fix: prevent buffer stripe index overflow (#2331)

Author: zeitlinger

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

@@ -43,7 +43,7 @@ class Buffer {
   }
 
   boolean append(double value) {
-    int index = Math.abs((int) Thread.currentThread().getId()) % stripedObservationCounts.length;
+    int index = stripeIndex(Thread.currentThread().getId(), stripedObservationCounts.length);
     AtomicLong observationCountForThread = stripedObservationCounts[index];
     long count = observationCountForThread.incrementAndGet();
     if ((count & bufferActiveBit) == 0) {
@@ -54,6 +54,10 @@ boolean append(double value) {
     }
   }
 
+  static int stripeIndex(long threadId, int stripeCount) {
+    return (int) Math.floorMod(threadId, stripeCount);
+  }
+
   private void doAppend(double amount) {
     appendLock.lock();
     try {

prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/metrics/BufferTest.java

@@ -0,0 +1,15 @@
+package io.prometheus.metrics.core.metrics;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class BufferTest {
+
+  @Test
+  void stripeIndexDoesNotOverflowWhenThreadIdNarrowsToIntegerMinValue() {
+    assertThat(Buffer.stripeIndex(2_147_483_648L, 3)).isEqualTo(2);
+    assertThat(Buffer.stripeIndex(2_147_483_648L, 6)).isEqualTo(2);
+    assertThat(Buffer.stripeIndex(2_147_483_648L, 12)).isEqualTo(8);
+  }
+}