test: parallel transactions are performed serially

crypto-ffi/bindings/jvm/src/test/kotlin/com/wire/crypto/MLSTest.kt

@@ -18,15 +18,13 @@
 
 package com.wire.crypto
 
-import kotlinx.coroutines.runBlocking
+import kotlinx.coroutines.*
 import kotlinx.coroutines.test.runTest
 import org.assertj.core.api.AssertionsForInterfaceTypes.assertThat
 import org.assertj.core.api.AssertionsForInterfaceTypes.assertThatNoException
 import java.nio.file.Files
-import kotlin.test.Test
-import kotlin.test.assertEquals
-import kotlin.test.assertFailsWith
-import kotlin.test.assertIs
+import kotlin.test.*
+import kotlin.time.Duration.Companion.milliseconds
 
 class MLSTest {
 
@@ -107,6 +105,39 @@ class MLSTest {
         cc.transaction { ctx -> ctx.createConversation(id) }
     }
 
+    @Test
+    fun parallel_transactions_are_performed_serially() = runTest() {
+        withContext(Dispatchers.Default) {
+            val (alice) = newClients(aliceId)
+            val jobs: MutableList<Job> = mutableListOf()
+            val token = "t"
+            val transactionCount = 3
+
+            // How this test ensures that transactions are performed serially:
+            // Each transaction gets the previous token string, adds one token at the end and stores it.
+            // If, for instance, the second and third transaction run in parallel they will both get same current
+            // token string "tt" and store "ttt".
+            // If they execute serially, one will store "ttt" and the other "tttt" (this is what we assert).
+
+            repeat(transactionCount) {
+                jobs += launch {
+                    alice.transaction { ctx ->
+                        delay(100.milliseconds)
+                        val data = ctx.getData()?.decodeToString()?.plus(token) ?: token
+                        ctx.setData(data.toByteArray())
+                    }
+                }
+            }
+            jobs.joinAll()
+
+            val result = alice.transaction { ctx ->
+                ctx.getData()?.decodeToString()
+            }
+
+            assertEquals(token.repeat(transactionCount), result, "Expected all transactions to complete")
+        }
+    }
+
     @Test
     fun getPublicKey_should_return_non_empty_result() = runTest {
         val (alice) = newClients(aliceId)