API: Implement nvim_win_set_buf() #9100

closes #9100

src/nvim/api/window.c

@@ -10,6 +10,7 @@
 #include "nvim/api/private/defs.h"
 #include "nvim/api/private/helpers.h"
 #include "nvim/vim.h"
+#include "nvim/buffer.h"
 #include "nvim/cursor.h"
 #include "nvim/window.h"
 #include "nvim/screen.h"
@@ -33,6 +34,41 @@ Buffer nvim_win_get_buf(Window window, Error *err)
   return win->w_buffer->handle;
 }
 
+/// Sets the current buffer in a window, without side-effects
+///
+/// @param window   Window handle
+/// @param buffer   Buffer handle
+/// @param[out] err Error details, if any
+void nvim_win_set_buf(Window window, Buffer buffer, Error *err)
+  FUNC_API_SINCE(5)
+{
+  win_T *win = find_window_by_handle(window, err), *save_curwin = curwin;
+  buf_T *buf = find_buffer_by_handle(buffer, err);
+  tabpage_T *tab = win_find_tabpage(win), *save_curtab = curtab;
+
+  if (!win || !buf) {
+    return;
+  }
+
+  if (switch_win(&save_curwin, &save_curtab, win, tab, false) == FAIL) {
+    api_set_error(err,
+                  kErrorTypeException,
+                  "Failed to switch to window %d",
+                  window);
+  }
+
+  try_start();
+  int result = do_buffer(DOBUF_GOTO, DOBUF_FIRST, FORWARD, buf->b_fnum, 0);
+  if (!try_end(err) && result == FAIL) {
+    api_set_error(err,
+                  kErrorTypeException,
+                  "Failed to set buffer %d",
+                  buffer);
+  }
+
+  restore_win(save_curwin, save_curtab, false);
+}
+
 /// Gets the cursor position in the window
 ///
 /// @param window   Window handle

test/functional/api/window_spec.lua

@@ -11,6 +11,7 @@ local NIL = helpers.NIL
 local meth_pcall = helpers.meth_pcall
 local meths = helpers.meths
 local command = helpers.command
+local expect_err = helpers.expect_err
 
 -- check if str is visible at the beginning of some line
 local function is_visible(str)
@@ -31,7 +32,7 @@ local function is_visible(str)
     return false
 end
 
-describe('api/win', function()
+describe('API/win', function()
   before_each(clear)
 
   describe('get_buf', function()
@@ -45,6 +46,21 @@ describe('api/win', function()
     end)
   end)
 
+  describe('set_buf', function()
+    it('works', function()
+      nvim('command', 'new')
+      local windows = nvim('list_wins')
+      neq(window('get_buf', windows[2]), window('get_buf', windows[1]))
+      window('set_buf', windows[2], window('get_buf', windows[1]))
+      eq(window('get_buf', windows[2]), window('get_buf', windows[1]))
+    end)
+
+    it('validates args', function()
+      expect_err('Invalid buffer id$', window, 'set_buf', nvim('get_current_win'), 23)
+      expect_err('Invalid window id$', window, 'set_buf', 23, nvim('get_current_buf'))
+    end)
+  end)
+
   describe('{get,set}_cursor', function()
     it('works', function()
       eq({1, 0}, curwin('get_cursor'))