fix(buffer_updates): save and restore current window cursor (#16732)

When a buffer update callback is called, textlock is active so buffer
text cannot be changed, but cursor can still be moved. This can cause
problems when the buffer update is in the middle of an operator, like
the one mentioned in #16729. The solution is to save cursor position and
restore it afterwards, like how cursor is saved and restored when
evaluating an <expr> mapping.

4 files changed, 40 insertions, 15 deletions

diff --git a/runtime/doc/api.txt b/runtime/doc/api.txt
index 3ca50dda15..2930f2314b 100644
--- a/runtime/doc/api.txt
+++ b/runtime/doc/api.txt
@@ -356,6 +356,7 @@ In-process Lua plugins can receive buffer updates in the form of Lua
 callbacks. These callbacks are called frequently in various contexts;
 |textlock| prevents changing buffer contents and window layout (use
 |vim.schedule()| to defer such operations to the main loop instead).
+Moving the cursor is allowed, but it is restored afterwards.
 
 |nvim_buf_attach()| will take keyword args for the callbacks. "on_lines" will
 receive parameters ("lines", {buf}, {changedtick}, {firstline}, {lastline},
diff --git a/src/nvim/api/private/helpers.h b/src/nvim/api/private/helpers.h
index b70452d7cb..1b82aeac34 100644
--- a/src/nvim/api/private/helpers.h
+++ b/src/nvim/api/private/helpers.h
@@ -155,8 +155,18 @@ typedef struct {
     msglist_T *private_msg_list; \
     msg_list = &private_msg_list; \
     private_msg_list = NULL; \
-    code \
-      msg_list = saved_msg_list;  /* Restore the exception context. */ \
+    code; \
+    msg_list = saved_msg_list;  /* Restore the exception context. */ \
+  } while (0)
+
+// Execute code with cursor position saved and restored and textlock active.
+#define TEXTLOCK_WRAP(code) \
+  do { \
+    const pos_T save_cursor = curwin->w_cursor; \
+    textlock++; \
+    code; \
+    textlock--; \
+    curwin->w_cursor = save_cursor; \
   } while (0)
 
 // Useful macro for executing some `code` for each item in an array.
diff --git a/src/nvim/buffer_updates.c b/src/nvim/buffer_updates.c
index 075ac2adbf..9543731c9b 100644
--- a/src/nvim/buffer_updates.c
+++ b/src/nvim/buffer_updates.c
@@ -188,9 +188,9 @@ void buf_updates_unload(buf_T *buf, bool can_reload)
       // the first argument is always the buffer handle
       args.items[0] = BUFFER_OBJ(buf->handle);
 
-      textlock++;
-      nlua_call_ref(thecb, keep ? "reload" : "detach", args, false, NULL);
-      textlock--;
+      TEXTLOCK_WRAP({
+        nlua_call_ref(thecb, keep ? "reload" : "detach", args, false, NULL);
+      });
     }
 
     if (keep) {
@@ -305,9 +305,11 @@ void buf_updates_send_changes(buf_T *buf, linenr_T firstline, int64_t num_added,
         args.items[6] = INTEGER_OBJ((Integer)deleted_codepoints);
         args.items[7] = INTEGER_OBJ((Integer)deleted_codeunits);
       }
-      textlock++;
-      Object res = nlua_call_ref(cb.on_lines, "lines", args, false, NULL);
-      textlock--;
+
+      Object res;
+      TEXTLOCK_WRAP({
+        res = nlua_call_ref(cb.on_lines, "lines", args, false, NULL);
+      });
 
       if (res.type == kObjectTypeBoolean && res.data.boolean == true) {
         buffer_update_callbacks_free(cb);
@@ -354,9 +356,10 @@ void buf_updates_send_splice(buf_T *buf, int start_row, colnr_T start_col, bcoun
       ADD_C(args, INTEGER_OBJ(new_col));
       ADD_C(args, INTEGER_OBJ(new_byte));
 
-      textlock++;
-      Object res = nlua_call_ref(cb.on_bytes, "bytes", args, false, NULL);
-      textlock--;
+      Object res;
+      TEXTLOCK_WRAP({
+        res = nlua_call_ref(cb.on_bytes, "bytes", args, false, NULL);
+      });
 
       if (res.type == kObjectTypeBoolean && res.data.boolean == true) {
         buffer_update_callbacks_free(cb);
@@ -389,10 +392,10 @@ void buf_updates_changedtick(buf_T *buf)
       // next argument is b:changedtick
       ADD_C(args, INTEGER_OBJ(buf_get_changedtick(buf)));
 
-      textlock++;
-      Object res = nlua_call_ref(cb.on_changedtick, "changedtick",
-                                 args, false, NULL);
-      textlock--;
+      Object res;
+      TEXTLOCK_WRAP({
+        res = nlua_call_ref(cb.on_changedtick, "changedtick", args, false, NULL);
+      });
 
       if (res.type == kObjectTypeBoolean && res.data.boolean == true) {
         buffer_update_callbacks_free(cb);
diff --git a/test/functional/lua/buffer_updates_spec.lua b/test/functional/lua/buffer_updates_spec.lua
index 2fd44b8b5f..b1b39501f7 100644
--- a/test/functional/lua/buffer_updates_spec.lua
+++ b/test/functional/lua/buffer_updates_spec.lua
@@ -317,7 +317,18 @@ describe('lua buffer event callbacks: on_lines', function()
     feed('1G0')
     feed('P')
     eq(meths.get_var('linesev'), { "lines", 1, 6, 0, 3, 3, 9 })
+  end)
 
+  it('calling nvim_buf_call() from callback does not cause Normal mode CTRL-A to misbehave #16729', function()
+    exec_lua([[
+      vim.api.nvim_buf_attach(0, false, {
+        on_lines = function(...)
+          vim.api.nvim_buf_call(0, function() end)
+        end,
+      })
+    ]])
+    feed('itest123<Esc><C-A>')
+    eq('test124', meths.get_current_line())
   end)
 end)