问golang sync.WaitGroup从未完成

EN

package mapreduce

import (
    "fmt"
    "sync"
)

func schedule(jobName string, mapFiles []string, nReduce int, phase jobPhase, registerChan chan string) {
    var ntasks int
    var n_other int // number of inputs (for reduce) or outputs (for map)
    switch phase {
    case mapPhase:
        ntasks = len(mapFiles)
        n_other = nReduce
    case reducePhase:
        ntasks = nReduce
        n_other = len(mapFiles)

    }

    fmt.Printf("Schedule: %v %v tasks (%d I/Os)\n", ntasks, phase, n_other)
    var wg sync.WaitGroup
    for i := 0; i < ntasks; i++ {
        worker := <-registerChan
        doTaskArg := DoTaskArgs{jobName, mapFiles[i], phase, i, n_other}
        wg.Add(1)
        go func() {
            defer wg.Done()

            done := call(worker, "Worker.DoTask", doTaskArg, nil)
            if done {
                registerChan <- worker
            } else {
                i = i - 1
            }

        }()
    }

    wg.Wait()

    fmt.Printf("Schedule: %v phase done\n", phase)
}