fix(api): auto-load buffers in nvim_buf_set_* operations (#35046)

fix(api): auto-load buffers in nvim_buf_set_* operations

Problem: Setting marks, lines, or text on unloaded buffers fails because
ml_line_count is not accurate until the buffer is loaded.

Solution: Add require_loaded_buffer() helper and use it in write operations
(nvim_buf_set_lines, nvim_buf_set_text, nvim_buf_set_mark). These now
auto-load buffers as needed.

1 files changed, 16 insertions, 0 deletions

diff --git a/test/functional/api/buffer_spec.lua b/test/functional/api/buffer_spec.lua
index 6282bbccd8..a167e7ad11 100644
--- a/test/functional/api/buffer_spec.lua
+++ b/test/functional/api/buffer_spec.lua
@@ -863,6 +863,7 @@ describe('api/buf', function()
       local new_bufnr = fn.bufnr('set_lines', true)
       lua_or_rpc.nvim_buf_set_lines(new_bufnr, 0, -1, false, {})
       eq({ '' }, lua_or_rpc.nvim_buf_get_lines(new_bufnr, 0, -1, false))
+      eq(true, api.nvim_buf_is_loaded(new_bufnr))
     end)
   end)
 
@@ -1809,6 +1810,14 @@ describe('api/buf', function()
       eq({ 'one', 'two' }, get_lines(0, 2, true))
     end)
 
+    it('auto-loads unloaded buffer', function()
+      local new_bufnr = fn.bufnr('set_text', true)
+      eq(false, api.nvim_buf_is_loaded(new_bufnr))
+      api.nvim_buf_set_text(new_bufnr, 0, 0, 0, -1, { 'foo' })
+      eq(true, api.nvim_buf_is_loaded(new_bufnr))
+      eq({ 'foo' }, api.nvim_buf_get_lines(new_bufnr, 0, -1, false))
+    end)
+
     describe('handles topline', function()
       local screen
       before_each(function()
@@ -2375,6 +2384,13 @@ describe('api/buf', function()
     it('fails when invalid buffer number is used', function()
       eq(false, pcall(api.nvim_buf_set_mark, 99, 'a', 1, 1, {}))
     end)
+    it('auto-loads unloaded buffer', function()
+      local new_bufnr = fn.bufnr('set_mark', true)
+      eq(false, api.nvim_buf_is_loaded(new_bufnr))
+      eq(true, api.nvim_buf_set_mark(new_bufnr, 'A', 0, 0, {}))
+      eq(true, api.nvim_buf_is_loaded(new_bufnr))
+      eq({ 0, 0 }, api.nvim_buf_get_mark(new_bufnr, 'A'))
+    end)
   end)
 
   describe('nvim_buf_del_mark', function()