raft: optimize string representation of Progress

Make it less verbose by omitting the values for the steady state.
Also rearrange the order so that information that is typically more
relevant is printed first.

raft/tracker/progress.go

@@ -14,7 +14,10 @@
 
 package tracker
 
-import "fmt"
+import (
+	"fmt"
+	"strings"
+)
 
 // Progress represents a follower’s progress in the view of the leader. Leader
 // maintains progresses of all followers, and sends entries to the follower
@@ -50,7 +53,7 @@ type Progress struct {
 	// RecentActive can be reset to false after an election timeout.
 	RecentActive bool
 
-	// ProbeSent is used while this follow is in StateProbe. When ProbeSent is
+	// ProbeSent is used while this follower is in StateProbe. When ProbeSent is
 	// true, raft should pause sending replication message to this peer until
 	// ProbeSent is reset. See ProbeAcked() and IsPaused().
 	ProbeSent bool
@@ -210,6 +213,25 @@ func (pr *Progress) IsPaused() bool {
 }
 
 func (pr *Progress) String() string {
-	return fmt.Sprintf("next = %d, match = %d, state = %s, waiting = %v, pendingSnapshot = %d, recentActive = %v, isLearner = %v",
-		pr.Next, pr.Match, pr.State, pr.IsPaused(), pr.PendingSnapshot, pr.RecentActive, pr.IsLearner)
+	var buf strings.Builder
+	fmt.Fprintf(&buf, "%s match=%d next=%d", pr.State, pr.Match, pr.Next)
+	if pr.IsLearner {
+		fmt.Fprint(&buf, " learner")
+	}
+	if pr.IsPaused() {
+		fmt.Fprint(&buf, " paused")
+	}
+	if pr.PendingSnapshot > 0 {
+		fmt.Fprintf(&buf, " pendingSnap=%d", pr.PendingSnapshot)
+	}
+	if !pr.RecentActive {
+		fmt.Fprintf(&buf, " inactive")
+	}
+	if n := pr.Inflights.Count(); n > 0 {
+		fmt.Fprintf(&buf, " inflight=%d", n)
+		if pr.Inflights.Full() {
+			fmt.Fprint(&buf, "[full]")
+		}
+	}
+	return buf.String()
 }

raft/tracker/progress_test.go

@@ -18,6 +18,25 @@ import (
 	"testing"
 )
 
+func TestProgressString(t *testing.T) {
+	ins := NewInflights(1)
+	ins.Add(123)
+	pr := &Progress{
+		Match:           1,
+		Next:            2,
+		State:           StateSnapshot,
+		PendingSnapshot: 123,
+		RecentActive:    false,
+		ProbeSent:       true,
+		IsLearner:       true,
+		Inflights:       ins,
+	}
+	const exp = `StateSnapshot match=1 next=2 learner paused pendingSnap=123 inactive inflight=1[full]`
+	if act := pr.String(); act != exp {
+		t.Errorf("exp: %s\nact: %s", exp, act)
+	}
+}
+
 func TestProgressIsPaused(t *testing.T) {
 	tests := []struct {
 		state  StateType