Add debug for sync chunk loads

This patch adds a tool to find calls to getChunkAt which would load
chunks, however it must be enabled by setting the startup flag
-Dpaper.debug-sync-loads=true

- To get a debug log for sync loads, the command is
  /paper syncloadinfo
- To clear clear the currently stored sync load info, use
  /paper syncloadinfo clear

paper-server/src/main/java/com/destroystokyo/paper/io/SyncLoadFinder.java

@@ -0,0 +1,177 @@
+package com.destroystokyo.paper.io;
+
+import com.google.gson.JsonArray;
+import com.google.gson.JsonObject;
+import com.mojang.datafixers.util.Pair;
+import it.unimi.dsi.fastutil.longs.Long2IntMap;
+import it.unimi.dsi.fastutil.longs.Long2IntOpenHashMap;
+import it.unimi.dsi.fastutil.objects.Object2ObjectOpenHashMap;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.WeakHashMap;
+import net.minecraft.world.level.ChunkPos;
+import net.minecraft.world.level.Level;
+
+public class SyncLoadFinder {
+
+    public static final boolean ENABLED = Boolean.getBoolean("paper.debug-sync-loads");
+
+    private static final WeakHashMap<Level, Object2ObjectOpenHashMap<ThrowableWithEquals, SyncLoadInformation>> SYNC_LOADS = new WeakHashMap<>();
+
+    private static final class SyncLoadInformation {
+
+        public int times;
+
+        public final Long2IntOpenHashMap coordinateTimes = new Long2IntOpenHashMap();
+    }
+
+    public static void clear() {
+        SYNC_LOADS.clear();
+    }
+
+    public static void logSyncLoad(final Level world, final int chunkX, final int chunkZ) {
+        if (!ENABLED) {
+            return;
+        }
+
+        final ThrowableWithEquals stacktrace = new ThrowableWithEquals(Thread.currentThread().getStackTrace());
+
+        SYNC_LOADS.compute(world, (final Level keyInMap, Object2ObjectOpenHashMap<ThrowableWithEquals, SyncLoadInformation> map) -> {
+            if (map == null) {
+                map = new Object2ObjectOpenHashMap<>();
+            }
+
+            map.compute(stacktrace, (ThrowableWithEquals keyInMap0, SyncLoadInformation valueInMap) -> {
+                if (valueInMap == null) {
+                    valueInMap = new SyncLoadInformation();
+                }
+
+                ++valueInMap.times;
+
+                valueInMap.coordinateTimes.compute(ChunkPos.asLong(chunkX, chunkZ), (Long keyInMap1, Integer valueInMap1) -> {
+                    return valueInMap1 == null ? Integer.valueOf(1) : Integer.valueOf(valueInMap1.intValue() + 1);
+                });
+
+                return valueInMap;
+            });
+
+            return map;
+        });
+    }
+
+    public static JsonObject serialize() {
+        final JsonObject ret = new JsonObject();
+
+        final JsonArray worldsData = new JsonArray();
+
+        for (final Map.Entry<Level, Object2ObjectOpenHashMap<ThrowableWithEquals, SyncLoadInformation>> entry : SYNC_LOADS.entrySet()) {
+            final Level world = entry.getKey();
+
+            final JsonObject worldData = new JsonObject();
+
+            worldData.addProperty("name", world.getWorld().getName());
+
+            final List<Pair<ThrowableWithEquals, SyncLoadInformation>> data = new ArrayList<>();
+
+            entry.getValue().forEach((ThrowableWithEquals stacktrace, SyncLoadInformation times) -> {
+                data.add(new Pair<>(stacktrace, times));
+            });
+
+            data.sort((Pair<ThrowableWithEquals, SyncLoadInformation> pair1, Pair<ThrowableWithEquals, SyncLoadInformation> pair2) -> {
+                return Integer.compare(pair2.getSecond().times, pair1.getSecond().times); // reverse order
+            });
+
+            final JsonArray stacktraces = new JsonArray();
+
+            for (Pair<ThrowableWithEquals, SyncLoadInformation> pair : data) {
+                final JsonObject stacktrace = new JsonObject();
+
+                stacktrace.addProperty("times", pair.getSecond().times);
+
+                final JsonArray traces = new JsonArray();
+
+                for (StackTraceElement element : pair.getFirst().stacktrace) {
+                    traces.add(String.valueOf(element));
+                }
+
+                stacktrace.add("stacktrace", traces);
+
+                final JsonArray coordinates = new JsonArray();
+
+                for (Long2IntMap.Entry coordinate : pair.getSecond().coordinateTimes.long2IntEntrySet()) {
+                    final long key = coordinate.getLongKey();
+                    final int times = coordinate.getIntValue();
+                    final ChunkPos chunkPos = new ChunkPos(key);
+                    coordinates.add("(" + chunkPos.x + "," + chunkPos.z + "): " + times);
+                }
+
+                stacktrace.add("coordinates", coordinates);
+
+                stacktraces.add(stacktrace);
+            }
+
+
+            worldData.add("stacktraces", stacktraces);
+            worldsData.add(worldData);
+        }
+
+        ret.add("worlds", worldsData);
+
+        return ret;
+    }
+
+    static final class ThrowableWithEquals {
+
+        private final StackTraceElement[] stacktrace;
+        private final int hash;
+
+        public ThrowableWithEquals(final StackTraceElement[] stacktrace) {
+            this.stacktrace = stacktrace;
+            this.hash = ThrowableWithEquals.hash(stacktrace);
+        }
+
+        public static int hash(final StackTraceElement[] stacktrace) {
+            int hash = 0;
+
+            for (int i = 0; i < stacktrace.length; ++i) {
+                hash *= 31;
+                hash += stacktrace[i].hashCode();
+            }
+
+            return hash;
+        }
+
+        @Override
+        public int hashCode() {
+            return this.hash;
+        }
+
+        @Override
+        public boolean equals(final Object obj) {
+            if (obj == null || obj.getClass() != this.getClass()) {
+                return false;
+            }
+
+            final ThrowableWithEquals other = (ThrowableWithEquals)obj;
+            final StackTraceElement[] otherStackTrace = other.stacktrace;
+
+            if (this.stacktrace.length != otherStackTrace.length || this.hash != other.hash) {
+                return false;
+            }
+
+            if (this == obj) {
+                return true;
+            }
+
+            for (int i = 0; i < this.stacktrace.length; ++i) {
+                if (!this.stacktrace[i].equals(otherStackTrace[i])) {
+                    return false;
+                }
+            }
+
+            return true;
+        }
+    }
+}