Elusive crash: terminated due to memory issue - ios

Question

I'm still trying to debug an elusive crash in my app. See here for my earlier post.
The app takes sound from the microphone, processes it, and continuously updates the display with the processed results. After running uneventfully for many minutes, the app will halt with Message from debugger: terminated due to memory issue. There is no stack trace.
The timing of the crash makes it appear that there is some finite resource that gets exhausted after so many minute of running. The time it takes to crash is quite uniform. The time to crash may vary unpredictably when I change something in my code, but as long as the code stays the same, the time to crash keeps approximately the same. On a recent set of 10 test runs, the time to crash varied between 1014 seconds and 1029 seconds.
The number of times the display gets updated is even more uniform. On that same set of 10 tests, the number of calls to UIView.draw varied from 15311 to 15322. That's a variation of 0.07 percent, as opposed to 1.5 percent in the time to crash.
It's not running out of memory. My code is written in Swift 3, so I'm not doing any explicit mallocs or frees. I've made my class references weak where needed. And I've tested under the Activity Monitor, Allocations, and Leaks Instruments under XCode. My program takes up 44.6 MiB, and it doesn't grow with time.
And I've been careful about thread safety when accessing shared data. All shared data is read and written on the same serial DispatchQueue.
I've traced the crash to a section of code that writes a byte array to disk, then reads in another array of bytes. Here's a simplified version of that code:
var inputBuf:Buffer = Buffer()
var outputBuf: Buffer = Buffer()
var fileHandle:FileHandle? = ...
struct Buffer {
let bufferSize = 16384
var fileIndex:Int = 0
var bytes:[UInt8]
init() {
bytes = [UInt8](repeating:0, count:bufferSize)
}
func save(fileHandle:FileHandle) {
fileHandle.seek(toFileOffset: UInt64(Int64(fileIndex)))
fileHandle.write(Data(bytes))
}
}
func bug()
{
outputBuf.save(fileHandle:fileHandle!)
fileHandle!.seek(toFileOffset: UInt64(inputBuf.fileIndex))
let data = fileHandle!.readData(ofLength: inputBuf.bufferSize )
for i in 0..<data.count {
inputBuf.bytes[i] = data[i] // May crash here
}
}
Usually the crash occurs during the loop that copies data from the result of the readData to my buffer. But on one occasion, the loop completed before the crash. That leads me to suspect the actual crash occurs on another thread. There's no stack trace, so my only debugging technique is to insert print statements in the code.
fileIndex is always between 0 and 2592500. I modified the code to close the FileHandle after use and create a new FileHandle when next needed. It did not affect the outcome.

Answer 1

It was the zombie detector! I turned off zombie detection and the app runs forever.