fix(api): load buffer first on nvim_buf_set_lines (#25823)

Fix #22670
Fix #8659

2 files changed, 31 insertions, 8 deletions

diff --git a/src/nvim/api/buffer.c b/src/nvim/api/buffer.c
index 64dddea5b8..86298efe08 100644
--- a/src/nvim/api/buffer.c
+++ b/src/nvim/api/buffer.c
@@ -359,6 +359,14 @@ void nvim_buf_set_lines(uint64_t channel_id, Buffer buffer, Integer start, Integ
     return;
   }
 
+  // loaded buffer first if it's not loaded
+  if (buf->b_ml.ml_mfp == NULL) {
+    if (!buf_ensure_loaded(buf)) {
+      api_set_error(err, kErrorTypeException, "Failed to load buffer");
+      return;
+    }
+  }
+
   bool oob = false;
   start = normalize_index(buf, start, true, &oob);
   end = normalize_index(buf, end, true, &oob);
@@ -396,10 +404,6 @@ void nvim_buf_set_lines(uint64_t channel_id, Buffer buffer, Integer start, Integ
     goto end;
   }
 
-  if (!buf_ensure_loaded(buf)) {
-    goto end;
-  }
-
   if (u_save_buf(buf, (linenr_T)(start - 1), (linenr_T)end) == FAIL) {
     api_set_error(err, kErrorTypeException, "Failed to save undo information");
     goto end;
@@ -537,6 +541,14 @@ void nvim_buf_set_text(uint64_t channel_id, Buffer buffer, Integer start_row, In
     return;
   }
 
+  // loaded buffer first if it's not loaded
+  if (buf->b_ml.ml_mfp == NULL) {
+    if (!buf_ensure_loaded(buf)) {
+      api_set_error(err, kErrorTypeException, "Failed to load buffer");
+      return;
+    }
+  }
+
   bool oob = false;
 
   // check range is ordered and everything!
@@ -645,10 +657,6 @@ void nvim_buf_set_text(uint64_t channel_id, Buffer buffer, Integer start_row, In
     goto end;
   }
 
-  if (!buf_ensure_loaded(buf)) {
-    goto end;
-  }
-
   // Small note about undo states: unlike set_lines, we want to save the
   // undo state of one past the end_row, since end_row is inclusive.
   if (u_save_buf(buf, (linenr_T)start_row - 1, (linenr_T)end_row + 1) == FAIL) {
diff --git a/test/functional/api/buffer_spec.lua b/test/functional/api/buffer_spec.lua
index 9833ebee4c..6ed9aa574a 100644
--- a/test/functional/api/buffer_spec.lua
+++ b/test/functional/api/buffer_spec.lua
@@ -713,6 +713,21 @@ describe('api/buf', function()
       feed('<c-w>p')
       eq(3, funcs.winnr())
     end)
+
+    it('set_lines on unloaded buffer #8659 #22670', function()
+      local bufnr = curbuf('get_number')
+      meths.buf_set_lines(bufnr, 0, -1, false, {'a', 'b', 'c'})
+      meths.buf_set_name(bufnr, 'set_lines')
+      finally(function()
+        os.remove('set_lines')
+      end)
+      command('write!')
+      command('new')
+      command('bunload! '..bufnr)
+      local new_bufnr = funcs.bufnr('set_lines', true)
+      meths.buf_set_lines(new_bufnr, 0, -1, false, {})
+      eq({''}, meths.buf_get_lines(new_bufnr, 0, -1, false))
+    end)
   end)
 
   describe('nvim_buf_set_text', function()