Fix error "Unable to stop reading in state Writing"

Author: Sergey Mashkov

kotlinx-coroutines-io/kotlinx-coroutines-io-jvm/src/main/kotlin/kotlinx/coroutines/experimental/io/ByteBufferChannel.kt

@@ -1826,7 +1826,9 @@ internal class ByteBufferChannel(
             if (closed != null || state === ReadWriteBufferState.Terminated) return visitor(TerminatedLookAhead)
             result = visitor(this)
             if (!state.idle && state !== ReadWriteBufferState.Terminated) {
-                restoreStateAfterRead()
+                if (state is ReadWriteBufferState.Reading || state is ReadWriteBufferState.ReadingWriting) {
+                    restoreStateAfterRead()
+                }
                 tryTerminate()
             }
         }

kotlinx-coroutines-io/kotlinx-coroutines-io-jvm/src/test/kotlin/kotlinx/coroutines/experimental/io/ByteBufferChannelLookAheadTest.kt

@@ -0,0 +1,44 @@
+package kotlinx.coroutines.experimental.io
+
+import kotlinx.coroutines.io.*
+import kotlin.test.*
+
+class ByteBufferChannelLookAheadTest : ByteChannelTestBase() {
+    @Test
+    fun testDoNothing() = runTest {
+        ch.lookAheadSuspend {
+        }
+    }
+
+    @Test
+    fun testDoNothingWhileWriting() = runTest {
+        ch.writeSuspendSession {
+            ch.lookAheadSuspend {
+            }
+        }
+    }
+
+    @Test
+    fun testDoNothingWhileWriting2() = runTest {
+        ch.lookAheadSuspend {
+            ch.writeSuspendSession {
+            }
+        }
+    }
+
+    @Test
+    fun testReadDuringWriting() = runTest {
+        ch.writeSuspendSession {
+            ch.lookAheadSuspend {
+                this@writeSuspendSession.request(1)!!.writeInt(777)
+                written(4)
+                flush()
+
+                val bb = request(0, 1)
+                assertNotNull(bb)
+                assertEquals(777, bb.getInt())
+                consumed(4)
+            }
+        }
+    }
+}