Fix testCDCIndexFileWriteOnSync and document cdc index file read edge…

… case

Patch by Yifan Cai; reviewed by Josh McKenzie for CASSANDRA-17416

doc/modules/cassandra/pages/operating/cdc.adoc

@@ -16,12 +16,17 @@ persisted to disk. Upon final segment flush, a second line with the
 human-readable word "COMPLETED" will be added to the _cdc.idx file
 indicating that Cassandra has completed all processing on the file.
 
-We we use an index file rather than just encouraging clients to parse
+We use an index file rather than just encouraging clients to parse
 the log realtime off a memory mapped handle as data can be reflected in
 a kernel buffer that is not yet persisted to disk. Parsing only up to
 the listed offset in the _cdc.idx file will ensure that you only parse
 CDC data for data that is durable.
 
+Please note that in rare chances, e.g. slow disk, it is possible for the
+consumer to read an empty value from the _cdc.idx file because update is
+achieved with first truncating the file then write to the file. In such
+case, the consumer should retry read the index file.
+
 A threshold of total disk space allowed is specified in the yaml at
 which time newly allocated CommitLogSegments will not allow CDC data
 until a consumer parses and removes files from the specified cdc_raw

test/unit/org/apache/cassandra/db/commitlog/CommitLogSegmentManagerCDCTest.java

@@ -24,7 +24,9 @@
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.*;
+import java.util.concurrent.TimeUnit;
 
+import com.google.monitoring.runtime.instrumentation.common.util.concurrent.Uninterruptibles;
 import org.apache.cassandra.io.util.File;
 import org.apache.cassandra.io.util.FileReader;
 import org.junit.Assert;
@@ -157,11 +159,28 @@ public void testCDCIndexFileWriteOnSync() throws IOException
         Assert.assertTrue("Index file not written: " + cdcIndexFile, cdcIndexFile.exists());
 
         // Read index value and confirm it's == end from last sync
-        BufferedReader in = new BufferedReader(new FileReader(cdcIndexFile));
-        String input = in.readLine();
-        Integer offset = Integer.parseInt(input);
-        Assert.assertEquals(syncOffset, (long)offset);
-        in.close();
+        String input = null;
+        // There could be a race between index file update (truncate & write) and read. See CASSANDRA-17416
+        // It is possible to read an empty line. In this case, re-try at most 5 times.
+        for (int i = 0; input == null && i < 5; i++)
+        {
+            if (i != 0) // add a little pause between each attempt
+                Uninterruptibles.sleepUninterruptibly(10, TimeUnit.MILLISECONDS);
+
+            try (BufferedReader in = new BufferedReader(new FileReader(cdcIndexFile)))
+            {
+                input = in.readLine();
+            }
+        }
+
+        if (input == null)
+        {
+            Assert.fail("Unable to read the CDC index file after several attempts");
+        }
+
+        int indexOffset = Integer.parseInt(input);
+        Assert.assertTrue("The offset read from CDC index file should be equal or larger than the offset after sync. See CASSANDRA-17416",
+                          syncOffset <= indexOffset);
     }
 
     @Test