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[FSSDK-11900] fix: potential concurrent issue for cmab decision #608

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Merged
merged 2 commits into from
Oct 31, 2025
Merged

[FSSDK-11900] fix: potential concurrent issue for cmab decision #608

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799ab5d
refactor: improve thread safety in getDecision function
muzahidul-opti Oct 21, 2025
a10e902
refactor: update getLockIndex algorithm
muzahidul-opti Oct 22, 2025
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33 changes: 24 additions & 9 deletions Sources/CMAB/CmabService.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -48,25 +48,40 @@ class DefaultCmabService: CmabService {
private let cmabCache: LruCache<String, CmabCacheValue>
private let logger = OPTLoggerFactory.getLogger()

private static let NUM_LOCKS = 1000
private let locks: [NSLock]

init(cmabClient: CmabClient, cmabCache: LruCache<String, CmabCacheValue>) {
self.cmabClient = cmabClient
self.cmabCache = cmabCache
self.locks = (0..<Self.NUM_LOCKS).map { _ in NSLock() }
}

private func getLockIndex(userId: String, ruleId: String) -> Int {
let combinedKey = userId + ruleId
let hashValue = MurmurHash3.hash32(key: combinedKey)
let lockIndex = Int(hashValue) % Self.NUM_LOCKS
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Copilot AI Oct 22, 2025

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The conversion from UInt32 to Int could result in a negative value on platforms where Int is 32-bit, leading to a potential crash when accessing the locks array. Use Int(hashValue % UInt32(Self.NUM_LOCKS)) to ensure the modulo operation happens before conversion.

Suggested change
let lockIndex = Int(hashValue) % Self.NUM_LOCKS
let lockIndex = Int(hashValue % UInt32(Self.NUM_LOCKS))

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return lockIndex
}

func getDecision(config: ProjectConfig,
userContext: OptimizelyUserContext,
ruleId: String,
options: [OptimizelyDecideOption]) -> Result<CmabDecision, Error> {
var result: Result<CmabDecision, Error>!
let semaphore = DispatchSemaphore(value: 0)
getDecision(config: config,
userContext: userContext,
ruleId: ruleId, options: options) { _result in
result = _result
semaphore.signal()
let lockIdx = getLockIndex(userId: userContext.userId, ruleId: ruleId)
let lock = locks[lockIdx]
return lock.withLock {
var result: Result<CmabDecision, Error>!
let semaphore = DispatchSemaphore(value: 0)
Comment on lines +73 to +75
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Copilot AI Oct 22, 2025

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Using a semaphore to block while holding a lock can lead to deadlock if the asynchronous callback in getDecision attempts to acquire the same lock. The lock should be released before waiting on the semaphore, or the async operation should be moved outside the lock scope.

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getDecision(config: config,
userContext: userContext,
ruleId: ruleId, options: options) { _result in
result = _result
semaphore.signal()
}
semaphore.wait()
return result
}
semaphore.wait()
return result
}

func getDecision(config: ProjectConfig,
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