Prevent write completion starvation for duplicates

When a duplicate output stream is detected, we must signal the
writeWinner (because the write exists) and onInsert (because it was
inserted) for an output stream creation. If we're racing, we should be
eventually convergent, but this absolutely fixes a hang which occurs on
this sentinel stream's return into getOutput, where the future might
never be triggered otherwise.

src/main/java/build/buildfarm/cas/cfc/CASFileCache.java

@@ -2637,6 +2637,9 @@ private CancellableOutputStream putImpl(
             onInsert,
             isReset);
     if (out == DUPLICATE_OUTPUT_STREAM) {
+      log.log(Level.FINER, format("duplicate output stream, completing %s for %s", key, writeId));
+      writeWinner.get();
+      onInsert.run();
       return null;
     }
     log.log(Level.FINER, format("entry %s is missing, downloading and populating", key));