Fix nested transactions with async bodies (#25)

Sources/Blackbird/BlackbirdDatabase.swift

@@ -591,8 +591,13 @@ extension Blackbird {
                 }
             }
 
+            private let asyncTransactionSemaphore = Blackbird.Semaphore(value: 1)
+
             // Exactly like the function below, but accepts an async action
             public func cancellableTransaction<R: Sendable>(_ action: (@Sendable (_ core: isolated Blackbird.Database.Core) async throws -> R) ) async throws -> Blackbird.TransactionResult<R> {
+                await asyncTransactionSemaphore.wait()
+                defer { asyncTransactionSemaphore.signal() }
+
                 if isClosed { throw Error.databaseIsClosed }
                 let transactionID = nextTransactionID
                 nextTransactionID += 1

Tests/BlackbirdTests/BlackbirdTests.swift

@@ -1030,7 +1030,26 @@ final class BlackbirdTestTests: XCTestCase, @unchecked Sendable {
         XCTAssert(all[2].id == 3)
         XCTAssert(all[2].a == "a2")
         XCTAssert(all[2].b == 201)
-
+    }
+
+    // To test bug #25: https://github.com/marcoarment/Blackbird/issues/25
+    func testConcurrentTransactions() async throws {
+        let db = try Blackbird.Database(path: sqliteFilename)
+        let semaphore = Blackbird.Semaphore(value: 0)
+
+        let numTasks = 1000
+
+        for i in 0..<numTasks {
+            Task {
+                try! await db.transaction { core in
+                    try? await Task.sleep(nanoseconds: UInt64(arc4random_uniform(10_000_000)))
+                    try! TestModel(id: Int64(i), title: TestData.randomTitle, url: TestData.randomURL).writeIsolated(to: db, core: core)
+                }
+                semaphore.signal()
+            }
+        }
+
+        for _ in 0..<numTasks { await semaphore.wait() }
     }
 
     func testCache() async throws {