From 27f4cc3cf724ad6f317eb732026584c427da80c6 Mon Sep 17 00:00:00 2001
From: "ievgen.degtiarenko" <ievgen.degtiarenko@elastic.co>
Date: Wed, 16 Feb 2022 17:29:48 +0100
Subject: [PATCH 1/3] update `readMap` to avoid resizing map during reading

This commit updates readMap to use helper method that presizes map
correctly. It guarantees no resizing when adding given numbers of
entries to a hashmap.
---
 .../java/org/elasticsearch/common/io/stream/StreamInput.java    | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/server/src/main/java/org/elasticsearch/common/io/stream/StreamInput.java b/server/src/main/java/org/elasticsearch/common/io/stream/StreamInput.java
index 2b757c6781b2d..a1e0fa6eef517 100644
--- a/server/src/main/java/org/elasticsearch/common/io/stream/StreamInput.java
+++ b/server/src/main/java/org/elasticsearch/common/io/stream/StreamInput.java
@@ -607,7 +607,7 @@ public String[] readOptionalStringArray() throws IOException {
      * If the returned map contains any entries it will be mutable. If it is empty it might be immutable.
      */
     public <K, V> Map<K, V> readMap(Writeable.Reader<K> keyReader, Writeable.Reader<V> valueReader) throws IOException {
-        return readMap(keyReader, valueReader, HashMap::new);
+        return readMap(keyReader, valueReader, Maps::newMapWithExpectedSize);
     }
 
     public <K, V> Map<K, V> readOrderedMap(Writeable.Reader<K> keyReader, Writeable.Reader<V> valueReader) throws IOException {

From 7bc45b783f9b10da2110373dea40b88be002a9f8 Mon Sep 17 00:00:00 2001
From: "ievgen.degtiarenko" <ievgen.degtiarenko@elastic.co>
Date: Wed, 16 Feb 2022 17:55:14 +0100
Subject: [PATCH 2/3] upd

---
 .../org/elasticsearch/common/io/stream/StreamInput.java   | 8 +++-----
 .../src/main/java/org/elasticsearch/common/util/Maps.java | 8 ++++++++
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/server/src/main/java/org/elasticsearch/common/io/stream/StreamInput.java b/server/src/main/java/org/elasticsearch/common/io/stream/StreamInput.java
index a1e0fa6eef517..2daf1a4527b0b 100644
--- a/server/src/main/java/org/elasticsearch/common/io/stream/StreamInput.java
+++ b/server/src/main/java/org/elasticsearch/common/io/stream/StreamInput.java
@@ -60,9 +60,7 @@
 import java.util.Collections;
 import java.util.Date;
 import java.util.EnumSet;
-import java.util.HashMap;
 import java.util.HashSet;
-import java.util.LinkedHashMap;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Locale;
@@ -607,11 +605,11 @@ public String[] readOptionalStringArray() throws IOException {
      * If the returned map contains any entries it will be mutable. If it is empty it might be immutable.
      */
     public <K, V> Map<K, V> readMap(Writeable.Reader<K> keyReader, Writeable.Reader<V> valueReader) throws IOException {
-        return readMap(keyReader, valueReader, Maps::newMapWithExpectedSize);
+        return readMap(keyReader, valueReader, Maps::newHashMapWithExpectedSize);
     }
 
     public <K, V> Map<K, V> readOrderedMap(Writeable.Reader<K> keyReader, Writeable.Reader<V> valueReader) throws IOException {
-        return readMap(keyReader, valueReader, LinkedHashMap::new);
+        return readMap(keyReader, valueReader, Maps::newLinkedHashMapWithExpectedSize);
     }
 
     private <K, V> Map<K, V> readMap(Writeable.Reader<K> keyReader, Writeable.Reader<V> valueReader, IntFunction<Map<K, V>> constructor)
@@ -784,7 +782,7 @@ private Map<String, Object> readLinkedHashMap() throws IOException {
         if (size9 == 0) {
             return Collections.emptyMap();
         }
-        Map<String, Object> map9 = new LinkedHashMap<>(size9);
+        Map<String, Object> map9 = Maps.newLinkedHashMapWithExpectedSize(size9);
         for (int i = 0; i < size9; i++) {
             map9.put(readString(), readGenericValue());
         }
diff --git a/server/src/main/java/org/elasticsearch/common/util/Maps.java b/server/src/main/java/org/elasticsearch/common/util/Maps.java
index 72fd2ead73ecb..052b6d08c72f8 100644
--- a/server/src/main/java/org/elasticsearch/common/util/Maps.java
+++ b/server/src/main/java/org/elasticsearch/common/util/Maps.java
@@ -252,9 +252,17 @@ private static Map<String, Object> flatten(List<Object> list, boolean ordered, S
      * @return a new pre-sized {@link HashMap}
      */
     public static <K, V> Map<K, V> newMapWithExpectedSize(int expectedSize) {
+        return newHashMapWithExpectedSize(expectedSize);
+    }
+
+    public static <K, V> Map<K, V> newHashMapWithExpectedSize(int expectedSize) {
         return new HashMap<>(capacity(expectedSize));
     }
 
+    public static <K, V> Map<K, V> newLinkedHashMapWithExpectedSize(int expectedSize) {
+        return new LinkedHashMap<>(capacity(expectedSize));
+    }
+
     static int capacity(int expectedSize) {
         assert expectedSize >= 0;
         return expectedSize < 2 ? expectedSize + 1 : (int) (expectedSize / 0.75 + 1.0);

From 2ab574a044f6141d2167e01a7efdec1304de679b Mon Sep 17 00:00:00 2001
From: Ievgen Degtiarenko <ievgen.degtiarenko@gmail.com>
Date: Mon, 21 Feb 2022 08:07:21 +0100
Subject: [PATCH 3/3] Update docs/changelog/84045.yaml

---
 docs/changelog/84045.yaml | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 docs/changelog/84045.yaml

diff --git a/docs/changelog/84045.yaml b/docs/changelog/84045.yaml
new file mode 100644
index 0000000000000..b45380f264d3f
--- /dev/null
+++ b/docs/changelog/84045.yaml
@@ -0,0 +1,5 @@
+pr: 84045
+summary: Update `readMap` to avoid resizing map during reading
+area: Infra/Core
+type: enhancement
+issues: []