Deserialize deprecated thread pools compatibly with V9

server/src/main/java/org/elasticsearch/threadpool/ThreadPool.java

@@ -941,7 +941,12 @@ public Info(String name, ThreadPoolType type, int min, int max, @Nullable TimeVa
 
         public Info(StreamInput in) throws IOException {
             name = in.readString();
-            type = ThreadPoolType.fromType(in.readString());
+            ThreadPoolType receivedType = ThreadPoolType.fromType(in.readString());
+            type = switch (receivedType) {
+                case DIRECT, FIXED_AUTO_QUEUE_SIZE -> ThreadPoolType.FIXED;
+                default -> receivedType;
+            };
+
             min = in.readInt();
             max = in.readInt();
             keepAlive = in.readOptionalTimeValue();

server/src/test/java/org/elasticsearch/nodesinfo/NodeInfoStreamingTests.java

@@ -138,7 +138,11 @@ private static NodeInfo createNodeInfo() {
             List<ThreadPool.Info> threadPoolInfos = new ArrayList<>(numThreadPools);
             for (int i = 0; i < numThreadPools; i++) {
                 threadPoolInfos.add(
-                    new ThreadPool.Info(randomAlphaOfLengthBetween(3, 10), randomFrom(ThreadPool.ThreadPoolType.values()), randomInt())
+                    new ThreadPool.Info(
+                        randomAlphaOfLengthBetween(3, 10),
+                        randomFrom(ThreadPool.ThreadPoolType.FIXED, ThreadPool.ThreadPoolType.SCALING),
+                        randomInt()
+                    )
                 );
             }
             threadPoolInfo = new ThreadPoolInfo(threadPoolInfos);

server/src/test/java/org/elasticsearch/threadpool/ThreadPoolSerializationTests.java

@@ -37,7 +37,7 @@ public class ThreadPoolSerializationTests extends ESTestCase {
     @Before
     public void setUp() throws Exception {
         super.setUp();
-        threadPoolType = randomFrom(ThreadPool.ThreadPoolType.values());
+        threadPoolType = randomFrom(ThreadPool.ThreadPoolType.FIXED, ThreadPool.ThreadPoolType.SCALING);
     }
 
     public void testThatQueueSizeSerializationWorks() throws Exception {
@@ -123,4 +123,15 @@ public void testThatThreadPoolTypeIsSerializedCorrectly() throws IOException {
 
         assertThat(newInfo.getThreadPoolType(), is(threadPoolType));
     }
+
+    public void testThatDeprecatedTypesDeserializedAsFixed() throws IOException {
+        ThreadPool.Info info = new ThreadPool.Info("foo", ThreadPool.ThreadPoolType.DIRECT);
+        output.setTransportVersion(TransportVersion.current());
+        info.writeTo(output);
+
+        StreamInput input = output.bytes().streamInput();
+        ThreadPool.Info newInfo = new ThreadPool.Info(input);
+
+        assertThat(newInfo.getThreadPoolType(), is(ThreadPool.ThreadPoolType.FIXED));
+    }
 }