Optimize SQLite3 access. Mainly separate read access from write access.

Based on tips from https://archive.is/Xfjh6#selection-257.0-278.0

db/db.go

@@ -4,6 +4,7 @@ import (
 	"database/sql"
 	"embed"
 	"fmt"
+	"runtime"
 
 	"github.com/mattn/go-sqlite3"
 	"github.com/navidrome/navidrome/conf"
@@ -24,8 +25,36 @@ var embedMigrations embed.FS
 
 const migrationsFolder = "migrations"
 
-func Db() *sql.DB {
-	return singleton.GetInstance(func() *sql.DB {
+type DB interface {
+	ReadDB() *sql.DB
+	WriteDB() *sql.DB
+	Close()
+}
+
+type db struct {
+	readDB  *sql.DB
+	writeDB *sql.DB
+}
+
+func (d *db) ReadDB() *sql.DB {
+	return d.readDB
+}
+
+func (d *db) WriteDB() *sql.DB {
+	return d.writeDB
+}
+
+func (d *db) Close() {
+	if err := d.readDB.Close(); err != nil {
+		log.Error("Error closing read DB", err)
+	}
+	if err := d.writeDB.Close(); err != nil {
+		log.Error("Error closing write DB", err)
+	}
+}
+
+func Db() DB {
+	return singleton.GetInstance(func() *db {
 		sql.Register(Driver+"_custom", &sqlite3.SQLiteDriver{
 			ConnectHook: func(conn *sqlite3.SQLiteConn) error {
 				return conn.RegisterFunc("SEEDEDRAND", hasher.HashFunc(), false)
@@ -38,21 +67,32 @@ func Db() *sql.DB {
 			conf.Server.DbPath = Path
 		}
 		log.Debug("Opening DataBase", "dbPath", Path, "driver", Driver)
-		instance, err := sql.Open(Driver+"_custom", Path)
+
+		rdb, err := sql.Open(Driver+"_custom", Path)
 		if err != nil {
 			panic(err)
 		}
-		return instance
+		rdb.SetMaxOpenConns(max(4, runtime.NumCPU()))
+
+		wdb, err := sql.Open(Driver+"_custom", Path)
+		if err != nil {
+			panic(err)
+		}
+		wdb.SetMaxOpenConns(1)
+		return &db{
+			readDB:  rdb,
+			writeDB: wdb,
+		}
 	})
 }
 
-func Close() error {
+func Close() {
 	log.Info("Closing Database")
-	return Db().Close()
+	Db().Close()
 }
 
 func Init() func() {
-	db := Db()
+	db := Db().WriteDB()
 
 	// Disable foreign_keys to allow re-creating tables in migrations
 	_, err := db.Exec("PRAGMA foreign_keys=off")
@@ -82,11 +122,7 @@ func Init() func() {
 		log.Fatal("Failed to apply new migrations", err)
 	}
 
-	return func() {
-		if err := Close(); err != nil {
-			log.Error("Error closing DB", err)
-		}
-	}
+	return Close
 }
 
 type statusLogger struct{ numPending int }