[6.8] Backport some fixes to CharArraysTest#testConstantTimeEquals() (#67168)

* Ensure char array test uses different values (#47238)

The test of constantTimeEquals could get unlucky and randomly produce
the same two strings. This commit tweaks the test to ensure the two
string are unique, and the loop inside constantTimeEquals is actually
executed (which requires the strings be of the same length).

* Fix CharArraysTests.testConstantTimeEquals() (#47346)

The change #47238 fixed a first issue (#47076) but introduced 
another one that can be reproduced using:

org.elasticsearch.common.CharArraysTests > testConstantTimeEquals FAILED

java.lang.StringIndexOutOfBoundsException: String index out of range: 1
at __randomizedtesting.SeedInfo.seed([DFCA64FE2C786BE3:ED987E883715C63B]:0)
at java.lang.String.substring(String.java:1963)
at org.elasticsearch.common.CharArraysTests.testConstantTimeEquals(CharArraysTests.java:74)

REPRODUCE WITH: ./gradlew ':libs:elasticsearch-core:test' --tests 
"org.elasticsearch.common.CharArraysTests.testConstantTimeEquals" 
-Dtests.seed=DFCA64FE2C786BE3 -Dtests.security.manager=true -Dtests.locale=fr-CA 
-Dtests.timezone=Pacific/Johnston -Dcompiler.java=12 -Druntime.java=8

that happens when the first randomized string has a length of 0.

Co-authored-by: Ryan Ernst <[email protected]>
Co-authored-by: Tanguy Leroux <[email protected]>

Author: 3 people

libs/core/src/test/java/org/elasticsearch/common/CharArraysTests.java

@@ -23,6 +23,8 @@
 
 import java.nio.charset.StandardCharsets;
 
+import static org.hamcrest.Matchers.is;
+
 public class CharArraysTests extends ESTestCase {
 
     public void testCharsToBytes() {
@@ -69,9 +71,13 @@ public void testConstantTimeEquals() {
         assertTrue(CharArrays.constantTimeEquals(value, value));
         assertTrue(CharArrays.constantTimeEquals(value.toCharArray(), value.toCharArray()));
 
-        final String other = randomAlphaOfLengthBetween(1, 32);
-        assertFalse(CharArrays.constantTimeEquals(value, other));
-        assertFalse(CharArrays.constantTimeEquals(value.toCharArray(), other.toCharArray()));
+        // we want a different string, so ensure the first character is different, but the same overall length
+        final int length = value.length();
+        final String other = length > 0 ? new String(randomAlphaOfLengthNotBeginningWith(value.substring(0, 1), length, length)) : "";
+        final boolean expectedEquals = length == 0;
+
+        assertThat("value: " + value + ", other: " + other, CharArrays.constantTimeEquals(value, other), is(expectedEquals));
+        assertThat(CharArrays.constantTimeEquals(value.toCharArray(), other.toCharArray()), is(expectedEquals));
     }
 
     private char[] randomAlphaOfLengthNotBeginningWith(String undesiredPrefix, int min, int max) {