learner
+1: StateProbe match=0 next=1 learner
 2: StateProbe match=0 next=2
 
 simple

confchange/testdata/simple_promote_demote.txt

@@ -4,20 +4,20 @@ simple
 v1
 ----
 voters=(1)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 
 simple
 v2
 ----
 voters=(1 2)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 2: StateProbe match=0 next=1
 
 simple
 v3
 ----
 voters=(1 2 3)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 2: StateProbe match=0 next=1
 3: StateProbe match=0 next=2
 
@@ -27,7 +27,7 @@ simple
 l1 v1
 ----
 voters=(1 2 3)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 2: StateProbe match=0 next=1
 3: StateProbe match=0 next=2
 
@@ -36,7 +36,7 @@ simple
 l2
 ----
 voters=(1 3) learners=(2)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 2: StateProbe match=0 next=1 learner
 3: StateProbe match=0 next=2
 
@@ -46,7 +46,7 @@ simple
 v2 l2
 ----
 voters=(1 3) learners=(2)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 2: StateProbe match=0 next=1 learner
 3: StateProbe match=0 next=2
 
@@ -55,6 +55,6 @@ simple
 v2
 ----
 voters=(1 2 3)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 2: StateProbe match=0 next=1
 3: StateProbe match=0 next=2

confchange/testdata/update.txt

@@ -6,18 +6,18 @@ simple
 v1
 ----
 voters=(1)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 
 simple
 v2 u1
 ----
 voters=(1 2)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 2: StateProbe match=0 next=1
 
 simple
 u1 u2 u3 u1 u2 u3
 ----
 voters=(1 2)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1
 2: StateProbe match=0 next=1

confchange/testdata/zero.txt

@@ -3,4 +3,4 @@ simple
 v1 r0 v0 l0
 ----
 voters=(1)
-1: StateProbe match=0 next=0
+1: StateProbe match=0 next=1

tracker/progress.go

@@ -28,7 +28,15 @@ import (
 // strewn around `*raft.raft`. Additionally, some fields are only used when in a
 // certain State. All of this isn't ideal.
 type Progress struct {
-	Match, Next uint64
+	// Match is the index up to which the follower's log is known to match the
+	// leader's.
+	Match uint64
+	// Next is the log index of the next entry to send to this follower. All
+	// entries with indices in (Match, Next) interval are already in flight.
+	//
+	// Invariant: 0 <= Match < Next.
+	Next uint64
+
 	// State defines how the leader should interact with the follower.
 	//
 	// When in StateProbe, leader sends at most one replication message
@@ -208,7 +216,6 @@ func (pr *Progress) MaybeDecrTo(rejected, matchHint uint64) bool {
 		return false
 	}
 
-	// Next index shall always be larger than match index.
 	pr.Next = max(min(rejected, matchHint+1), pr.Match+1)
 	pr.MsgAppFlowPaused = false
 	return true