go - 使用 BLPOP 处理 Redis 队列会导致单元测试中出现竞争条件？

Question

我正在尝试实现一个先进先出的任​​务队列，如Go语言中 Redis 电子书的第 6.4.1 章所述。出于测试目的，我将一个CommandExecutor接口传递给“worker”函数，如下所示：

package service

import (
    "context"

    "github.com/gomodule/redigo/redis"
    "github.com/pkg/errors"
    "github.com/sirupsen/logrus"
)

const commandsQueue = "queuedCommands:"

var pool = redis.Pool{
    MaxIdle:   50,
    MaxActive: 1000,
    Dial: func() (redis.Conn, error) {
        conn, err := redis.Dial("tcp", ":6379")
        if err != nil {
            logrus.WithError(err).Fatal("initialize Redis pool")
        }
        return conn, err
    },
}

// CommandExecutor executes a command
type CommandExecutor interface {
    Execute(string) error
}

func processQueue(ctx context.Context, done chan<- struct{}, executor CommandExecutor) error {
    rc := pool.Get()
    defer rc.Close()

    for {
        select {
        case <-ctx.Done():
            done <- struct{}{}
            return nil
        default:
            // If the commands queue does not exist, BLPOP blocks until another client
            // performs an LPUSH or RPUSH against it. The timeout argument of zero is
            // used to block indefinitely.
            reply, err := redis.Strings(rc.Do("BLPOP", commandsQueue, 0))
            if err != nil {
                logrus.WithError(err).Errorf("BLPOP %s %d", commandsQueue, 0)
                return errors.Wrapf(err, "BLPOP %s %d", commandsQueue, 0)
            }

            if len(reply) < 2 {
                logrus.Errorf("Expected a reply of length 2, got one of length %d", len(reply))
                return errors.Errorf("Expected a reply of length 2, got one of length %d", len(reply))
            }

            // BLPOP returns a two-element multi-bulk with the first element being the
            // name of the key where an element was popped and the second element
            // being the value of the popped element (cf. https://redis.io/commands/blpop#return-value)
            if err := executor.Execute(reply[1]); err != nil {
                return errors.Wrapf(err, "execute scheduled command: %s", reply[0])
            }
            done <- struct{}{}
        }
    }
}

我制作了一个小型示例存储库https://github.com/kurtpeek/process-queue，其中包含此代码以及单元测试的尝试。对于单元测试，我有两个相同的测试（名称不同）：

package service

import (
    "context"
    "testing"

    "github.com/stretchr/testify/assert"
    "github.com/stretchr/testify/require"
)

func TestProcessQueue(t *testing.T) {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()

    executor := &CommandExecutorMock{
        ExecuteFunc: func(string) error {
            return nil
        },
    }

    done := make(chan struct{})
    go processQueue(ctx, done, executor)

    rc := pool.Get()
    defer rc.Close()

    _, err := rc.Do("RPUSH", commandsQueue, "foobar")
    require.NoError(t, err)

    <-done

    assert.Exactly(t, 1, len(executor.ExecuteCalls()))
    assert.Exactly(t, "foobar", executor.ExecuteCalls()[0].In1)
}

func TestProcessQueue2(t *testing.T) {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()

    executor := &CommandExecutorMock{
        ExecuteFunc: func(string) error {
            return nil
        },
    }

    done := make(chan struct{})
    go processQueue(ctx, done, executor)

    rc := pool.Get()
    defer rc.Close()

    _, err := rc.Do("RPUSH", commandsQueue, "foobar")
    require.NoError(t, err)

    <-done

    assert.Exactly(t, 1, len(executor.ExecuteCalls()))
    assert.Exactly(t, "foobar", executor.ExecuteCalls()[0].In1)
}

其中CommandExecutorMock是使用 生成的moq。如果我单独运行每个测试，它们会通过：

~/g/s/g/k/process-queue> go test ./... -v -run TestProcessQueue2
=== RUN   TestProcessQueue2
--- PASS: TestProcessQueue2 (0.00s)
PASS
ok      github.com/kurtpeek/process-queue/service   0.243s

但是，如果我运行所有测试，第二个会超时：

~/g/s/g/k/process-queue> 
go test ./... -v -timeout 10s
=== RUN   TestProcessQueue
--- PASS: TestProcessQueue (0.00s)
=== RUN   TestProcessQueue2
panic: test timed out after 10s

似乎在第二个测试运行时，在第一个测试中启动的 goroutine 仍在运行并BLPOP从队列中读取命令，因此<-done第二个测试中的行无限期阻塞。尽管调用cancel()了第一个测试的父上下文。

我如何“隔离”这些测试，以便它们在一起运行时都通过？（我尝试将-p 1标志传递给go test但无济于事）。

标签： goconcurrencyredisrace-condition

Answer 1

尽管在第一个测试的父上下文上调用了 cancel()。

在写入done和调用之间有一些时间cancel()，这意味着第一个测试可能（并且确实）进入第二for/select次迭代而不是退出 on <-ctx.Done()。更具体地说，测试代码在取消之前包含 2 个断言：

    assert.Exactly(t, 1, len(executor.ExecuteCalls()))
    assert.Exactly(t, "foobar", executor.ExecuteCalls()[0].In1)

然后才defer cancel()开始，这似乎为时已晚，无法取消第一个 go 例程的上下文。

如果您cancel()在读取之前移动 call done，则测试通过：

func TestProcessQueue(t *testing.T) {
    ctx, cancel := context.WithCancel(context.Background())

    executor := &CommandExecutorMock{
        ExecuteFunc: func(string) error {
            return nil
        },
    }

    done := make(chan struct{})
    go processQueue(ctx, done, executor)

    rc := pool.Get()
    defer rc.Close()

    _, err := rc.Do("RPUSH", commandsQueue, "foobar")
    require.NoError(t, err)

    cancel() // note this change right here
    <-done

    assert.Exactly(t, 1, len(executor.ExecuteCalls()))
    assert.Exactly(t, "foobar", executor.ExecuteCalls()[0].In1)
}