Properly reuse JsonStringBuilders in CharArrayPool

Fixes #1455

formats/json/jvmMain/src/kotlinx/serialization/json/internal/CharArrayPool.kt

@@ -19,7 +19,7 @@ internal object CharArrayPool {
          * and the pool will be populated in the 'release'
          */
         val candidate = synchronized(this) {
-            arrays.lastOrNull()?.also { charsTotal -= it.size }
+            arrays.removeLastOrNull()?.also { charsTotal -= it.size }
         }
         return candidate ?: CharArray(128)
     }

formats/json/jvmTest/src/kotlinx/serialization/json/ParallelJsonStressTest.kt

@@ -0,0 +1,22 @@
+package kotlinx.serialization.json
+
+import kotlinx.coroutines.*
+import kotlinx.serialization.builtins.*
+import kotlinx.serialization.test.*
+import org.junit.*
+import kotlin.concurrent.*
+import kotlin.random.*
+
+class ParallelJsonStressTest : JsonTestBase() {
+    private val iterations = 1_000_000
+
+    @Test
+    fun testDecodeInParallel() = runBlocking<Unit> {
+        for (i in 1..1000) {
+            launch(Dispatchers.Default) {
+                val value = (1..10000).map { Random.nextDouble() }
+                assertSerializedAndRestored(value, ListSerializer(Double.serializer()))
+            }
+        }
+    }
+}