[hle/kernel] Fix OutputDebugString formatting (#3744)

Fixes the debug outputs by buffering it, should not effect any normal game as the flush thread only get's created once the function has actually been called.

Turning it from:
```
[   9.707449] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString: SDK Assertion Failure: 'hipc::ResultSessionClosed::Includes(result)' in CloseClientSessionHandleSafely() at pid=81, tid=76(Main
[   9.707642] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString: Thread)

[   9.707791] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString: [SF-Internal]
[   9.707827] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:

[   9.708202] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString: Stack trace:

[   9.708880] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:   0x00000000866B5170 (unknown)

[   9.708911] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:   0x00000000866B42BC (unknown)

[   9.708916] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:   0x00000000866B4AEC (unknown)

[   9.708919] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:   0x00000000866B2F20 (unknown)

[   9.708922] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:   0x00000000866B19BC (unknown)

[   9.708925] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:   0x0000000086875C64 (unknown)

[   9.708927] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:   0x00000000868752E0 (unknown)

[   9.708929] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:   0x0000000086874C1C (unknown)

[   9.708932] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:   0x00000000866FBD1C (unknown)

[   9.708944] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:   0x00000000866FBDCC (unknown)

[   9.708967] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:

[   9.709035] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString: Related modules:

[   9.709061] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:   base               size               name/path

[   9.709349] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:   0x000000008665D000 0x0000000000E50000 nnSdkEn

[   9.709431] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:17:OutputDebugString:

```
Into:
```
[   5.895512] Debug.Emulated <Info> core\hle\kernel\svc\svc_debug_string.cpp:47:FlushDbgLoop:
SDK Assertion Failure: 'hipc::ResultSessionClosed::Includes(result)' in CloseClientSessionHandleSafely() at pid=81, tid=76(MainThread)
[SF-Internal]
Stack trace:
  0x00000000866B5170 (unknown)
  0x00000000866B42BC (unknown)
  0x00000000866B4AEC (unknown)
  0x00000000866B2F20 (unknown)
  0x00000000866B19BC (unknown)
  0x0000000086875C64 (unknown)
  0x00000000868752E0 (unknown)
  0x0000000086874C1C (unknown)
  0x00000000866FBD1C (unknown)
  0x00000000866FBDCC (unknown)

Related modules:
  base               size               name/path
  0x000000008665D000 0x0000000000E50000 nnSdkEn

```

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3744
Reviewed-by: crueter <crueter@eden-emu.dev>
Co-authored-by: smiRaphi <neogt404@gmail.com>
Co-committed-by: smiRaphi <neogt404@gmail.com>

1 files changed, 70 insertions, 3 deletions

diff --git a/src/core/hle/kernel/svc/svc_debug_string.cpp b/src/core/hle/kernel/svc/svc_debug_string.cpp
index 00b65429be..92cc334a81 100644
--- a/src/core/hle/kernel/svc/svc_debug_string.cpp
+++ b/src/core/hle/kernel/svc/svc_debug_string.cpp
@@ -1,6 +1,13 @@
+// SPDX-FileCopyrightText: Copyright 2026 Eden Emulator Project
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 // SPDX-FileCopyrightText: Copyright 2023 yuzu Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
+#include <condition_variable>
+#include <mutex>
+#include <thread>
+
 #include "core/core.h"
 #include "core/hle/kernel/k_thread.h"
 #include "core/hle/kernel/svc.h"
@@ -8,14 +15,62 @@
 
 namespace Kernel::Svc {
 
+constexpr auto MAX_MSG_TIME = std::chrono::milliseconds(250);
+const auto MAX_MSG_SIZE = 0x1000;
+
+static std::string msg_buffer;
+static std::mutex msg_mutex;
+static std::condition_variable msg_cv;
+static std::chrono::steady_clock::time_point last_msg_time;
+static bool worker_running = true;
+static std::unique_ptr<std::thread> flush_thread;
+static std::once_flag start_flag;
+
+static void FlushDbgLoop() {
+    while (true) {
+        std::unique_lock lock(msg_mutex);
+
+        msg_cv.wait(lock, [] { return !msg_buffer.empty() || !worker_running; });
+        if (!worker_running && msg_buffer.empty()) break;
+
+        auto timeout = last_msg_time + MAX_MSG_TIME;
+        bool woke_early = msg_cv.wait_until(lock, timeout, [] {
+            return msg_buffer.size() >= MAX_MSG_SIZE || !worker_running;
+        });
+
+        if (!woke_early || msg_buffer.size() >= MAX_MSG_SIZE || !worker_running) {
+            if (!msg_buffer.empty()) {
+                // Remove trailing newline as LOG_INFO adds that anyways
+                if (msg_buffer.back() == '\n')
+                    msg_buffer.pop_back();
+
+                LOG_INFO(Debug_Emulated, "\n{}", msg_buffer);
+                msg_buffer.clear();
+            }
+            if (!worker_running) break;
+        }
+    }
+}
+
 /// Used to output a message on a debug hardware unit - does nothing on a retail unit
 Result OutputDebugString(Core::System& system, u64 address, u64 len) {
     R_SUCCEED_IF(len == 0);
 
-    std::string str(len, '\0');
-    GetCurrentMemory(system.Kernel()).ReadBlock(address, str.data(), str.size());
-    LOG_INFO(Debug_Emulated, "{}", str);
+    // Only start the thread the very first time this function is called
+    std::call_once(start_flag, [] {
+        flush_thread = std::make_unique<std::thread>(FlushDbgLoop);
+    });
 
+    {
+        std::lock_guard lock(msg_mutex);
+        const auto old_size = msg_buffer.size();
+        msg_buffer.resize(old_size + len);
+        GetCurrentMemory(system.Kernel()).ReadBlock(address, msg_buffer.data() + old_size, len);
+
+        last_msg_time = std::chrono::steady_clock::now();
+    }
+
+    msg_cv.notify_one();
     R_SUCCEED();
 }
 
@@ -27,4 +82,16 @@ Result OutputDebugString64From32(Core::System& system, uint32_t debug_str, uint3
     R_RETURN(OutputDebugString(system, debug_str, len));
 }
 
+struct BufferAutoFlush {
+    ~BufferAutoFlush() {
+        {
+            std::lock_guard lock(msg_mutex);
+            worker_running = false;
+        }
+        msg_cv.notify_all();
+        if (flush_thread && flush_thread->joinable()) flush_thread->join();
+    }
+};
+static BufferAutoFlush auto_flusher;
+
 } // namespace Kernel::Svc