KAFKA-20292 [1/N]: Prepare to split TargetAssignmentBuilders (apache#22485)

To accommodate asynchronous assignments, such as those from client-side
assignors and assignors offloaded to background threads, we want to
split the TargetAssignmentBuilders into two: one builder for building
the target assignment and another for building the target assignment
records. Client-side assignors will only use the second builder.

Both builders require an up-to-date view of group members at the time
they are run. In the non-offloaded case, this is the same view. However,
the view needs to include the unwritten member operations from the
ongoing heartbeat request. Currently the operations are applied within
the TargetAssignmentBuilders. To avoid duplicating the logic once the
TargetAssignmentBuilders are split, we would like to lift it out and
pass the TargetAssignmentBuilders the updated view of members and
assignments.

Add an OverlayMap class, to be used to provide the updated views of
members and assignments.

Reviewers: Dongnuo Lyu <dlyu@confluent.io>, David Jacot
 <david.jacot@gmail.com>

Author: squah-confluent

group-coordinator/src/main/java/org/apache/kafka/coordinator/group/util/OverlayMap.java

@@ -0,0 +1,193 @@
+/*
+ * 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.kafka.coordinator.group.util;
+
+import java.util.AbstractMap;
+import java.util.AbstractSet;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.NoSuchElementException;
+import java.util.Objects;
+import java.util.Set;
+
+/**
+ * A map which wraps an underlying base map and accepts incremental updates
+ * that overlay on top of it. This class expects the underlying base map to
+ * be immutable.
+ *
+ * <p>Null values are not supported.
+ *
+ * @param <K> The key type.
+ * @param <V> The value type.
+ */
+public class OverlayMap<K, V> extends AbstractMap<K, V> {
+    private final Map<K, V> base;
+
+    /** Entries whose keys are not in the base. */
+    private final Map<K, V> additions = new HashMap<>();
+
+    /** Entries whose keys are in the base, with a value that supersedes the base value. */
+    private final Map<K, V> replacements = new HashMap<>();
+
+    /** Keys that are in the base but have been removed. */
+    private final Set<Object> removals = new HashSet<>();
+
+    private Set<Entry<K, V>> entrySet;
+
+    public OverlayMap(Map<K, V> base) {
+        this.base = Objects.requireNonNull(base);
+    }
+
+    @Override
+    public int size() {
+        return base.size() + additions.size() - removals.size();
+    }
+
+    @Override
+    public boolean isEmpty() {
+        return additions.isEmpty() && removals.size() == base.size();
+    }
+
+    @Override
+    public boolean containsKey(Object key) {
+        if (additions.containsKey(key)) return true;
+        if (replacements.containsKey(key)) return true;
+        if (removals.contains(key)) return false;
+        return base.containsKey(key);
+    }
+
+    @Override
+    public V get(Object key) {
+        if (additions.containsKey(key)) return additions.get(key);
+        if (replacements.containsKey(key)) return replacements.get(key);
+        if (removals.contains(key)) return null;
+        return base.get(key);
+    }
+
+    @Override
+    public V put(K key, V value) {
+        Objects.requireNonNull(value);
+
+        if (additions.containsKey(key)) {
+            return additions.put(key, value);
+        }
+        if (replacements.containsKey(key)) {
+            return replacements.put(key, value);
+        }
+        if (removals.remove(key)) {
+            replacements.put(key, value);
+            return null;
+        }
+        if (base.containsKey(key)) {
+            replacements.put(key, value);
+            return base.get(key);
+        }
+        additions.put(key, value);
+        return null;
+    }
+
+    @Override
+    public V remove(Object key) {
+        if (additions.containsKey(key)) {
+            return additions.remove(key);
+        }
+        if (replacements.containsKey(key)) {
+            V prev = replacements.remove(key);
+            removals.add(key);
+            return prev;
+        }
+        if (removals.contains(key)) {
+            return null;
+        }
+        if (base.containsKey(key)) {
+            removals.add(key);
+            return base.get(key);
+        }
+        return null;
+    }
+
+    @Override
+    public void clear() {
+        additions.clear();
+        replacements.clear();
+        removals.clear();
+        removals.addAll(base.keySet());
+    }
+
+    @Override
+    public Set<Entry<K, V>> entrySet() {
+        if (entrySet != null) return entrySet;
+
+        entrySet = new AbstractSet<>() {
+            @Override
+            public Iterator<Entry<K, V>> iterator() {
+                return new Iterator<>() {
+                    private final Iterator<Entry<K, V>> baseIterator = base.entrySet().iterator();
+                    private final Iterator<Entry<K, V>> additionsIterator = additions.entrySet().iterator();
+                    private Entry<K, V> next = null;
+
+                    @Override
+                    public boolean hasNext() {
+                        if (next != null) return true;
+                        while (baseIterator.hasNext()) {
+                            Entry<K, V> entry = baseIterator.next();
+                            if (replacements.containsKey(entry.getKey())) {
+                                next = Map.entry(entry.getKey(), replacements.get(entry.getKey()));
+                            } else {
+                                next = entry;
+                            }
+                            if (removals.contains(entry.getKey())) continue;
+                            return true;
+                        }
+                        if (additionsIterator.hasNext()) {
+                            next = additionsIterator.next();
+                            return true;
+                        }
+                        next = null;
+                        return false;
+                    }
+
+                    @Override
+                    public Entry<K, V> next() {
+                        if (!hasNext()) throw new NoSuchElementException();
+                        Entry<K, V> result = next;
+                        next = null;
+                        return result;
+                    }
+                };
+            }
+
+            @Override
+            public int size() {
+                return OverlayMap.this.size();
+            }
+        };
+        return entrySet;
+    }
+
+    @Override
+    public String toString() {
+        return "OverlayMap(" +
+            "base=" + base +
+            ", additions=" + additions +
+            ", replacements=" + replacements +
+            ", removals=" + removals +
+            ')';
+    }
+}