Fix concurrency issue in jdk provider (#26)

(cherry picked from commit 2837434)

cryptography-providers/jdk/src/jvmMain/kotlin/pooling.kt

@@ -16,11 +16,9 @@ internal sealed class Pooled<T>(protected val instantiate: () -> T) {
         private val pooled = ArrayDeque<T>()
 
         override fun get(): T {
-            synchronized(this) {
-                pooled.firstOrNull()
-            }?.let { return it }
-
-            return instantiate()
+            return synchronized(this) {
+                pooled.removeLastOrNull()
+            } ?: instantiate()
         }
 
         override fun put(value: T) {

cryptography-providers/jdk/src/jvmTest/kotlin/PoolingTest.kt

@@ -0,0 +1,50 @@
+package dev.whyoleg.cryptography.providers.jdk
+
+import kotlinx.coroutines.*
+import kotlinx.coroutines.test.*
+import kotlin.test.*
+
+class PoolingTest {
+
+    @Test
+    fun testSequentialUseCachedPool() {
+        val pool = Pooled.Cached(::Any)
+        val first = pool.use { it }
+        val second = pool.use { it }
+        assertSame(first, second)
+        val third = pool.use { it }
+        assertSame(second, third)
+    }
+
+    @Test
+    fun testOverlappingUseCachedPool() {
+        val pool = Pooled.Cached(::Any)
+        val first = pool.use { it }
+        pool.use { i1 ->
+            assertSame(first, i1)
+            pool.use { i2 ->
+                assertNotSame(i1, i2)
+                pool.use { i3 ->
+                    assertNotSame(i2, i3)
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testConcurrentUseCachedPool() = runTest {
+        val pool = Pooled.Cached(::Any)
+        val first = pool.use { it }
+        val instances = List(3) {
+            async {
+                pool.use { instance ->
+                    delay(1000)
+                    instance
+                }
+            }
+        }.awaitAll()
+        assertSame(first, instances[0])
+        assertNotSame(instances[0], instances[1])
+        assertNotSame(instances[1], instances[2])
+    }
+}