Fix handling of fractional byte size value settings (#37172)

This commit addresses an issue when setting a byte size value setting
using a value that has a fractional component when converted to its
string representation. For example, trying to set a byte size value
setting to a value of 1536 bytes is problematic because internally this
is converted to the string "1.5k". When we go to get this setting, we
try to parse "1.5k" back to a byte size value, which does not support
fractional values. The problem is that internally we are relying on a
method which loses the unit when doing the string conversion. Instead,
we are going to use a method that does not lose the unit and therefore
we can roundtrip from the byte size value to the string and back to the
byte size value.

server/src/main/java/org/elasticsearch/common/settings/Settings.java

@@ -861,8 +861,8 @@ public final class Settings implements ToXContentFragment {
          * @param byteSizeValue The setting value
          * @return The builder
          */
-        public Builder put(String key, ByteSizeValue byteSizeValue) {
-            return put(key, byteSizeValue.toString());
+        public Builder put(final String key, final ByteSizeValue byteSizeValue) {
+            return put(key, byteSizeValue.getStringRep());
         }
 
         /**

server/src/test/java/org/elasticsearch/common/settings/SettingsTests.java

@@ -25,6 +25,8 @@ import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.io.stream.BytesStreamOutput;
 import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.common.unit.ByteSizeUnit;
+import org.elasticsearch.common.unit.ByteSizeValue;
 import org.elasticsearch.common.unit.TimeValue;
 import org.elasticsearch.common.xcontent.ToXContent;
 import org.elasticsearch.common.xcontent.XContentBuilder;
@@ -726,4 +728,20 @@ public class SettingsTests extends ESTestCase {
         assertThat(actual, equalTo(expected));
     }
 
+    public void testFractionalByteSizeValue() {
+        final Setting<ByteSizeValue> setting =
+                Setting.byteSizeSetting("key", ByteSizeValue.parseBytesSizeValue(randomIntBetween(1, 16) + "k", "key"));
+        final ByteSizeValue expected = new ByteSizeValue(randomNonNegativeLong(), ByteSizeUnit.BYTES);
+        final Settings settings = Settings.builder().put("key", expected).build();
+        /*
+         * Previously we would internally convert the byte size value to a string using a method that tries to be smart about the units
+         * (e.g., 1024 bytes would be converted to 1kb). However, this had a problem in that, for example, 1536 bytes would be converted to
+         * 1.5k. Then, 1.5k could not be converted back to a ByteSizeValue because ByteSizeValues do not support fractional components.
+         * Effectively this test is then asserting that we no longer make this mistake when doing the internal string conversion. Instead,
+         * we convert to a string using a method that does not lose the original unit.
+         */
+        final ByteSizeValue actual = setting.get(settings);
+        assertThat(actual, equalTo(expected));
+    }
+
 }