PaperMC/Spigot-Server-Patches/0390-Elide-lock-in-DataWatcher.patch

From 51fd7997bca218ef386063113130b7108f779682 Mon Sep 17 00:00:00 2001
From: Spottedleaf <Spottedleaf@users.noreply.github.com>
Date: Sat, 11 May 2019 08:19:27 -0700
Subject: [PATCH] Elide lock in DataWatcher

The lock in DataWatcher is used to prevent concurrent modifications
to the 'd' field (entries in MCP). However any modifications to
this map only occur on initialization of an Entity in its
constructor. This modification is write-locked.

Every other access is through a readlock, which allows
the threads to pass if there is no thread holding the
writelock.

Since the writelock is only obtained in the constructor
of the Entity, the further readlocks are actually
useless (which get obtained on set, get, etc calls).

diff --git a/src/main/java/net/minecraft/server/DataWatcher.java b/src/main/java/net/minecraft/server/DataWatcher.java
index f224043d8e..bbea8ef726 100644
--- a/src/main/java/net/minecraft/server/DataWatcher.java
+++ b/src/main/java/net/minecraft/server/DataWatcher.java
@@ -23,7 +23,7 @@ public class DataWatcher {
     private static final Map<Class<? extends Entity>, Integer> b = Maps.newHashMap();
     private final Entity c;
     private final Int2ObjectOpenHashMap<DataWatcher.Item<?>> d = new Int2ObjectOpenHashMap<>(); // Paper
-    private final ReadWriteLock e = new ReentrantReadWriteLock();
+    //private final ReadWriteLock e = new ReentrantReadWriteLock(); // Paper - not required
     private boolean f = true;
     private boolean g;
 
@@ -88,30 +88,14 @@ public class DataWatcher {
     private <T> void registerObject(DataWatcherObject<T> datawatcherobject, T t0) {
         DataWatcher.Item<T> datawatcher_item = new DataWatcher.Item<>(datawatcherobject, t0);
 
-        this.e.writeLock().lock();
+        //this.e.writeLock().lock(); // Paper - not required
         this.d.put(datawatcherobject.a(), datawatcher_item);
         this.f = false;
-        this.e.writeLock().unlock();
+        //this.e.writeLock().unlock(); // Paper - not required
     }
 
     private <T> DataWatcher.Item<T> b(DataWatcherObject<T> datawatcherobject) {
-        this.e.readLock().lock();
-
-        DataWatcher.Item datawatcher_item;
-
-        try {
-            datawatcher_item = (DataWatcher.Item) this.d.get(datawatcherobject.a());
-        } catch (Throwable throwable) {
-            CrashReport crashreport = CrashReport.a(throwable, "Getting synched entity data");
-            CrashReportSystemDetails crashreportsystemdetails = crashreport.a("Synched entity data");
-
-            crashreportsystemdetails.a("Data ID", (Object) datawatcherobject);
-            throw new ReportedException(crashreport);
-        } finally {
-            this.e.readLock().unlock();
-        }
-
-        return datawatcher_item;
+        return (DataWatcher.Item<T>)this.d.get(datawatcherobject.a()); // Paper - avoid lock and try catch, get() does not fail
     }
 
     public <T> T get(DataWatcherObject<T> datawatcherobject) {
@@ -158,7 +142,7 @@ public class DataWatcher {
         List<DataWatcher.Item<?>> list = null;
 
         if (this.g) {
-            this.e.readLock().lock();
+            //this.e.readLock().lock(); // Paper - not required
             Iterator iterator = this.d.values().iterator();
 
             while (iterator.hasNext()) {
@@ -174,7 +158,7 @@ public class DataWatcher {
                 }
             }
 
-            this.e.readLock().unlock();
+            //this.e.readLock().unlock(); // Paper - not required
         }
 
         this.g = false;
@@ -182,7 +166,7 @@ public class DataWatcher {
     }
 
     public void a(PacketDataSerializer packetdataserializer) throws IOException {
-        this.e.readLock().lock();
+        //this.e.readLock().lock(); // Paper - not required
         Iterator iterator = this.d.values().iterator();
 
         while (iterator.hasNext()) {
@@ -191,7 +175,7 @@ public class DataWatcher {
             a(packetdataserializer, datawatcher_item);
         }
 
-        this.e.readLock().unlock();
+        //this.e.readLock().unlock(); // Paper - not required
         packetdataserializer.writeByte(255);
     }
 
@@ -199,7 +183,7 @@ public class DataWatcher {
     public List<DataWatcher.Item<?>> c() {
         List<DataWatcher.Item<?>> list = null;
 
-        this.e.readLock().lock();
+        //this.e.readLock().lock(); // Paper - not required
 
         DataWatcher.Item datawatcher_item;
 
@@ -210,7 +194,7 @@ public class DataWatcher {
             }
         }
 
-        this.e.readLock().unlock();
+        //this.e.readLock().unlock(); // Paper - not required
         return list;
     }
 
@@ -261,7 +245,7 @@ public class DataWatcher {
 
     public void e() {
         this.g = false;
-        this.e.readLock().lock();
+        //this.e.readLock().lock(); // Paper - not required
         Iterator iterator = this.d.values().iterator();
 
         while (iterator.hasNext()) {
@@ -270,7 +254,7 @@ public class DataWatcher {
             datawatcher_item.a(false);
         }
 
-        this.e.readLock().unlock();
+        //this.e.readLock().unlock(); // Paper - not required
     }
 
     public static class Item<T> {
-- 
2.21.0