Fix max_primary_shard_size resize factor math (#86897)

Author: joegallo

docs/changelog/86897.yaml

@@ -0,0 +1,5 @@
+pr: 86897
+summary: Fix `max_primary_shard_size` resize factor math
+area: ILM+SLM
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/action/admin/indices/shrink/ResizeNumberOfShardsCalculator.java

@@ -134,7 +134,7 @@ protected static int calculateAcceptableNumberOfShards(final int sourceIndexShar
                     }
                 }
             } else {
-                for (int i = 1; i < num; i++) {
+                for (int i = 1; i <= num; i++) {
                     if (sourceIndexShardsNum % i == 0 && minShardsNum <= i) {
                         return i;
                     }

server/src/test/java/org/elasticsearch/action/admin/indices/shrink/ResizeNumberOfShardsCalculatorTests.java

@@ -91,15 +91,16 @@ public void testSplitInputs() {
     }
 
     public void testCalculateTargetShardsNumberInShrink() {
-        assertEquals(calculateAcceptableNumberOfShards(0, 0), 1);
-        assertEquals(calculateAcceptableNumberOfShards(10, 0), 1);
-        assertEquals(calculateAcceptableNumberOfShards(10, 1), 1);
-        assertEquals(calculateAcceptableNumberOfShards(10, 2), 2);
-        assertEquals(calculateAcceptableNumberOfShards(10, 3), 5);
-        assertEquals(calculateAcceptableNumberOfShards(10, 6), 10);
-        assertEquals(calculateAcceptableNumberOfShards(10, 11), 10);
-        assertEquals(calculateAcceptableNumberOfShards(59, 21), 59);
-        assertEquals(calculateAcceptableNumberOfShards(60, 21), 30);
-        assertEquals(calculateAcceptableNumberOfShards(60, 31), 60);
+        assertEquals(1, calculateAcceptableNumberOfShards(0, 0));
+        assertEquals(3, calculateAcceptableNumberOfShards(9, 2));
+        assertEquals(1, calculateAcceptableNumberOfShards(10, 0));
+        assertEquals(1, calculateAcceptableNumberOfShards(10, 1));
+        assertEquals(2, calculateAcceptableNumberOfShards(10, 2));
+        assertEquals(5, calculateAcceptableNumberOfShards(10, 3));
+        assertEquals(10, calculateAcceptableNumberOfShards(10, 6));
+        assertEquals(10, calculateAcceptableNumberOfShards(10, 11));
+        assertEquals(59, calculateAcceptableNumberOfShards(59, 21));
+        assertEquals(30, calculateAcceptableNumberOfShards(60, 21));
+        assertEquals(60, calculateAcceptableNumberOfShards(60, 31));
     }
 }