Initial release

Sources/KnotchyCore/Session/BackendChain.swift

@@ -0,0 +1,49 @@
+import Synchronization
+
+/// Runs async jobs one at a time, in order, so backend operations never overlap.
+@MainActor
+final class BackendChain {
+    private var tail: Task<Void, Never>?
+    private(set) var isClosed = false
+
+    func enqueue(_ job: @escaping @MainActor () async -> Void) {
+        guard !isClosed else { return }
+        let previous = tail
+        tail = Task {
+            await previous?.value
+            guard !self.isClosed else { return }
+            await job()
+        }
+    }
+
+    func close() {
+        isClosed = true
+    }
+
+    func waitUntilIdle() async {
+        while let current = tail {
+            await current.value
+            if tail == current { return }
+        }
+    }
+}
+
+/// Every VM ID from its `start` being queued until its destroy is confirmed.
+/// Lock-protected: the quit path reads it off the main actor.
+public final class LiveIDs: Sendable {
+    private let ids = Mutex<Set<String>>([])
+
+    public init() {}
+
+    public var snapshot: Set<String> {
+        ids.withLock { $0 }
+    }
+
+    func insert(_ id: String) {
+        _ = ids.withLock { $0.insert(id) }
+    }
+
+    func remove(_ id: String) {
+        _ = ids.withLock { $0.remove(id) }
+    }
+}