Easy: Fix XctoolOutputParsing exception on empty JSON

Summary:
If xctool is in a bad state, it can close stdout before printing
anything (facebookarchive/xctool#656)

That makes the JSON parser in `XctoolOutputParsing` fail:

  com.google.gson.JsonIOException: java.io.EOFException: End of input at line 1 column 1
                                   at com.google.gson.JsonStreamParser.hasNext(JsonStreamParser.java:109)
                                   at com.facebook.buck.apple.XctoolOutputParsing.streamOutputFromReader(XctoolOutputParsing.java:147)
                                   at com.facebook.buck.apple.AppleTest$AppleTestXctoolStdoutReader.readStdout(AppleTest.java:138)
                                   at com.facebook.buck.apple.XctoolRunTestsStep.execute(XctoolRunTestsStep.java:292)
                                   at com.facebook.buck.step.DefaultStepRunner.runStepForBuildTarget(DefaultStepRunner.java:63)
                                   at com.facebook.buck.step.DefaultStepRunner$1.call(DefaultStepRunner.java:92)
                                   at java.util.concurrent.FutureTask.run(FutureTask.java:266)
                                   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
                                   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
                                   at java.lang.Thread.run(Thread.java:745)
  Caused by: java.io.EOFException: End of input at line 1 column 1
         at com.google.gson.stream.JsonReader.nextNonWhitespace(JsonReader.java:1377)
         at com.google.gson.stream.JsonReader.consumeNonExecutePrefix(JsonReader.java:1514)
         at com.google.gson.stream.JsonReader.doPeek(JsonReader.java:523)
         at com.google.gson.stream.JsonReader.peek(JsonReader.java:414)
         at com.google.gson.JsonStreamParser.hasNext(JsonStreamParser.java:105)
         ... 9 more

This fixes the issue by cleanly handling exceptions from `hasNext()` and just
ignoring the rest of the output.

Test Plan: Unit tests updated. Confirmed test failed before fix and passed after fix.

Reviewed By: ryu2

fb-gh-sync-id: 3d3a2e0

src/com/facebook/buck/apple/XctoolOutputParsing.java

@@ -144,12 +144,12 @@ public static void streamOutputFromReader(
       XctoolEventCallback eventCallback) {
     Gson gson = new Gson();
     JsonStreamParser streamParser = new JsonStreamParser(reader);
-    while (streamParser.hasNext()) {
-      try {
+    try {
+      while (streamParser.hasNext()) {
         dispatchEventCallback(gson, streamParser.next(), eventCallback);
-      } catch (JsonParseException e) {
-        LOG.warn(e, "Couldn't parse xctool JSON stream");
       }
+    } catch (JsonParseException e) {
+      LOG.warn(e, "Couldn't parse xctool JSON stream");
     }
   }
 

test/com/facebook/buck/apple/XctoolOutputParsingTest.java

@@ -33,6 +33,7 @@
 
 import org.junit.Test;
 
+import java.io.StringReader;
 import java.io.Reader;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
@@ -268,6 +269,15 @@ public void streamingSimpleFailure() throws Exception {
     assertThat(endOcunitEvent.succeeded, is(false));
   }
 
+  @Test
+  public void streamingEmptyReaderDoesNotCauseFailure() {
+    final List<Object> streamedObjects = new ArrayList<>();
+    XctoolOutputParsing.streamOutputFromReader(
+        new StringReader(""),
+        eventCallbackAddingEventsToList(streamedObjects));
+    assertThat(streamedObjects, is(empty()));
+  }
+
   @Test
   public void validEventParsesToStatusMessage() {
     XctoolOutputParsing.StatusEvent statusEvent = new XctoolOutputParsing.StatusEvent();