feat: ignore swapfile for running Nvim processes #25336

Problem:
The swapfile "E325: ATTENTION" dialog is displayed when editing a file
already open in another (running) Nvim. Usually this behavior is
annoying and irrelevant:
- "Recover" and the other options ("Open readonly", "Quit", "Abort") are
  almost never wanted.
- swapfiles are less relevant for "multi-Nvim" since 'autoread' is
  enabled by default.
  - Even less relevant if user enables 'autowrite'.

Solution:
Define a default SwapExists handler which does the following:
1. If the swapfile is owned by a running Nvim process, automatically
   chooses "(E)dit anyway" (caveat: this creates a new, extra swapfile,
   which is mostly harmless and ignored except by `:recover` or `nvim -r`.
2. Shows a 1-line "ignoring swapfile..." message.
3. Users can disable the default SwapExists handler via `autocmd! nvim_swapfile`.

2 files changed, 19 insertions, 2 deletions

diff --git a/runtime/lua/vim/_editor.lua b/runtime/lua/vim/_editor.lua
index 7f09fc8038..1ba7d6163d 100644
--- a/runtime/lua/vim/_editor.lua
+++ b/runtime/lua/vim/_editor.lua
@@ -1147,11 +1147,28 @@ function vim._init_default_autocmds()
       end
     end,
   })
+
   vim.api.nvim_create_autocmd({ 'CmdwinEnter' }, {
     pattern = '[:>]',
     group = vim.api.nvim_create_augroup('nvim_cmdwin', {}),
     command = 'syntax sync minlines=1 maxlines=1',
   })
+
+  vim.api.nvim_create_autocmd({ 'SwapExists' }, {
+    pattern = '*',
+    group = vim.api.nvim_create_augroup('nvim_swapfile', {}),
+    callback = function()
+      local info = vim.fn.swapinfo(vim.v.swapname)
+      local user = vim.uv.os_get_passwd().username
+      local iswin = 1 == vim.fn.has('win32')
+      if info.error or info.pid <= 0 or (not iswin and info.user ~= user) then
+        vim.v.swapchoice = '' -- Show the prompt.
+        return
+      end
+      vim.v.swapchoice = 'e' -- Choose "(E)dit".
+      vim.notify(('W325: Ignoring swapfile from Nvim process %d'):format(info.pid))
+    end,
+  })
 end
 
 function vim._init_defaults()
diff --git a/runtime/lua/vim/_meta/vimfn.lua b/runtime/lua/vim/_meta/vimfn.lua
index 55c324d739..5a3a5f3890 100644
--- a/runtime/lua/vim/_meta/vimfn.lua
+++ b/runtime/lua/vim/_meta/vimfn.lua
@@ -9312,8 +9312,8 @@ function vim.fn.swapfilelist() end
 ---   user  user name
 ---   host  host name
 ---   fname  original file name
----   pid  PID of the Vim process that created the swap
----     file
+---   pid  PID of the Nvim process that created the swap
+---     file, or zero if not running.
 ---   mtime  last modification time in seconds
 ---   inode  Optional: INODE number of the file
 ---   dirty  1 if file was modified, 0 if not