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/io/papermc/paper/command/PaperCommand.java

@@ -38,6 +38,7 @@ public final class PaperCommand extends Command {
         commands.put(Set.of("reload"), new ReloadCommand());
         commands.put(Set.of("version"), new VersionCommand());
         commands.put(Set.of("dumpplugins"), new DumpPluginsCommand());
+        commands.put(Set.of("syncloadinfo"), new SyncLoadInfoCommand());
 
         return commands.entrySet().stream()
             .flatMap(entry -> entry.getKey().stream().map(s -> Map.entry(s, entry.getValue())))

paper-server/src/main/java/io/papermc/paper/command/subcommands/SyncLoadInfoCommand.java

@@ -0,0 +1,88 @@
+package io.papermc.paper.command.subcommands;
+
+import com.destroystokyo.paper.io.SyncLoadFinder;
+import com.google.gson.JsonObject;
+import com.google.gson.internal.Streams;
+import com.google.gson.stream.JsonWriter;
+import io.papermc.paper.command.CommandUtil;
+import io.papermc.paper.command.PaperSubcommand;
+import java.io.File;
+import java.io.FileOutputStream;
+import java.io.PrintStream;
+import java.io.StringWriter;
+import java.nio.charset.StandardCharsets;
+import java.time.LocalDateTime;
+import java.time.format.DateTimeFormatter;
+import java.util.List;
+
+import net.kyori.adventure.text.event.ClickEvent;
+import net.kyori.adventure.text.event.HoverEvent;
+import net.minecraft.server.MinecraftServer;
+import org.bukkit.command.CommandSender;
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.framework.qual.DefaultQualifier;
+
+import static net.kyori.adventure.text.Component.text;
+import static net.kyori.adventure.text.format.NamedTextColor.GRAY;
+import static net.kyori.adventure.text.format.NamedTextColor.GREEN;
+import static net.kyori.adventure.text.format.NamedTextColor.RED;
+import static net.kyori.adventure.text.format.NamedTextColor.WHITE;
+
+@DefaultQualifier(NonNull.class)
+public final class SyncLoadInfoCommand implements PaperSubcommand {
+    @Override
+    public boolean execute(final CommandSender sender, final String subCommand, final String[] args) {
+        this.doSyncLoadInfo(sender, args);
+        return true;
+    }
+
+    @Override
+    public List<String> tabComplete(final CommandSender sender, final String subCommand, final String[] args) {
+        return CommandUtil.getListMatchingLast(sender, args, "clear");
+    }
+
+    private static final DateTimeFormatter FORMATTER = DateTimeFormatter.ofPattern("yyyy-MM-dd_HH.mm.ss");
+
+    private void doSyncLoadInfo(final CommandSender sender, final String[] args) {
+        if (!SyncLoadFinder.ENABLED) {
+            String systemFlag = "-Dpaper.debug-sync-loads=true";
+            sender.sendMessage(text().color(RED).append(text("This command requires the server startup flag '")).append(
+                text(systemFlag, WHITE).clickEvent(ClickEvent.copyToClipboard(systemFlag))
+                                       .hoverEvent(HoverEvent.showText(text("Click to copy the system flag")))).append(
+                text("' to be set.")));
+            return;
+        }
+
+        if (args.length > 0 && args[0].equals("clear")) {
+            SyncLoadFinder.clear();
+            sender.sendMessage(text("Sync load data cleared.", GRAY));
+            return;
+        }
+
+        File file = new File(new File(new File("."), "debug"),
+            "sync-load-info-" + FORMATTER.format(LocalDateTime.now()) + ".txt");
+        file.getParentFile().mkdirs();
+        sender.sendMessage(text("Writing sync load info to " + file, GREEN));
+
+
+        try {
+            final JsonObject data = SyncLoadFinder.serialize();
+
+            StringWriter stringWriter = new StringWriter();
+            JsonWriter jsonWriter = new JsonWriter(stringWriter);
+            jsonWriter.setIndent(" ");
+            jsonWriter.setLenient(false);
+            Streams.write(data, jsonWriter);
+
+            try (
+                PrintStream out = new PrintStream(new FileOutputStream(file), false, StandardCharsets.UTF_8)
+            ) {
+                out.print(stringWriter);
+            }
+            sender.sendMessage(text("Successfully written sync load information!", GREEN));
+        } catch (Throwable thr) {
+            sender.sendMessage(text("Failed to write sync load information! See the console for more info.", RED));
+            MinecraftServer.LOGGER.warn("Error occurred while dumping sync chunk load info", thr);
+        }
+    }
+}