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Catch race condition in PostHogFileBackedQueue.deleteFiles #218

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merged 3 commits into from
Oct 22, 2024
Merged

Catch race condition in PostHogFileBackedQueue.deleteFiles #218

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7a0e151
Catch race condition in PostHogFileBackedQueue.deleteFiles
jverkoey Oct 17, 2024
5324f29
Update ReadWriteLock.
jverkoey Oct 17, 2024
bbcbf8b
Address feedback.
jverkoey Oct 17, 2024
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11 changes: 8 additions & 3 deletions PostHog/PostHogFileBackedQueue.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -100,9 +100,14 @@ class PostHogFileBackedQueue {

private func deleteFiles(_ count: Int) {
for _ in 0 ..< count {
if items.isEmpty { return }
let removed = items.remove(at: 0) // We always remove from the top of the queue

guard let removed: String = _items.mutate({ items in
if items.isEmpty {
return nil
}
return items.remove(at: 0) // We always remove from the top of the queue
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How did you reproduce this issue since the whole block is locked with the isFlushingLock and isFlushing flag? Can you pass the specific steps to reproduce the issue so we can check if the very same issue happens in more places as well?
I don't see how the remove(at: 0) would fail if the internal items list isn't modified anywhere else outside of this lock.

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I think this call here is outside the isFlushingLock

func add(_ event: PostHogEvent) {
        if fileQueue.depth >= config.maxQueueSize {
            hedgeLog("Queue is full, dropping oldest event")
            // first is always oldest
            fileQueue.delete(index: 0)
        }

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Mmm in this case we might have a bug deleting the wrong file as well.
I think that part isn't locked to not block the calling thread on add, but we should do something about it then.
Can be a different issue/PR though, not blocking this one as its avoiding a crash already

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Yeah, we definitely shouldn't wait on a lock here. Thing is that we can't assume that this is aways called on main thread either.

Maybe it's best to try and recreate this crash with unit tests with background queue and DispatchQueue.concurrentPerform(iterations:)

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Checking the thread is easy (Thread.isMainThread), but independently of the thread, the add method should be run within the dispatchQueue.async block and then it's ok to lock with isFlushingLock and await its availability.
Again, another PR, fixing the crash here is more important now, but a follow-up with the improvement above would be cool.

posthog-ios/PostHog/PostHogQueue.swift

Line 212 in 1bb1709

fileQueue.delete(index: 0)
is an edge case so I bet rarely happens

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fwiw I only got one crash report with this call stack.

}) else {
continue
}
deleteSafely(queue.appendingPathComponent(removed))
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}
}
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5 changes: 3 additions & 2 deletions PostHog/Utils/ReadWriteLock.swift
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Expand Up @@ -50,10 +50,11 @@ public final class ReadWriteLock<Value> {
/// The lock will be acquired once for writing before invoking the closure.
///
/// - Parameter closure: The closure with the mutable value.
public func mutate(_ closure: (inout Value) -> Void) {
public func mutate<T>(_ closure: (inout Value) -> T) -> T {
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pthread_rwlock_wrlock(&rwlock)
closure(&value)
let result = closure(&value)
pthread_rwlock_unlock(&rwlock)
return result
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}
}

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