如何讓 goroutine 與匿名函數一起工作，在循環中返回值

Question

我正在編寫一個自定義腳本來從 RackSpace cloudfiles 容器中獲取數據並列出給定容器中的所有文件（容器有大約 1 億個文件），我一直致力於並行化代碼，但目前卡住了。

// function to read data from channel and display
// currently just displaying, but there will be allot of processing done on this data
func extractObjectItemsFromList(objListChan <-chan []string) {
    fmt.Println("ExtractObjectItemsFromList")
    for _, c := range <-objListChan {
        fmt.Println(urlPrefix, c, "\t", count)
    }
}


func main()

// fetching data using flags
ao := gophercloud.AuthOptions{
    Username: *userName,
    APIKey:   *apiKey,
}

provider, err := rackspace.AuthenticatedClient(ao)
client, err := rackspace.NewObjectStorageV1(provider,gophercloud.EndpointOpts{
    Region: *region,
})
if err != nil {
    logFatal(err)
}

// We have the option of filtering objects by their attributes
opts := &objects.ListOpts{
    Full:   true,
    Prefix: *prefix,
}

var objectListChan = make(chan []string)
go extractObjectItemsFromList(objectListChan)

// Retrieve a pager (i.e. a paginated collection)
pager := objects.List(client, *containerName, opts)


// Not working
// By default EachPage contains 10000 records
// Define an anonymous function to be executed on each page's iteration
lerr := pager.EachPage(func(page pagination.Page) (bool, error) {       // Get a slice of objects.Object structs
    objectList, err := objects.ExtractNames(page)
    if err != nil {
        logFatal(err)
    }
    for _, o := range objectList {
        _ = o
    }
    objectListChan <- objectList
    return true, nil
})
if lerr != nil {
    logFatal(lerr)
}
//---------------------------------------------------
//       below code is working
//---------------------------------------------------

// working, but only works inside the loop, this keeps on fetching new pages and showing new records, 10000 per page
// By default EachPage contains 10000 records
// Define an anonymous function to be executed on each page's iteration
lerr := pager.EachPage(func(page pagination.Page) (bool, error) {       // Get a slice of objects.Object structs
    objectList, err := objects.ExtractNames(page)
    if err != nil {
        logFatal(err)
    }
    for _, o := range objectList {
        fmt.Println(o)
    }
    return true, nil
})
if lerr != nil {
    logFatal(lerr)
}

顯示前 10000 條記錄，但隨后卡住了，什么也沒有發生。 如果我不使用 channel 而只是運行普通循環，它就可以正常工作，這會破壞並行化的目的。

Answer 1

for _, c := range <-objListChan {
    fmt.Println(urlPrefix, c, "\t", count)
}

您的異步工作者從頻道中彈出一個列表，對其進行迭代並退出。 您需要有兩個循環：一個讀取通道（ range objListChan ），另一個 - 讀取（剛剛檢索到的）對象列表。