Do not touch thread-locals if they were never set in UndispatchedCoroutine

Author: qwwdfsad

kotlinx-coroutines-core/jvm/src/CoroutineContext.kt

@@ -179,7 +179,9 @@ internal actual class UndispatchedCoroutine<in T>actual constructor (
      *     // and it also calls saveThreadContext and clearThreadContext
      * }
      */
-    private var threadStateToRecover = ThreadLocal<Pair<CoroutineContext, Any?>>()
+    private val threadStateToRecover = ThreadLocal<Pair<CoroutineContext, Any?>>()
+    @Volatile
+    private var threadLocalIsSet = false // Better than nullable thread-local for easier debugging
 
     init {
         /*
@@ -213,11 +215,12 @@ internal actual class UndispatchedCoroutine<in T>actual constructor (
     }
 
     fun saveThreadContext(context: CoroutineContext, oldValue: Any?) {
+        threadLocalIsSet = true // Specify that thread-local is touched at all
         threadStateToRecover.set(context to oldValue)
     }
 
     fun clearThreadContext(): Boolean {
-        if (threadStateToRecover.get() == null) {
+        if (threadLocalIsSet && threadStateToRecover.get() == null) {
             threadStateToRecover.remove()
             return false
         }
@@ -226,10 +229,12 @@ internal actual class UndispatchedCoroutine<in T>actual constructor (
     }
 
     override fun afterResume(state: Any?) {
-        threadStateToRecover.get()?.let { (ctx, value) ->
-            restoreThreadContext(ctx, value)
+        if (threadLocalIsSet) {
+            threadStateToRecover.get()?.let { (ctx, value) ->
+                restoreThreadContext(ctx, value)
+            }
+            threadStateToRecover.remove()
         }
-        threadStateToRecover.remove()
         // resume undispatched -- update context but stay on the same dispatcher
         val result = recoverResult(state, uCont)
         withContinuationContext(uCont, null) {