Home > Article > Backend Development > How to fix this issue: Panic: Synchronization: Negative WaitGroup counter
I sometimes run into this problem after running it again and again. I know this is related to counters. It throws this error when the done() method of sync.waitgroup is called more times than the add() method is called.
how to solve this problem?
My code creates batches of size 4 and does some processing on each batch, but I'm having trouble resolving this panic.
package main import ( "fmt" "sync" ) func main() { // create input channel input := make(chan int) // create wait group var wg sync.waitgroup // start batcher goroutine wg.add(1) go batcher(input, &wg) // send input values to the batcher for i := 1; i <= 10; i++ { input <- i } // close input channel close(input) // wait for batcher goroutine to finish wg.wait() } func batcher(input chan int, wg *sync.waitgroup) { // create batch channel with buffer of size 4 batch := make(chan int, 4) // create channel to synchronize worker goroutines done := make(chan bool) // create wait group for worker goroutines var workerwg sync.waitgroup // start worker goroutines for i := 0; i < 4; i++ { workerwg.add(1) go worker(batch, &workerwg, done) } // read input values and send to batch for value := range input { batch <- value if len(batch) == 4 { // wait for worker goroutines to finish processing batch workerwg.wait() // send batch to worker goroutines for i := 0; i < 4; i++ { workerwg.add(1) go sendbatch(batch, &workerwg, done) } } } // wait for worker goroutines to finish processing remaining batch workerwg.wait() // close done channel to notify that all batches have been processed close(done) wg.done() } func sendbatch(batch chan int, workerwg *sync.waitgroup, done chan bool) { // process batch for value := range batch { fmt.println("processing value:", value) } // notify worker goroutines that batch has been processed workerwg.done() select { case done <- true: default: // done channel has been closed } } func worker(batch chan int, workerwg *sync.waitgroup, done chan bool) { // process batches received from batch channel for batch := range batch { // process batch fmt.println("processing batch:", batch) workerwg.done() } // notify batcher goroutine that worker goroutine has finished select { case done <- true: default: // done channel has been closed } }
Basic code for writing batch programs:
package main import ( "fmt" "sync" ) func main() { input := make(chan int) output := make(chan []int) var wg sync.waitgroup wg.add(2) // start the batcher goroutine go func() { batch := []int{} for value := range input { batch = append(batch, value) if len(batch) == 4 { output <- batch batch = []int{} } } if len(batch) > 0 { output <- batch } close(output) wg.done() }() // start the worker goroutine go func() { for batch := range output { sum := 0 for _, value := range batch { sum += value } fmt.printf("sum of batch %v: %d\n", batch, sum) } wg.done() }() // send input values to the batcher for _, v := range []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10} { input <- v } close(input) // wait for both goroutines to finish wg.wait() }
Sum of batch [1 2 3 4]: 10 Sum of batch [5 6 7 8]: 26 Sum of batch [9 10]: 19
The early design is a bit complex, I will try to expand on this basic design.
According to this code:
for i := 0; i < 4; i++ { workerwg.add(1) go worker(batch, &workerwg, done) }
I think workerwg.done()
should be moved outside the loop:
func worker(batch chan int, workerWg *sync.WaitGroup, done chan bool) { + defer workerWg.Done() // process batches received from batch channel for batch := range batch { // process batch fmt.Println("Processing batch:", batch) - workerWg.Done() } // notify batcher goroutine that worker goroutine has finished select { case done <- true: default: // done channel has been closed } }
But batch
is not closed in the demo. So in effect, the goroutine will run forever until the program ends.
I don’t know if there are any other questions. The design is too complicated. Complex code is difficult to understand and error-prone. Consider redesigning it.
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