Docker Swarm代码分析笔记（10）——Cluster.validatePendingEngine

Cluster.validatePendingEngine是实际上用来连接Docker engine的代码：

// validatePendingEngine connects to the engine,
func (c *Cluster) validatePendingEngine(engine *cluster.Engine) bool {
    // Attempt a connection to the engine. Since this is slow, don't get a hold
    // of the lock yet.
    if err := engine.Connect(c.TLSConfig); err != nil {
        log.WithFields(log.Fields{"Addr": engine.Addr}).Debugf("Failed to validate pending node: %s", err)
        return false
    }

    // The following is critical and fast. Grab a lock.
    c.Lock()
    defer c.Unlock()

    // Only validate engines from pendingEngines list
    if _, exists := c.pendingEngines[engine.Addr]; !exists {
        return false
    }

    // Make sure the engine ID is unique.
    if old, exists := c.engines[engine.ID]; exists {
        if old.Addr != engine.Addr {
            log.Errorf("ID duplicated. %s shared by %s and %s", engine.ID, old.Addr, engine.Addr)
            // Keep this engine in pendingEngines table and show its error.
            // If it's ID duplication from VM clone, user see this message and can fix it.
            // If the engine is rebooted and get new IP from DHCP, previous address will be removed
            // from discovery after a while.
            // In both cases, retry may fix the problem.
            engine.HandleIDConflict(old.Addr)
        } else {
            log.Debugf("node %q (name: %q) with address %q is already registered", engine.ID, engine.Name, engine.Addr)
            engine.Disconnect()
            // Remove it from pendingEngines table
            delete(c.pendingEngines, engine.Addr)
        }
        return false
    }

    // Engine validated, move from pendingEngines table to engines table
    delete(c.pendingEngines, engine.Addr)
    // set engine state to healthy, and start refresh loop
    engine.ValidationComplete()
    c.engines[engine.ID] = engine

    log.Infof("Registered Engine %s at %s", engine.Name, engine.Addr)
    return true
}

Cluster.validatePendingEngine永远操作Cluster.pendingEngines里的Engine，如果连接Docker engine成功，就会把这个Engine移到Cluster.engines这个map里。