Insufficient space allocated to copy array contents - arrays

Question

I am building up some code to request multiple information from a database in order to have a time table in my front end incl. multiple DB requests.
The problem is that with one particular request where . am using swiftKuery and DispatchGroups i receive ooccasionally but not always an error message in my XCode. This can not be reconstructed by different request but just sometimes happens.
here is a snippet of my code.
var profWorkDaysBreak = [time_workbreaks]()
let groupServiceWorkDayBreaks = DispatchGroup()
...
///WorkdaysBreakENTER AsyncCall
//UnreliableCode ?
profWorkDays.forEach {workDay in
groupServiceWorkDayBreaks.enter()
time_workbreaks.getAll(weekDayId: workDay.id) { results, error in
if let error = error {
print(error)
}
if let results = results {
profWorkDaysBreak.append(contentsOf: results) // The error happens here !
}
groupServiceWorkDayBreaks.leave()
}
}
...
groupServiceWorkDayBreaks.wait()
The results and profWorkDaysBreak variables are the same just sometimes i receive the message:
Fatal error: Insufficient space allocated to copy array contents
This leads to a stop of the execution.
I assume, that maybe the loop might sometimes finish an earlier execution in the DispatchGroup ??? but this is the only think i have as an idea....

Answer 1

Most likely this is caused by some race conditions due to the fact that you modify the array from multiple threads. And if two threads happen to try and alter the array at the same time, you get into problems.
Make sure you serialize the access to the array, this should solve the problem. You can use a semaphore for that:
var profWorkDaysBreak = [time_workbreaks]()
let groupServiceWorkDayBreaks = DispatchGroup()
let semaphore = DispatchSemaphore(value: 0)
...
profWorkDays.forEach { workDay in
groupServiceWorkDayBreaks.enter()
time_workbreaks.getAll(weekDayId: workDay.id) { results, error in
if let error = error {
print(error)
}
if let results = results {
// acquire the lock, perform the non-thread safe operation, release the lock
semaphore.wait()
profWorkDaysBreak.append(contentsOf: results) // The error happens here !
semaphore.signal()
}
groupServiceWorkDayBreaks.leave()
}
}
...
groupServiceWorkDayBreaks.wait()
The semaphore here acts like a mutex, allowing at most one thread to operate on the array. Also I would like to emphasise the fact that the lock should be hold for the least amount of time possible, so that the other threads don't have to wait for too much.

Answer 2

Here is the only way i got my code running reliable so far..
i skipped contents of completely and just went to a forEach Loop
var profWorkDaysBreak = [time_workbreaks]()
let groupServiceWorkDayBreaks = DispatchGroup()
...
///WorkdaysBreakENTER AsyncCall
//UnreliableCode ?
profWorkDays.forEach {workDay in
groupServiceWorkDayBreaks.enter()
time_workbreaks.getAll(weekDayId: workDay.id) { results, error in
if let error = error {
print(error)
}
if let results = results {
results.forEach {profWorkDayBreak in
profWorkDaysBreak.append(profWorkDayBreak)
}
/*
//Alternative causes error !
profWorkDaysBreak.append(contentsOf: results)
*/
}
groupServiceWorkDayBreaks.leave()
}
}
...
groupServiceWorkDayBreaks.wait()