Integration of TOS: Add lazy reload interface to common.

hadoop-cloud-storage-project/hadoop-tos/src/main/java/org/apache/hadoop/fs/tosfs/common/LazyReload.java

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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hadoop.fs.tosfs.common;
+
+import java.util.Iterator;
+import java.util.function.Supplier;
+
+public class LazyReload<T> implements Iterable<T> {
+  private final Supplier<Reload<T>> reload;
+
+  public LazyReload(Supplier<Reload<T>> reload) {
+    this.reload = reload;
+  }
+
+  @Override
+  public Iterator<T> iterator() {
+    return new LazyReloadIter<>(reload.get());
+  }
+}

hadoop-cloud-storage-project/hadoop-tos/src/main/java/org/apache/hadoop/fs/tosfs/common/LazyReloadIter.java

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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hadoop.fs.tosfs.common;
+
+import org.apache.hadoop.util.Lists;
+
+import java.util.Iterator;
+import java.util.List;
+import java.util.NoSuchElementException;
+
+public class LazyReloadIter<T> implements Iterator<T> {
+  private final List<T> buf = Lists.newArrayList();
+  private final Reload<T> reload;
+  private int cur = 0;
+  private boolean exhausted = false;
+
+  public LazyReloadIter(Reload<T> reload) {
+    this.reload = reload;
+  }
+
+  @Override
+  public boolean hasNext() {
+    if (exhausted && buf.isEmpty()) {
+      return false;
+    }
+    if (cur >= buf.size()) {
+      // Reset the buffer and load more elements.
+      buf.clear();
+      cur = 0;
+      // Reload the next batch.
+      boolean exhausted = reload.fill(buf);
+      while (buf.isEmpty() && !exhausted) {
+        exhausted = reload.fill(buf);
+      }
+
+      if (exhausted) {
+        this.exhausted = true;
+      }
+
+      return !buf.isEmpty();
+    }
+    return true;
+  }
+
+  @Override
+  public T next() {
+    if (hasNext()) {
+      return buf.get(cur++);
+    } else {
+      throw new NoSuchElementException();
+    }
+  }
+}
+

hadoop-cloud-storage-project/hadoop-tos/src/main/java/org/apache/hadoop/fs/tosfs/common/Reload.java

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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hadoop.fs.tosfs.common;
+
+import java.util.List;
+
+public interface Reload<T> {
+  /**
+   * @param buf contains the filled data
+   * @return true if no need to fill again since the source is exhausted, otherwise false.
+   */
+  boolean fill(List<T> buf);
+}