DBZ-8163 Clarify equals case & positive/negative infinity

src/main/java/io/debezium/connector/vitess/pipeline/txmetadata/ShardLineage.java

@@ -41,10 +41,10 @@ public static Long getInheritedEpoch(String shardString, ShardEpochMap shardEpoc
 
     private static class Shard {
 
-        // A string lexicographically less than all other strings
-        public static final String NEGATIVE_INFINITY = "";
-        // A string lexicographically greater than all other strings
-        public static final String POSITIVE_INFINITY = "\uFFFF";
+        // A string of a single char that is lexicographically less than all other chars
+        public static final String NEGATIVE_INFINITY = String.valueOf(Character.MIN_VALUE);
+        // A string of a single char that is lexicographically greater than all other chars
+        public static final String POSITIVE_INFINITY = String.valueOf(Character.MAX_VALUE);
 
         private final String lowerBound;
         private final String upperBound;

src/test/java/io/debezium/connector/vitess/pipeline/txmetadata/ShardLineageTest.java

@@ -46,6 +46,17 @@ public void shouldGetInheritedEpoch_MultiShard_SplitOneShard() {
         assertThat(epoch3).isEqualTo(parentEpoch2 + 1);
     }
 
+    @Test
+    public void shouldGetInheritedEpoch_MultiShard_SplitOneShard_UpperEqualsLowerBound() {
+        Long parentEpoch = 3L;
+        Long parentEpoch2 = 1L;
+        ShardEpochMap shardEpochMap = new ShardEpochMap(Map.of("-80", parentEpoch, "80-", parentEpoch2));
+        // The lower bound (80) is equal to a previous shard's upper bound (-80)
+        Long epoch2 = ShardLineage.getInheritedEpoch("80-c0", shardEpochMap);
+        // This is not a descendant so assert the epoch is from the actual parent (which is less than the other parent)
+        assertThat(epoch2).isEqualTo(parentEpoch2 + 1);
+    }
+
     @Test
     public void shouldGetInheritedEpoch_TwoToFourShards() {
         Long parentEpoch = 1L;