fix(socket): --listen silently truncates long paths #40625

Problem:
A `--listen` path longer than the system socket path limit (~104 bytes on
macOS, 108 on Linux) is silently truncated by `uv_pipe_bind()`. Nvim either
serves a socket at a different path than `v:servername` reports, or fails with
an error that blames the full untruncated path (confusing):

    nvim: Failed to --listen: address already in use: "<full path>"

Steps to reproduce:

    $ nvim --listen /var/folders/g7/9y_ydbnj2fs_fvp8xf44p8gc0000gn/T//nvim/-Users-rpatterson-Projects-src-github.com-neovide-neovide --embed -p
    nvim: Failed to --listen: address already in use: "/var/folders/g7/9y_ydbnj2fs_fvp8xf44p8gc0000gn/T//nvim/-Users-rpatterson-Projects-src-github.com-neovide-neovide"

    $ ls /var/folders/g7/9y_ydbnj2fs_fvp8xf44p8gc0000gn/T//nvim/-Users-rpatterson-Projects-src-github.com-neovide-neovide
    "/var/folders/g7/9y_ydbnj2fs_fvp8xf44p8gc0000gn/T//nvim/-Users-rpatterson-Projects-src-github.com-neovide-neovide": No such file or directory (os error 2)

    $ ls -l /var/folders/g7/9y_ydbnj2fs_fvp8xf44p8gc0000gn/T//nvim/
    srwxr-xr-x@ - rpatterson 31 Mar 10:24 -Users-rpatterson-Projects-src-github.com-neovid

Solution:
Bind with `uv_pipe_bind2()` and `UV_PIPE_NO_TRUNCATE` (libuv 1.46+), so
a too-long path fails up front with the actual reason:

    nvim: Failed to --listen: invalid argument: "<full path>"

src/nvim/event/socket.c

@@ -169,7 +169,8 @@ int socket_watcher_start(SocketWatcher *watcher, int backlog, socket_cb cb)
     }
     uv_freeaddrinfo(watcher->uv.tcp.addrinfo);
   } else {
-    result = uv_pipe_bind(&watcher->uv.pipe.handle, watcher->addr);
+    result = uv_pipe_bind2(&watcher->uv.pipe.handle, watcher->addr, strlen(watcher->addr),
+                           UV_PIPE_NO_TRUNCATE);
 
     // If bind failed with EACCES/EADDRINUSE, check if socket is stale
     if (result == UV_EACCES || result == UV_EADDRINUSE) {
@@ -196,7 +197,8 @@ int socket_watcher_start(SocketWatcher *watcher, int backlog, socket_cb cb)
           watcher->stream->data = watcher;
 
           // Retry bind with fresh handle
-          result = uv_pipe_bind(&watcher->uv.pipe.handle, watcher->addr);
+          result = uv_pipe_bind2(&watcher->uv.pipe.handle, watcher->addr, strlen(watcher->addr),
+                                 UV_PIPE_NO_TRUNCATE);
         }
       } else {
         // Socket is alive - this is a real error

test/functional/core/server_spec.lua

@@ -364,6 +364,15 @@ describe('startup --listen', function()
       ('nvim.*: Failed to %%-%%-listen: [^:]+ already [^:]+: "%s"'):format(vim.pesc(in_use))
     )
     _test({ '--listen', '/' }, nil, 'nvim.*: Failed to %-%-listen: [^:]+: "/"')
+    if not is_os('win') then
+      -- Too-long path is rejected, not silently truncated. #38623
+      local too_long = './Xtest-listen-' .. ('x'):rep(192)
+      _test(
+        { '--listen', too_long },
+        nil,
+        ('nvim.*: Failed to %%-%%-listen: invalid argument: "%s"'):format(vim.pesc(too_long))
+      )
+    end
     _test(
       { '--listen', 'https://example.com' },
       nil,

test/functional/options/defaults_spec.lua

@@ -376,8 +376,8 @@ describe('XDG defaults', function()
       if not is_os('win') then
         -- Broken XDG vars cause serverstart() to fail (except on Windows, where servernames are not
         -- informed by $XDG_STATE_HOME).
-        t.matches('Failed to start server: no such file or directory', t.pcall_err(fn.serverstart))
-        assert_log('Failed to start server: no such file or directory: /X/X/X', testlog, 10)
+        t.matches('Failed to start server: invalid argument', t.pcall_err(fn.serverstart))
+        assert_log('Failed to start server: invalid argument: /X/X/X', testlog, 10)
       end
 
       local vimruntime, libdir = vimruntime_and_libdir()
@@ -962,6 +962,13 @@ describe('stdpath()', function()
   it('$NVIM_APPNAME relative path', function()
     local tmpdir = t.tmpname(false)
     t.mkdir(tmpdir)
+    -- Short runtime dir so the autogenerated server socket fits in sun_path even
+    -- when $TMPDIR is deep (otherwise the server fails to bind, see #38623).
+    local xdg_run = 'Xtest-appname-run'
+    mkdir(xdg_run)
+    finally(function()
+      rmdir(xdg_run)
+    end)
 
     clear({
       args_rm = { '--listen' },
@@ -969,6 +976,7 @@ describe('stdpath()', function()
         NVIM_APPNAME = 'relative/appname',
         NVIM_LOG_FILE = testlog,
         TMPDIR = tmpdir,
+        XDG_RUNTIME_DIR = xdg_run,
       },
     })