From 721eaf37603d3e66c274172428fb012950cb78c2 Mon Sep 17 00:00:00 2001
From: ame <amelia@amyy.cc>
Date: Mon, 2 Feb 2026 03:40:54 -0600
Subject: buffer return prev on :set

---
 docs/thread/buffer.md       | 1 +
 library/lullaby/thread.lua  | 2 +-
 src/thread.c                | 4 +++-
 tests/units/buffer-swap.lua | 8 ++++++++
 4 files changed, 13 insertions(+), 2 deletions(-)
 create mode 100644 tests/units/buffer-swap.lua

diff --git a/docs/thread/buffer.md b/docs/thread/buffer.md
index 65af0c0..fbd6678 100644
--- a/docs/thread/buffer.md
+++ b/docs/thread/buffer.md
@@ -36,6 +36,7 @@ copies the value in the buffer to the current state
 buffer:set(V)
 
 sets the value in the buffer
+returns the old value of the buffer
 
 ### buffer:mod
 
diff --git a/library/lullaby/thread.lua b/library/lullaby/thread.lua
index 43e897d..fc29e90 100644
--- a/library/lullaby/thread.lua
+++ b/library/lullaby/thread.lua
@@ -65,7 +65,7 @@ function buffer.get(T) end
 ---sets the value of the buffer
 ---@param T buffer-table
 ---@param value any
----@return nil
+---@return any prev previous table value
 function buffer.set(T, value) end
 
 ---calls a function with a parameter that is the value of the buffer, return the new value of the buffer
diff --git a/src/thread.c b/src/thread.c
index 555a62b..1e58aef 100644
--- a/src/thread.c
+++ b/src/thread.c
@@ -392,12 +392,14 @@ int _buffer_get(lua_State* L){
   return 1;
 }
 
-#warning "how do you handle gc for the new object?"
 int _buffer_set(lua_State* L){
   struct thread_buffer *buffer = lua_touserdata(L, 1);
   pthread_mutex_lock(&*buffer->lock);
+  luaI_deepcopy(buffer->L, L, SKIP_LOCALS | STRIP_GC);
   lua_settop(buffer->L, 0); 
+  lua_pushvalue(L, -2);
   luaI_deepcopy(L, buffer->L, SKIP_LOCALS | STRIP_GC);
+  lua_pop(L, 1);
   pthread_mutex_unlock(&*buffer->lock);
 
   return 1;
diff --git a/tests/units/buffer-swap.lua b/tests/units/buffer-swap.lua
new file mode 100644
index 0000000..50b5b7c
--- /dev/null
+++ b/tests/units/buffer-swap.lua
@@ -0,0 +1,8 @@
+local a = 539
+local b = "meow meow mrrp!"
+
+local buffer = llby.thread.buffer(a)
+
+local c = buffer:set(b)
+
+return c == a and b == buffer:get()
-- 
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