Add java test for keyMayExists.

Author: rhubner

db/db_impl/db_impl.cc

@@ -3830,7 +3830,6 @@ bool DBImpl::KeyMayExist(const ReadOptions& read_options,
   // not present in block_cache, the return value will be Status::Incomplete.
   // In this case, key may still exist in the table.
   if (value_found != nullptr && s.IsIncomplete()) {
-    // falsify later if key-may-exist but can't fetch value
     *value_found = false;
   }
   return s.ok() || s.IsIncomplete();

java/src/test/java/org/rocksdb/KeyMayExistTest.java

@@ -24,6 +24,8 @@ public class KeyMayExistTest {
   @Rule
   public TemporaryFolder dbFolder = new TemporaryFolder();
 
+  @Rule public TemporaryFolder dbFolder2 = new TemporaryFolder();
+
   @Rule public ExpectedException exceptionRule = ExpectedException.none();
 
   List<ColumnFamilyDescriptor> cfDescriptors;
@@ -541,4 +543,40 @@ public void keyMayExistNonUnicodeString() throws RocksDBException {
     exists = db.keyMayExist("key".getBytes(UTF_8), null);
     assertThat(exists).isTrue();
   }
+
+  /*
+    This test was created to verify bug with keyMaye exist and value_found.
+    It's probabilistic and expect that SST files are not loaded immediately to memory
+    after database opening.
+   */
+  @Test
+  public void keyMayExistNoCache() throws RocksDBException {
+    byte[] key = "key".getBytes(UTF_8);
+    byte[] value = "value".getBytes(UTF_8);
+
+    try (RocksDB db2 = RocksDB.open(dbFolder2.getRoot().getAbsolutePath())) {
+      db2.put(key, value);
+      db2.compactRange();
+    }
+
+    try (RocksDB db2 = RocksDB.open(dbFolder2.getRoot().getAbsolutePath())) {
+      Holder<byte[]> holder = new Holder<>();
+
+      boolean exists = db2.keyMayExist("key".getBytes(UTF_8), holder);
+
+      assertThat(exists).isTrue();
+
+      // keys is not in cache, so returned value must be null;
+      assertThat(holder.getValue()).isNull();
+
+      byte[] returned_value = db2.get(key);
+
+      assertThat(returned_value).isEqualTo(value);
+
+      exists = db2.keyMayExist("key".getBytes(UTF_8), holder);
+      assertThat(exists).isTrue();
+      // key is in cache, it should return value
+      assertThat(holder.getValue()).isEqualTo(value);
+    }
+  }
 }