chore: Finish goodbye handling. (#130)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> **Low Risk**
> Low risk: primarily adds logging and threads a `reason` string through
`GOODBYE` status results; behavior changes are limited to observability
and an extra status field.
> 
> **Overview**
> FDv2 `GOODBYE` handling now **captures and exposes the server-provided
reason** end-to-end: `FDv2SourceResult.Status` adds a `reason` field
(with `getReason()`), and `FDv2SourceResult.goodbye()` now stores that
reason instead of discarding it.
> 
> Both polling (`PollingBase`) and streaming
(`StreamingSynchronizerImpl`) paths now **log an info message** when a
`goodbye` is received and pass the extracted reason into
`FDv2SourceResult.goodbye(...)`. Tests are updated to assert `reason` is
null for non-goodbye statuses and equals the expected string for goodbye
events.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
b20c3c4. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Co-authored-by: Todd Anderson <127344469+tanderson-ld@users.noreply.github.com>

Author: kinyoklion

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/PollingBase.java

@@ -138,7 +138,9 @@ protected CompletableFuture<FDv2SourceResult> poll(Selector selector, boolean on
                         return oneShot ? FDv2SourceResult.terminalError(info, fdv1Fallback) : FDv2SourceResult.interrupted(info, fdv1Fallback);
                     }
                     case GOODBYE:
-                        return FDv2SourceResult.goodbye(((FDv2ProtocolHandler.FDv2ActionGoodbye) res).getReason(), fdv1Fallback);
+                        String reason = ((FDv2ProtocolHandler.FDv2ActionGoodbye) res).getReason();
+                        logger.info("Goodbye was received from the LaunchDarkly connection with reason: '{}'.", reason);
+                        return FDv2SourceResult.goodbye(reason, fdv1Fallback);
                     case NONE:
                         break;
                     case INTERNAL_ERROR: {

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/StreamingSynchronizerImpl.java

@@ -282,8 +282,9 @@ private void handleMessage(MessageEvent event) {
                 break;
 
             case GOODBYE:
-                FDv2ProtocolHandler.FDv2ActionGoodbye goodbye = (FDv2ProtocolHandler.FDv2ActionGoodbye) action;
-                result = FDv2SourceResult.goodbye(goodbye.getReason(), getFallback(event));
+                String reason = ((FDv2ProtocolHandler.FDv2ActionGoodbye) action).getReason();
+                logger.info("Goodbye was received from the LaunchDarkly connection with reason: '{}'.", reason);
+                result = FDv2SourceResult.goodbye(reason, getFallback(event));
                 // We drop this current connection and attempt to restart the stream.
                 restartStream();
                 break;

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/datasources/FDv2SourceResult.java

@@ -54,6 +54,8 @@ public static class Status {
         private final State state;
         private final DataSourceStatusProvider.ErrorInfo errorInfo;
 
+        private final String reason;
+
         public State getState() {
             return state;
         }
@@ -62,9 +64,34 @@ public DataSourceStatusProvider.ErrorInfo getErrorInfo() {
             return errorInfo;
         }
 
-        public Status(State state, DataSourceStatusProvider.ErrorInfo errorInfo) {
+        Status(State state, DataSourceStatusProvider.ErrorInfo errorInfo, String reason) {
             this.state = state;
             this.errorInfo = errorInfo;
+            this.reason = reason;
+        }
+
+        public static Status goodbye(String reason) {
+            return new Status(State.GOODBYE, null, reason);
+        }
+
+        public static Status interrupted(DataSourceStatusProvider.ErrorInfo errorInfo) {
+            return new Status(State.INTERRUPTED, errorInfo, null);
+        }
+
+        public static Status terminalError(DataSourceStatusProvider.ErrorInfo errorInfo) {
+            return new Status(State.TERMINAL_ERROR, errorInfo, null);
+        }
+
+        public static Status shutdown() {
+            return new Status(State.SHUTDOWN, null, null);
+        }
+
+        /**
+         * If the state is GOODBYE, then this will be the reason. Otherwise, it will be null.
+         * @return the reason, or null
+         */
+        public String getReason() {
+            return reason;
         }
     }
 
@@ -97,7 +124,7 @@ public static FDv2SourceResult interrupted(DataSourceStatusProvider.ErrorInfo er
     public static FDv2SourceResult interrupted(DataSourceStatusProvider.ErrorInfo errorInfo, boolean fdv1Fallback, Function<Void, Void> completionCallback) {
         return new FDv2SourceResult(
             null,
-            new Status(State.INTERRUPTED, errorInfo),
+            Status.interrupted(errorInfo),
             ResultType.STATUS,
             fdv1Fallback,
             completionCallback);
@@ -109,7 +136,7 @@ public static FDv2SourceResult shutdown() {
 
     public static FDv2SourceResult shutdown(Function<Void, Void> completionCallback) {
         return new FDv2SourceResult(null,
-            new Status(State.SHUTDOWN, null),
+            Status.shutdown(),
             ResultType.STATUS,
             false,
             completionCallback);
@@ -121,7 +148,7 @@ public static FDv2SourceResult terminalError(DataSourceStatusProvider.ErrorInfo
 
     public static FDv2SourceResult terminalError(DataSourceStatusProvider.ErrorInfo errorInfo, boolean fdv1Fallback, Function<Void, Void> completionCallback) {
         return new FDv2SourceResult(null,
-            new Status(State.TERMINAL_ERROR, errorInfo),
+            Status.terminalError(errorInfo),
             ResultType.STATUS,
             fdv1Fallback,
             completionCallback);
@@ -145,10 +172,9 @@ public static FDv2SourceResult goodbye(String reason, boolean fdv1Fallback) {
     }
 
     public static FDv2SourceResult goodbye(String reason, boolean fdv1Fallback, Function<Void, Void> completionCallback) {
-        // TODO: Goodbye reason.
         return new FDv2SourceResult(
             null,
-            new Status(State.GOODBYE, null),
+            Status.goodbye(reason),
             ResultType.STATUS,
             fdv1Fallback,
             completionCallback);

lib/sdk/server/src/test/java/com/launchdarkly/sdk/server/StreamingSynchronizerImplTest.java

@@ -144,6 +144,7 @@ public void httpRecoverableError() throws Exception {
             assertEquals(FDv2SourceResult.ResultType.STATUS, result.getResultType());
             assertEquals(FDv2SourceResult.State.INTERRUPTED, result.getStatus().getState());
             assertEquals(DataSourceStatusProvider.ErrorKind.ERROR_RESPONSE, result.getStatus().getErrorInfo().getKind());
+            assertNull(result.getStatus().getReason());
 
             synchronizer.close();
         }
@@ -244,6 +245,7 @@ public void shutdownBeforeEventReceived() throws Exception {
             assertEquals(FDv2SourceResult.ResultType.STATUS, result.getResultType());
             assertEquals(FDv2SourceResult.State.SHUTDOWN, result.getStatus().getState());
             assertNull(result.getStatus().getErrorInfo());
+            assertNull(result.getStatus().getReason());
         }
     }
 
@@ -322,6 +324,7 @@ public void goodbyeEventInResponse() throws Exception {
             assertNotNull(result1);
             assertEquals(FDv2SourceResult.ResultType.STATUS, result1.getResultType());
             assertEquals(FDv2SourceResult.State.GOODBYE, result1.getStatus().getState());
+            assertEquals("service-unavailable", result1.getStatus().getReason());
 
             // Second result should be a changeset from the restarted stream
             CompletableFuture<FDv2SourceResult> result2Future = synchronizer.next();