Avoid polling intermediate levels.

litestream/api.go

@@ -44,7 +44,6 @@ func NewReplica(name string, client litestream.ReplicaClient, options ReplicaOpt
 	if options.PollInterval <= 0 {
 		options.PollInterval = DefaultPollInterval
 	}
-	options.MinLevel = max(0, min(options.MinLevel, litestream.SnapshotLevel))
 
 	liteMtx.Lock()
 	defer liteMtx.Unlock()

litestream/example_test.go

@@ -0,0 +1,47 @@
+package litestream_test
+
+import (
+	"log"
+	"time"
+
+	"github.com/benbjohnson/litestream/s3"
+	"github.com/ncruces/go-sqlite3/driver"
+	_ "github.com/ncruces/go-sqlite3/embed"
+	"github.com/ncruces/go-sqlite3/litestream"
+)
+
+func ExampleNewReplica() {
+	client := s3.NewReplicaClient()
+	client.Bucket = "test-bucket"
+	client.Path = "fruits.db"
+
+	litestream.NewReplica("fruits.db", client, litestream.ReplicaOptions{
+		PollInterval: 5 * time.Second,
+	})
+
+	db, err := driver.Open("file:fruits.db?vfs=litestream")
+	if err != nil {
+		log.Fatalln(err)
+	}
+	defer db.Close()
+
+	for {
+		time.Sleep(time.Second)
+		rows, err := db.Query("SELECT * FROM fruits")
+		if err != nil {
+			log.Fatalln(err)
+		}
+
+		for rows.Next() {
+			var name, color string
+			err := rows.Scan(&name, &color)
+			if err != nil {
+				log.Fatalln(err)
+			}
+			log.Println(name, color)
+		}
+
+		log.Println("===")
+		rows.Close()
+	}
+}

litestream/vfs.go

@@ -214,10 +214,7 @@ func (f *liteDB) pollReplica(ctx context.Context) (*pageIndex, ltx.TXID, error)
 		return f.pages, f.txids[0], nil
 	}
 
-	// Updating from MinLevel to SnapshotLevel is non-racy,
-	// since LTX files are compacted into higher levels
-	// before the lower level LTX files are deleted.
-	for level := f.opts.MinLevel; level <= litestream.SnapshotLevel; level++ {
+	for level := range pollLevels(f.opts.MinLevel) {
 		if err := f.updateLevel(ctx, level); err != nil {
 			f.opts.Logger.Error("cannot poll replica", "error", err)
 			return nil, 0, err
@@ -285,6 +282,23 @@ func (f *liteDB) updateInfo(ctx context.Context, info *ltx.FileInfo) error {
 	return nil
 }
 
+func pollLevels(minLevel int) (r []int) {
+	// Updating from lower to upper levels is non-racy,
+	// since LTX files are compacted into higher levels
+	// before the lower level LTX files are deleted.
+
+	// Also, only level 0 compactions and snapshots delete files,
+	// so the intermediate levels never need to be updated.
+
+	if minLevel <= 0 {
+		return append(r, 0, 1, litestream.SnapshotLevel)
+	}
+	if minLevel >= litestream.SnapshotLevel {
+		return append(r, litestream.SnapshotLevel)
+	}
+	return append(r, minLevel, litestream.SnapshotLevel)
+}
+
 // Type aliases; these are a mouthful.
 type pageIndex = wbt.Tree[uint32, ltx.PageIndexElem]
 type levelTXIDs = [litestream.SnapshotLevel + 1]ltx.TXID

litestream/vfs_test.go

@@ -1,47 +1,33 @@
-package litestream_test
+package litestream
 
 import (
-	"log"
-	"time"
+	"slices"
+	"strconv"
+	"testing"
 
-	"github.com/benbjohnson/litestream/s3"
-	"github.com/ncruces/go-sqlite3/driver"
+	"github.com/benbjohnson/litestream"
 	_ "github.com/ncruces/go-sqlite3/embed"
-	"github.com/ncruces/go-sqlite3/litestream"
 )
 
-func ExampleNewReplica() {
-	client := s3.NewReplicaClient()
-	client.Bucket = "test-bucket"
-	client.Path = "fruits.db"
-
-	litestream.NewReplica("fruits.db", client, litestream.ReplicaOptions{
-		PollInterval: 5 * time.Second,
-	})
-
-	db, err := driver.Open("file:fruits.db?vfs=litestream")
-	if err != nil {
-		log.Fatalln(err)
+func Test_pollLevels(t *testing.T) {
+	tests := []struct {
+		minLevel int
+		want     []int
+	}{
+		{minLevel: -1, want: []int{0, 1, litestream.SnapshotLevel}},
+		{minLevel: 0, want: []int{0, 1, litestream.SnapshotLevel}},
+		{minLevel: 1, want: []int{1, litestream.SnapshotLevel}},
+		{minLevel: 2, want: []int{2, litestream.SnapshotLevel}},
+		{minLevel: 3, want: []int{3, litestream.SnapshotLevel}},
+		{minLevel: litestream.SnapshotLevel, want: []int{litestream.SnapshotLevel}},
+		{minLevel: litestream.SnapshotLevel + 1, want: []int{litestream.SnapshotLevel}},
 	}
-	defer db.Close()
-
-	for {
-		time.Sleep(time.Second)
-		rows, err := db.Query("SELECT * FROM fruits")
-		if err != nil {
-			log.Fatalln(err)
-		}
-
-		for rows.Next() {
-			var name, color string
-			err := rows.Scan(&name, &color)
-			if err != nil {
-				log.Fatalln(err)
+	for _, tt := range tests {
+		t.Run(strconv.Itoa(tt.minLevel), func(t *testing.T) {
+			got := pollLevels(tt.minLevel)
+			if !slices.Equal(got, tt.want) {
+				t.Errorf("pollLevels() = %v, want %v", got, tt.want)
 			}
-			log.Println(name, color)
-		}
-
-		log.Println("===")
-		rows.Close()
+		})
 	}
 }