Make randomInstantBetween return in range [minInstant, maxInstant] (#114177)

randomInstantBetween can produce a result which is not within the [minInstant, maxInstant] range. This occurs when the epoch second picked matches the min bound and the nanos are below the min nanos, or the second picked matches the max bound seconds and nanos are above the max bound nanos. This change fixes the function by setting a bound on which nano values can be picked if the min or max epoch second value is picked.

Author: parkertimmins

docs/changelog/114177.yaml

@@ -0,0 +1,5 @@
+pr: 114177
+summary: "Make `randomInstantBetween` always return value in range [minInstant, `maxInstant]`"
+area: Infra/Metrics
+type: bug
+issues: []

test/framework/src/main/java/org/elasticsearch/test/ESTestCase.java

@@ -900,10 +900,11 @@ public static long randomLongBetween(long min, long max) {
      * @return a random instant between a min and a max value with a random nanosecond precision
      */
     public static Instant randomInstantBetween(Instant minInstant, Instant maxInstant) {
-        return Instant.ofEpochSecond(
-            randomLongBetween(minInstant.getEpochSecond(), maxInstant.getEpochSecond()),
-            randomLongBetween(0, 999999999)
-        );
+        long epochSecond = randomLongBetween(minInstant.getEpochSecond(), maxInstant.getEpochSecond());
+        long minNanos = epochSecond == minInstant.getEpochSecond() ? minInstant.getNano() : 0;
+        long maxNanos = epochSecond == maxInstant.getEpochSecond() ? maxInstant.getNano() : 999999999;
+        long nanos = randomLongBetween(minNanos, maxNanos);
+        return Instant.ofEpochSecond(epochSecond, nanos);
     }
 
     /**