Fix race condition in json decoder (#995)

* Fix race condition in json decoder

* Update CHANGELOG.md

CHANGELOG.md

@@ -15,6 +15,7 @@
 #### Fixed
 - Fixed error message output for `minimumXcodeGenVersion`. [#967](https://github.com/yonaskolb/XcodeGen/pull/967) @joshwalker
 - Remove force-unwrapping causing crash for `LegacyTarget`s [#982](https://github.com/yonaskolb/XcodeGen/pull/982) @jcolicchio
+- Fixed a race condition in an internal JSON decoder, which would occasionally fail with an error like `Parsing project spec failed: Error Domain=Unspecified error Code=0`. [#995](https://github.com/yonaskolb/XcodeGen/pull/995) @elliottwilliams
 
 #### Internal
 - Updated to Yams 4.0.0 [#984](https://github.com/yonaskolb/XcodeGen/pull/984) @swiftty

Sources/ProjectSpec/Decoding.swift

@@ -10,36 +10,21 @@ extension Dictionary where Key: JSONKey {
         }
         if parallel {
             let defaultError = NSError(domain: "Unspecified error", code: 0, userInfo: nil)
-            var itemResults: [Result<T, Error>] = Array(repeating: .failure(defaultError), count: dictionary.count)
-            var ops: [BlockOperation] = []
-            var idx: Int = 0
-            for (key, _) in dictionary {
-                ops.append(BlockOperation { [idx] in
+            let keys = Array(dictionary.keys)
+            var itemResults: [Result<T, Error>] = Array(repeating: .failure(defaultError), count: keys.count)
+            itemResults.withUnsafeMutableBufferPointer { buffer in
+                DispatchQueue.concurrentPerform(iterations: dictionary.count) { idx in
                     do {
+                        let key = keys[idx]
                         let jsonDictionary: JSONDictionary = try dictionary.json(atKeyPath: .key(key))
                         let item = try T(name: key, jsonDictionary: jsonDictionary)
-                        itemResults[idx] = .success(item)
+                        buffer[idx] = .success(item)
                     } catch {
-                        itemResults[idx] = .failure(error)
+                        buffer[idx] = .failure(error)
                     }
-                })
-                idx += 1
-            }
-            let queue = OperationQueue()
-            queue.qualityOfService = .userInteractive
-            queue.maxConcurrentOperationCount = 8
-            queue.addOperations(ops, waitUntilFinished: true)
-            var items = ContiguousArray<T>()
-            items.reserveCapacity(itemResults.count)
-            for result in itemResults {
-                switch result {
-                case .failure(let error):
-                    throw error
-                case .success(let item):
-                    items.append(item)
                 }
             }
-            return Array(items)
+            return try itemResults.map { try $0.get() }
         } else {
             var items: [T] = []
             for (key, _) in dictionary {