fix deadlock (#1152)

* fix deadlock

Signed-off-by: Gregor Zeitlinger <[email protected]>

* fix deadlock

Signed-off-by: Gregor Zeitlinger <[email protected]>

---------

Signed-off-by: Gregor Zeitlinger <[email protected]>

Author: zeitlinger

prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/Buffer.java

@@ -57,6 +57,7 @@ void reset() {
     reset = true;
   }
 
+  @SuppressWarnings("ThreadPriorityCheck")
   <T extends DataPointSnapshot> T run(
       Function<Long, Boolean> complete,
       Supplier<T> createResult,
@@ -70,24 +71,26 @@ <T extends DataPointSnapshot> T run(
       // Signal that the buffer is active.
       Long expectedCount = observationCount.getAndAdd(bufferActiveBit);
 
-      appendLock.lock();
-      try {
-        while (!complete.apply(expectedCount)) {
-          // Wait until all in-flight threads have added their observations to the buffer.
-          bufferFilled.await();
-        }
-        result = createResult.get();
+      while (!complete.apply(expectedCount)) {
+        // Wait until all in-flight threads have added their observations to the histogram
+        // we can't use a condition here, because the other thread doesn't have a lock as it's on
+        // the fast path.
+        Thread.yield();
+      }
+      result = createResult.get();
 
-        // Signal that the buffer is inactive.
-        int expectedBufferSize;
-        if (reset) {
-          expectedBufferSize =
-              (int) ((observationCount.getAndSet(0) & ~bufferActiveBit) - expectedCount);
-          reset = false;
-        } else {
-          expectedBufferSize = (int) (observationCount.addAndGet(bufferActiveBit) - expectedCount);
-        }
+      // Signal that the buffer is inactive.
+      int expectedBufferSize;
+      if (reset) {
+        expectedBufferSize =
+            (int) ((observationCount.getAndSet(0) & ~bufferActiveBit) - expectedCount);
+        reset = false;
+      } else {
+        expectedBufferSize = (int) (observationCount.addAndGet(bufferActiveBit) - expectedCount);
+      }
 
+      appendLock.lock();
+      try {
         while (bufferPos < expectedBufferSize) {
           // Wait until all in-flight threads have added their observations to the buffer.
           bufferFilled.await();