2 files changed, 35 insertions, 0 deletions
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/BuildEventStreamer.java b/src/main/java/com/google/devtools/build/lib/runtime/BuildEventStreamer.java
index 6cb2821d28..7c54ec4600 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/BuildEventStreamer.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/BuildEventStreamer.java
@@ -19,6 +19,7 @@ import static com.google.devtools.build.lib.events.Event.of;
 import static com.google.devtools.build.lib.events.EventKind.PROGRESS;
 
 import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Strings;
 import com.google.common.collect.HashMultimap;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Multimap;
@@ -515,6 +516,10 @@ public class BuildEventStreamer implements EventHandler {
         out = outErrProvider.getOut();
         err = outErrProvider.getErr();
       }
+      if (Strings.isNullOrEmpty(out) && Strings.isNullOrEmpty(err)) {
+        // Nothing to flush; avoid generating an unneeded progress event.
+        return;
+      }
       if (announcedEvents != null) {
         updateEvent = flushStdoutStderrEvent(out, err);
       } else {
diff --git a/src/test/java/com/google/devtools/build/lib/runtime/BuildEventStreamerTest.java b/src/test/java/com/google/devtools/build/lib/runtime/BuildEventStreamerTest.java
index 541843b9fc..81b4328eaf 100644
--- a/src/test/java/com/google/devtools/build/lib/runtime/BuildEventStreamerTest.java
+++ b/src/test/java/com/google/devtools/build/lib/runtime/BuildEventStreamerTest.java
@@ -699,6 +699,36 @@ public class BuildEventStreamerTest extends FoundationTestCase {
   }
 
   @Test
+  public void testNoopFlush() throws Exception {
+    // Verify that the streamer ignores a flush, if neither stream produces any output.
+    RecordingBuildEventTransport transport = new RecordingBuildEventTransport();
+    BuildEventStreamer streamer =
+        new BuildEventStreamer(ImmutableSet.<BuildEventTransport>of(transport), reporter);
+    BuildEventStreamer.OutErrProvider outErr =
+        Mockito.mock(BuildEventStreamer.OutErrProvider.class);
+    String stdoutMsg = "Some text that was written to stdout.";
+    String stderrMsg = "The UI text that bazel wrote to stderr.";
+    when(outErr.getOut()).thenReturn(stdoutMsg).thenReturn("");
+    when(outErr.getErr()).thenReturn(stderrMsg).thenReturn("");
+    BuildEvent startEvent =
+        new GenericBuildEvent(
+            testId("Initial"),
+            ImmutableSet.<BuildEventId>of(ProgressEvent.INITIAL_PROGRESS_UPDATE));
+
+    streamer.registerOutErrProvider(outErr);
+    streamer.buildEvent(startEvent);
+    assertThat(transport.getEvents()).hasSize(1);
+    streamer.flush(); // Output, so a new progress event has to be added
+    assertThat(transport.getEvents()).hasSize(2);
+    streamer.flush(); // No further output, so no additional event should be generated.
+    assertThat(transport.getEvents()).hasSize(2);
+
+    assertThat(transport.getEvents().get(0)).isEqualTo(startEvent);
+    assertThat(transport.getEventProtos().get(1).getProgress().getStdout()).isEqualTo(stdoutMsg);
+    assertThat(transport.getEventProtos().get(1).getProgress().getStderr()).isEqualTo(stderrMsg);
+  }
+
+  @Test
   public void testEarlyFlushBadInitialEvent() throws Exception {
     // Verify that an early flush works correctly with an unusual start event.
     // In this case, we expect 3 events in the stream, in that order: