apoorvmittal10 commented on code in PR #19581:
URL: https://github.com/apache/kafka/pull/19581#discussion_r2075600196


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clients/src/test/java/org/apache/kafka/common/record/MemoryRecordsTest.java:
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@@ -1068,6 +1072,149 @@ public void testUnsupportedCompress() {
         });
     }
 
+    @ParameterizedTest
+    @ArgumentsSource(MemoryRecordsArgumentsProvider.class)
+    public void testSlice(Args args) {
+        // Create a MemoryRecords instance with multiple batches. Prior 
RecordBatch.MAGIC_VALUE_V2,
+        // every append in a batch is a new batch. After 
RecordBatch.MAGIC_VALUE_V2, we can have multiple
+        // batches in a single MemoryRecords instance. Though with 
compression, we can have multiple
+        // appends resulting in a single batch prior 
RecordBatch.MAGIC_VALUE_V2 as well.
+        LinkedHashMap<Long, Integer> recordsPerOffset = new LinkedHashMap<>();
+        recordsPerOffset.put(args.firstOffset, 3);
+        recordsPerOffset.put(args.firstOffset + 6L, 8);
+        recordsPerOffset.put(args.firstOffset + 15L, 4);
+        MemoryRecords records = createMemoryRecords(args, recordsPerOffset);
+
+        // Test slicing from start
+        MemoryRecords sliced = records.slice(0, records.sizeInBytes());
+        assertEquals(records.sizeInBytes(), sliced.sizeInBytes());
+        assertEquals(records.validBytes(), sliced.validBytes());
+        TestUtils.checkEquals(records.batches(), sliced.batches());
+
+        List<RecordBatch> items = batches(records);
+        // Test slicing first message.
+        RecordBatch first = items.get(0);
+        sliced = records.slice(first.sizeInBytes(), records.sizeInBytes() - 
first.sizeInBytes());
+        assertEquals(records.sizeInBytes() - first.sizeInBytes(), 
sliced.sizeInBytes());
+        assertEquals(items.subList(1, items.size()), batches(sliced), "Read 
starting from the second message");
+        assertTrue(sliced.validBytes() <= sliced.sizeInBytes());
+
+        // Read from second message and size is past the end of the file.
+        sliced = records.slice(first.sizeInBytes(), records.sizeInBytes());
+        assertEquals(records.sizeInBytes() - first.sizeInBytes(), 
sliced.sizeInBytes());
+        assertEquals(items.subList(1, items.size()), batches(sliced), "Read 
starting from the second message");
+        assertTrue(sliced.validBytes() <= sliced.sizeInBytes());
+
+        // Read from second message and position + size overflows.
+        sliced = records.slice(first.sizeInBytes(), Integer.MAX_VALUE);
+        assertEquals(records.sizeInBytes() - first.sizeInBytes(), 
sliced.sizeInBytes());
+        assertEquals(items.subList(1, items.size()), batches(sliced), "Read 
starting from the second message");
+        assertTrue(sliced.validBytes() <= sliced.sizeInBytes());
+
+        // Read a single message starting from second message.
+        RecordBatch second = items.get(1);
+        sliced = records.slice(first.sizeInBytes(), second.sizeInBytes());
+        assertEquals(second.sizeInBytes(), sliced.sizeInBytes());
+        assertEquals(Collections.singletonList(second), batches(sliced), "Read 
a single message starting from the second message");
+
+        // Read from already sliced view.
+        List<RecordBatch> remainingItems = IntStream.range(0, 
items.size()).filter(i -> i != 0 && i != 
1).mapToObj(items::get).collect(Collectors.toList());
+        int remainingSize = 
remainingItems.stream().mapToInt(RecordBatch::sizeInBytes).sum();
+        sliced = records.slice(first.sizeInBytes(), records.sizeInBytes() - 
first.sizeInBytes())
+                        .slice(second.sizeInBytes(), records.sizeInBytes() - 
first.sizeInBytes() - second.sizeInBytes());
+        assertEquals(remainingSize, sliced.sizeInBytes());
+        assertEquals(remainingItems, batches(sliced), "Read starting from the 
third message");
+
+        // Read from second message and size is past the end of the file on 
the already sliced view.
+        sliced = records.slice(1, records.sizeInBytes() - 1)

Review Comment:
   Yes, that's true. And test case also verifies that though the first slice 
was random but subequent slice on same with correct batch boundary resulted in 
correct data.



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