What is the purpose of this PR

• This PR fixes a flaky test caused by the non-determinism of the amount of time that the unit test testWildCardTtlScan() takes to run.
• In other words, the test may randomly fail or pass when run on a slow enough machine.

Reproduce the test failure

• Run test testWildCardTtlScan() for many times in a slow enough machine.
• To reproduce the failure more often, we can add Thread.sleep(1000); after getting the current time (line 862) and before creating the object of StoreScanner(line 877) in the TestStoreScanner.java file to simulate a slow machine.

Expected result:

• The test should run successfully regardless of the amount of time between getting current time (line 862) and creating the object of StoreScanner (line 877), regardless of the speed of running tests.

Actual result:

• We can get the stack trace of test failure as below

 java.lang.AssertionError: expected:<2> but was:<0>
    at org.junit.Assert.fail(Assert.java:89)
    at org.junit.Assert.failNotEquals(Assert.java:835)
    at org.junit.Assert.assertEquals(Assert.java:647)
    at org.junit.Assert.assertEquals(Assert.java:633)
    at org.apache.hadoop.hbase.regionserver.TestStoreScanner.testWildCardTtlScan(TestStoreScanner.java:856)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)

Why the test fails

• The time to create the keyValue array is different from that of creating the StoreScanner object.

  i.e., for line 175 in file StoreScanner.java:

  this.oldestUnexpiredTS = scan.isRaw() ? 0L : now - scanInfo.getTtl();

  the variable "now" is different from the variable "now" that line 836 gets in the TestStoreScanner.java file:

  long now = System.currentTimeMillis();

Fix

Create a new constructor for StoreScanner and modify the internal constructor to receive a new parameter, which enables us to set the start time of finding kvs.