--- a/net/minecraft/server/players/GameProfileCache.java
+++ b/net/minecraft/server/players/GameProfileCache.java
@@ -60,6 +60,11 @@
     @Nullable
     private Executor executor;
 
+    // Paper start - Fix GameProfileCache concurrency
+    protected final java.util.concurrent.locks.ReentrantLock stateLock = new java.util.concurrent.locks.ReentrantLock();
+    protected final java.util.concurrent.locks.ReentrantLock lookupLock = new java.util.concurrent.locks.ReentrantLock();
+    // Paper end - Fix GameProfileCache concurrency
+
     public GameProfileCache(GameProfileRepository profileRepository, File cacheFile) {
         this.profileRepository = profileRepository;
         this.file = cacheFile;
@@ -67,11 +72,13 @@
     }
 
     private void safeAdd(GameProfileCache.GameProfileInfo entry) {
+        try { this.stateLock.lock(); // Paper - Fix GameProfileCache concurrency
         GameProfile gameprofile = entry.getProfile();
 
         entry.setLastAccess(this.getNextOperation());
         this.profilesByName.put(gameprofile.getName().toLowerCase(Locale.ROOT), entry);
         this.profilesByUUID.put(gameprofile.getId(), entry);
+        } finally { this.stateLock.unlock(); } // Paper - Fix GameProfileCache concurrency
     }
 
     private static Optional<GameProfile> lookupGameProfile(GameProfileRepository repository, String name) {
@@ -85,10 +92,12 @@
                 }
 
                 public void onProfileLookupFailed(String s1, Exception exception) {
-                    atomicreference.set((Object) null);
+                    atomicreference.set(null); // CraftBukkit - decompile error
                 }
             };
 
+        if (!org.apache.commons.lang3.StringUtils.isBlank(name) // Paper - Don't lookup a profile with a blank name
+                && io.papermc.paper.configuration.GlobalConfiguration.get().proxies.isProxyOnlineMode()) // Paper - Add setting for proxy online mode status
             repository.findProfilesByNames(new String[]{name}, profilelookupcallback);
             GameProfile gameprofile = (GameProfile) atomicreference.get();
 
@@ -105,7 +114,7 @@
     }
 
     private static boolean usesAuthentication() {
-        return GameProfileCache.usesAuthentication;
+        return io.papermc.paper.configuration.GlobalConfiguration.get().proxies.isProxyOnlineMode(); // Paper - Add setting for proxy online mode status
     }
 
     public void add(GameProfile profile) {
@@ -117,15 +126,29 @@
         GameProfileCache.GameProfileInfo usercache_usercacheentry = new GameProfileCache.GameProfileInfo(profile, date);
 
         this.safeAdd(usercache_usercacheentry);
-        this.save();
+        if( !org.spigotmc.SpigotConfig.saveUserCacheOnStopOnly ) this.save(true); // Spigot - skip saving if disabled // Paper - Perf: Async GameProfileCache saving
     }
 
     private long getNextOperation() {
         return this.operationCount.incrementAndGet();
     }
 
+    // Paper start
+    public @Nullable GameProfile getProfileIfCached(String name) {
+        try { this.stateLock.lock(); // Paper - Fix GameProfileCache concurrency
+        GameProfileCache.GameProfileInfo entry = this.profilesByName.get(name.toLowerCase(Locale.ROOT));
+        if (entry == null) {
+            return null;
+        }
+        entry.setLastAccess(this.getNextOperation());
+        return entry.getProfile();
+        } finally { this.stateLock.unlock(); } // Paper - Fix GameProfileCache concurrency
+    }
+    // Paper end
+
     public Optional<GameProfile> get(String name) {
         String s1 = name.toLowerCase(Locale.ROOT);
+        boolean stateLocked = true; try { this.stateLock.lock(); // Paper - Fix GameProfileCache concurrency
         GameProfileCache.GameProfileInfo usercache_usercacheentry = (GameProfileCache.GameProfileInfo) this.profilesByName.get(s1);
         boolean flag = false;
 
@@ -141,19 +164,24 @@
         if (usercache_usercacheentry != null) {
             usercache_usercacheentry.setLastAccess(this.getNextOperation());
             optional = Optional.of(usercache_usercacheentry.getProfile());
+            stateLocked = false; this.stateLock.unlock(); // Paper - Fix GameProfileCache concurrency
         } else {
-            optional = GameProfileCache.lookupGameProfile(this.profileRepository, s1);
+            stateLocked = false; this.stateLock.unlock(); // Paper - Fix GameProfileCache concurrency
+            try { this.lookupLock.lock(); // Paper - Fix GameProfileCache concurrency
+            optional = GameProfileCache.lookupGameProfile(this.profileRepository, name); // CraftBukkit - use correct case for offline players
+            } finally { this.lookupLock.unlock(); } // Paper - Fix GameProfileCache concurrency
             if (optional.isPresent()) {
                 this.add((GameProfile) optional.get());
                 flag = false;
             }
         }
 
-        if (flag) {
-            this.save();
+        if (flag && !org.spigotmc.SpigotConfig.saveUserCacheOnStopOnly) { // Spigot - skip saving if disabled
+            this.save(true); // Paper - Perf: Async GameProfileCache saving
         }
 
         return optional;
+        } finally { if (stateLocked) {  this.stateLock.unlock(); } } // Paper - Fix GameProfileCache concurrency
     }
 
     public CompletableFuture<Optional<GameProfile>> getAsync(String username) {
@@ -167,7 +195,7 @@
             } else {
                 CompletableFuture<Optional<GameProfile>> completablefuture1 = CompletableFuture.supplyAsync(() -> {
                     return this.get(username);
-                }, Util.backgroundExecutor().forName("getProfile")).whenCompleteAsync((optional, throwable) -> {
+                }, Util.PROFILE_EXECUTOR).whenCompleteAsync((optional, throwable) -> { // Paper - don't submit BLOCKING PROFILE LOOKUPS to the world gen thread
                     this.requests.remove(username);
                 }, this.executor);
 
@@ -178,6 +206,7 @@
     }
 
     public Optional<GameProfile> get(UUID uuid) {
+        try { this.stateLock.lock(); // Paper - Fix GameProfileCache concurrency
         GameProfileCache.GameProfileInfo usercache_usercacheentry = (GameProfileCache.GameProfileInfo) this.profilesByUUID.get(uuid);
 
         if (usercache_usercacheentry == null) {
@@ -186,6 +215,7 @@
             usercache_usercacheentry.setLastAccess(this.getNextOperation());
             return Optional.of(usercache_usercacheentry.getProfile());
         }
+        } finally { this.stateLock.unlock(); } // Paper - Fix GameProfileCache concurrency
     }
 
     public void setExecutor(Executor executor) {
@@ -208,7 +238,7 @@
 
             label54:
             {
-                ArrayList arraylist;
+                List<GameProfileCache.GameProfileInfo> arraylist; // CraftBukkit - decompile error
 
                 try {
                     JsonArray jsonarray = (JsonArray) this.gson.fromJson(bufferedreader, JsonArray.class);
@@ -217,7 +247,7 @@
                         DateFormat dateformat = GameProfileCache.createDateFormat();
 
                         jsonarray.forEach((jsonelement) -> {
-                            Optional optional = GameProfileCache.readGameProfile(jsonelement, dateformat);
+                            Optional<GameProfileCache.GameProfileInfo> optional = GameProfileCache.readGameProfile(jsonelement, dateformat); // CraftBukkit - decompile error
 
                             Objects.requireNonNull(list);
                             optional.ifPresent(list::add);
@@ -250,6 +280,11 @@
             }
         } catch (FileNotFoundException filenotfoundexception) {
             ;
+        // Spigot Start
+        } catch (com.google.gson.JsonSyntaxException | NullPointerException ex) {
+            GameProfileCache.LOGGER.warn( "Usercache.json is corrupted or has bad formatting. Deleting it to prevent further issues." );
+            this.file.delete();
+        // Spigot End
         } catch (JsonParseException | IOException ioexception) {
             GameProfileCache.LOGGER.warn("Failed to load profile cache {}", this.file, ioexception);
         }
@@ -257,14 +292,15 @@
         return list;
     }
 
-    public void save() {
+    public void save(boolean asyncSave) { // Paper - Perf: Async GameProfileCache saving
         JsonArray jsonarray = new JsonArray();
         DateFormat dateformat = GameProfileCache.createDateFormat();
 
-        this.getTopMRUProfiles(1000).forEach((usercache_usercacheentry) -> {
+        this.listTopMRUProfiles(org.spigotmc.SpigotConfig.userCacheCap).forEach((usercache_usercacheentry) -> { // Spigot // Paper - Fix GameProfileCache concurrency
             jsonarray.add(GameProfileCache.writeGameProfile(usercache_usercacheentry, dateformat));
         });
         String s = this.gson.toJson(jsonarray);
+        Runnable save = () -> { // Paper - Perf: Async GameProfileCache saving
 
         try {
             BufferedWriter bufferedwriter = Files.newWriter(this.file, StandardCharsets.UTF_8);
@@ -289,13 +325,32 @@
         } catch (IOException ioexception) {
             ;
         }
+        // Paper start - Perf: Async GameProfileCache saving
+        };
+        if (asyncSave) {
+            io.papermc.paper.util.MCUtil.scheduleAsyncTask(save);
+        } else {
+            save.run();
+        }
+        // Paper end - Perf: Async GameProfileCache saving
 
     }
 
     private Stream<GameProfileCache.GameProfileInfo> getTopMRUProfiles(int limit) {
-        return ImmutableList.copyOf(this.profilesByUUID.values()).stream().sorted(Comparator.comparing(GameProfileCache.GameProfileInfo::getLastAccess).reversed()).limit((long) limit);
+        // Paper start - Fix GameProfileCache concurrency
+        return this.listTopMRUProfiles(limit).stream();
     }
 
+    private List<GameProfileCache.GameProfileInfo> listTopMRUProfiles(int limit) {
+        try {
+            this.stateLock.lock();
+            return this.profilesByUUID.values().stream().sorted(Comparator.comparing(GameProfileCache.GameProfileInfo::getLastAccess).reversed()).limit(limit).toList();
+        } finally {
+            this.stateLock.unlock();
+        }
+    }
+    // Paper end - Fix GameProfileCache concurrency
+
     private static JsonElement writeGameProfile(GameProfileCache.GameProfileInfo entry, DateFormat dateFormat) {
         JsonObject jsonobject = new JsonObject();