Go - parallel iteration

• Inner goroutine can send event to outer goroutine by channel
• Ex. Pass value to inner routine function
• Declare a variable in inner goroutine function, use outer variable will be wrong because that variable value will be changed in for loop
• Ex. Send error to errorChannel from inner goroutine。may goroutine leak.
• Solution is to use a buffered channel to prevent block.
• Another solution is to use abother goroutine to get error.
• Need sync.WaitGroup to make sure items to process all done. Wait done in a goroutine when main channel is used to receive msg from goroutine
• Add must be callled before goroutine start, otherwise cant make sure add happens before done
• Done same as Add(-1)