go源码阅读:database/sql包

简介

• database/sql最主要还是实现了连接池逻辑
• go源码版本: v1.16
• 源码仓库: https://github.com/golang/go/tree/go1.16.6/src/database/sql

sql.DB的一些关键逻辑

• Open会返回DB对象并开启一条connectionOpener线程

  • connectionOpener主要处理下面提到的”当前连接数大于maxOpen会陷入等待”的连接资源请求

• DB的核心方法: conn(ctx context.Context, strategy connReuseStrategy) (*driverConn, error)

  • 优先返回连接池里的连接
  • 当前连接数大于maxOpen会陷入等待, 等待取决于ctx, 也即由调用方控制
  • 其余情况则返回一个新连接

• DB的连接池: freeConn []*driverConn

• DB的核心方法: putConnDBLocked(conn, err) bool

  • 如果当前连接数大于maxOpen则直接返回false, 上层看到false则直接关闭该连接conn
  • 优先满足正在等待连接资源请求的(connRequests是一个map,可以看出请求连接资源并无优先级的说法)
  • 如果MaxIdleConns < len(freeConn), 即连接池满了, 则直接返回false, 上层看到false则直接关闭该连接conn
  • 否则丢到连接池freeConn中

• 清理线程(connectionCleaner):

  • SetConnMaxLifetime,SetConnMaxIdleTime时才会去起唯一的一条清理线程
  • 清理线程定期清理连接池freeConn的连接, 根据maxLifetime和createAt清理, 也根据maxIdleTime和returnedAt清理
  • 有趣的是清理线程并不理会MaxOpen和MaxIdleConns是多少, 只关注MaxLifetime和MaxIdleTime, 反正过期了就清理

• sql执行方法如Query,Exec,Ping等, 都会先去调conn获取连接, 再用其连接执行sql, 最后将putConnDBLocked(conn), (query是等rows全部scan完close再putConnDBLocked(conn))

• 关于resetSession最终的去处, 总的来说就是reset了个寂寞, 最终居然只是conn.SetReadDeadline(time.Time{})?

  // go-sql-driver/mysql/connection.go里面的
  // Write packet buffer 'data'
  func (mc *mysqlConn) writePacket(data []byte) error {
      pktLen := len(data) - 4
  
      if pktLen > mc.maxAllowedPacket {
          return ErrPktTooLarge
      }
  
      // Perform a stale connection check. We only perform this check for
      // the first query on a connection that has been checked out of the
      // connection pool: a fresh connection from the pool is more likely
      // to be stale, and it has not performed any previous writes that
      // could cause data corruption, so it's safe to return ErrBadConn
      // if the check fails.
      if mc.reset {
          mc.reset = false
          conn := mc.netConn
          if mc.rawConn != nil {
              conn = mc.rawConn
          }
          var err error
          // If this connection has a ReadTimeout which we've been setting on
          // reads, reset it to its default value before we attempt a non-blocking
          // read, otherwise the scheduler will just time us out before we can read
          if mc.cfg.ReadTimeout != 0 {
              err = conn.SetReadDeadline(time.Time{})
          }
          if err == nil && mc.cfg.CheckConnLiveness {
              err = connCheck(conn)
          }
          if err != nil {
              errLog.Print("closing bad idle connection: ", err)
              mc.Close()
              return driver.ErrBadConn
          }
      }
      ... ...
  }

关于statement

1. 关于query带?的sql的逻辑: func (db *DB) queryDC:
   • 对于带了args的query, 且InterpolateParams=false, 则driver会返回driver.ErrSkip, 此时会走到si, err = ctxDriverPrepare(ctx, dc.ci, query), 做完prepare与ctxDriverStmtQuery, 将si放到Rows里面, Rows读完Close后si会跟着Close