数据库主从切换过程中Druid没法获取连接错误

背景：

今天dba在进行DB的主从切换，导致应用一直报错，获取不到DB连接，druid的错误信息如下：

Could not open JDBC Connection for transaction; nested exception is com.alibaba.druid.pool.GetConnectionTimeoutException: wait millis 15000, active 20
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可以看到活跃连接数active=20, 而应用配置中设置的maxActive=100，远远没有达到最大的连接数，为什么就创建不了连接呢？

查找真相

查看和db连接有关的错误日志，发现如下

ERROR com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread (DruidDataSource.java:1713) - create connection holder error
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
The last packet successfully received from the server was 0 milliseconds ago. 
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错误信息展示CreateConnectionThread 创建连接线程报错，那问题就变成了猜测连接线程已经被中止了，导致没法创建连接，使用jstack命令导出应用线程，发现确实CreateConnectionThread 线程确实不存在了，所以问题已经明朗，是因为CreateConnectionThread 线程中止了，导致再也无法创建连接。

对CreateConnectionThread 线程的代码进行检查：

public void run() {
        initedLatch.countDown();

        int errorCount = 0;
        for (;;) {
            // addLast
            try {
                lock.lockInterruptibly();
            } catch (InterruptedException e2) {
                break;
            }

            try {
                // 必须存在线程等待，才创建连接
                if (poolingCount >= notEmptyWaitThreadCount) {
                    empty.await();
                }

                // 防止创建超过maxActive数量的连接
                if (activeCount + poolingCount >= maxActive) {
                    empty.await();
                    continue;
                }

            } catch (InterruptedException e) {
                lastCreateError = e;
                lastErrorTimeMillis = System.currentTimeMillis();
                break;
            } finally {
                lock.unlock();
            }

            Connection connection = null;

            try {
                connection = createPhysicalConnection();
            } catch (SQLException e) {
                LOG.error("create connection error", e);

                errorCount++;

                if (errorCount > connectionErrorRetryAttempts && timeBetweenConnectErrorMillis > 0) {
                    if (breakAfterAcquireFailure) {
                        break;
                    }

                    try {
                        Thread.sleep(timeBetweenConnectErrorMillis);
                    } catch (InterruptedException interruptEx) {
                        break;
                    }
                }
            } catch (RuntimeException e) {
                LOG.error("create connection error", e);
                continue;
            } catch (Error e) {
                LOG.error("create connection error", e);
                break;
            }

            if (connection == null) {
                continue;
            }

            DruidConnectionHolder holder = null;
            try {
                holder = new DruidConnectionHolder(DruidDataSource.this, connection);
            } catch (SQLException ex) {// 主从切换过程中，这里有报错，导致线程中止
                LOG.error("create connection holder error", ex);
                break;
            }

            lock.lock();
            try {
                connections[poolingCount++] = holder;

                if (poolingCount > poolingPeak) {
                    poolingPeak = poolingCount;
                    poolingPeakTime = System.currentTimeMillis();
                }

                errorCount = 0; // reset errorCount

                notEmpty.signal();
                notEmptySignalCount++;
            } finally {
                lock.unlock();
            }
        }
    }
}
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我使用的版本是druid 1.7，升级到最新的1.15版本后，这个bug已经没有解决了