fix(file): Race condition during buffer creation (fixes sindrets#380)

Problem:    If there are two or more file instances for the same rev,
            and one of them get destroyed while another is still waiting
            for the VCS adapter to provide the file content, we get an
            error once the waiting instance tries to use the now deleted
            buffer.

Solution:   Revalidate buffer after data is resolved.

Author: sindrets

lua/diffview/vcs/file.lua

@@ -237,16 +237,21 @@ File.create_buffer = async.wrap(function(self, callback)
     return
   end
 
+  -- Create buffer and set name *before* calling `produce_data()` to ensure
+  -- that multiple file instances won't ever try to create the same file.
   self.bufnr = api.nvim_create_buf(false, false)
-  local bufopts = vim.deepcopy(File.bufopts)
-
   api.nvim_buf_set_name(self.bufnr, fullname)
 
   local err, lines = await(self:produce_data())
   if err then error(table.concat(err, "\n")) end
 
   await(async.scheduler())
 
+  -- Revalidate buffer in case the file was destroyed before `produce_data()`
+  -- returned.
+  if not api.nvim_buf_is_valid(self.bufnr) then return end
+  local bufopts = vim.deepcopy(File.bufopts)
+
   if self.rev.type == RevType.STAGE and self.rev.stage == 0 then
     self.blob_hash = self.adapter:file_blob_hash(self.path)
     bufopts.modifiable = true
@@ -267,21 +272,19 @@ File.create_buffer = async.wrap(function(self, callback)
     api.nvim_buf_set_option(self.bufnr, option, value)
   end
 
-  if api.nvim_buf_is_valid(self.bufnr) then
-    local last_modifiable = vim.bo[self.bufnr].modifiable
-    local last_modified = vim.bo[self.bufnr].modified
-    vim.bo[self.bufnr].modifiable = true
-    api.nvim_buf_set_lines(self.bufnr, 0, -1, false, lines)
+  local last_modifiable = vim.bo[self.bufnr].modifiable
+  local last_modified = vim.bo[self.bufnr].modified
+  vim.bo[self.bufnr].modifiable = true
+  api.nvim_buf_set_lines(self.bufnr, 0, -1, false, lines)
 
-    api.nvim_buf_call(self.bufnr, function()
-      vim.cmd("filetype detect")
-    end)
+  api.nvim_buf_call(self.bufnr, function()
+    vim.cmd("filetype detect")
+  end)
 
-    vim.bo[self.bufnr].modifiable = last_modifiable
-    vim.bo[self.bufnr].modified = last_modified
-    self:post_buf_created()
-    callback(self.bufnr)
-  end
+  vim.bo[self.bufnr].modifiable = last_modifiable
+  vim.bo[self.bufnr].modified = last_modified
+  self:post_buf_created()
+  callback(self.bufnr)
   ---@diagnostic enable: invisible
 end)