Prevent interleaved slow interrupt stack traces

It's possible that there are multiple outstanding interrupts. When this
happens avoid interleaving their stack traces.

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MOS_MIGRATED_REVID=134477540

src/main/java/com/google/devtools/build/lib/util/ThreadUtils.java

diff --git a/src/main/java/com/google/devtools/build/lib/util/ThreadUtils.java b/src/main/java/com/google/devtools/build/lib/util/ThreadUtils.java
index 35b70c3..7f74e2e 100644
--- a/src/main/java/com/google/devtools/build/lib/util/ThreadUtils.java
+++ b/src/main/java/com/google/devtools/build/lib/util/ThreadUtils.java
@@ -28,7 +28,7 @@
   }
 
   /** Write a thread dump to the blaze.INFO log if interrupt took too long. */
-  public static void warnAboutSlowInterrupt() {
+  public static synchronized void warnAboutSlowInterrupt() {
     LOG.warning("Interrupt took too long. Dumping thread state.");
     for (Map.Entry <Thread, StackTraceElement[]> e : Thread.getAllStackTraces().entrySet()) {
       Thread t = e.getKey();