fix(path): open swapfile when using device path #39982

Problem:
Can't open swapfile when using a device path starting with `//?/`,
because `?` is a reserved char on Widnows.

Solution:
For device UNC paths, replace `//?/UNC/` and `//./UNC/` with `//`.
For other device paths, just strip their prefix (i.e. `//?/`, `//./`).
This aligns swapfile naming for device paths with regular UNC and DOS
paths.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

src/nvim/memline.c

@@ -1425,11 +1425,22 @@ char *make_percent_swname(char *dir, char *dir_end, const char *name)
   FUNC_ATTR_NONNULL_ARG(1, 2)
 {
   String fixed_fname;
-  fixed_fname.data = fix_fname(name != NULL ? name : "");
-  if (fixed_fname.data == NULL) {
+  char *fname = fix_fname(name != NULL ? name : "");
+  if (fname == NULL) {
     return NULL;
   }
 
+  FileInfo file_info;
+  if (!os_fileinfo2(fname, &file_info)) {
+    xfree(fname);
+    return NULL;
+  }
+  fixed_fname.data = fname + file_info.root_off;
+  if (file_info.type == kPathDeviceUNC) {
+    assert(file_info.root_off >= 2);
+    fixed_fname.data -= 2;
+    fixed_fname.data[0] = '/';  // Fixup //?/UNC/server/ path: "C/server/..." -> "//server/..."
+  }
   char *p;
   for (p = fixed_fname.data; *p != NUL; MB_PTR_ADV(p)) {
     if (vim_ispathsep(*p)) {
@@ -1442,7 +1453,7 @@ char *make_percent_swname(char *dir, char *dir_end, const char *name)
   p = &dir_end[-1];
   *p = NUL;
   String d = concat_fnames(cbuf_as_string(dir, (size_t)(p - dir)), fixed_fname, true);
-  xfree(fixed_fname.data);
+  xfree(fname);
 
   return d.data;
 }

src/nvim/os/fs.c

@@ -1188,6 +1188,40 @@ bool os_fileinfo(const char *path, FileInfo *file_info)
   return os_stat(path, &(file_info->stat)) == kLibuvSuccess;
 }
 
+/// Identify Windows path type and provide root offset for now, examples:
+/// - "//?/C:/foo" => type is `kPathDevice` root offset is 4
+/// - "//?/UNC/localhost/C$/foo" => type is `kPathDeviceUNC`, root offset is 8
+///
+/// TODO(ntdiary): Could be extended for path.c cleanup and path normalization
+/// logic. Eventually merge this with `os_fileinfo`
+///
+/// TODO(justinmk): return value is meaningless currently.
+bool os_fileinfo2(const char *path, FileInfo *info)
+  FUNC_ATTR_NONNULL_ALL
+{
+  CLEAR_POINTER(info);
+#ifdef MSWIN
+  size_t len = strlen(path);
+  if (len < 4) {
+    return true;
+  }
+  if (vim_ispathsep_nocolon(path[0])
+      && vim_ispathsep_nocolon(path[1])
+      && (path[2] == '?' || path[2] == '.')
+      && vim_ispathsep_nocolon(path[3])) {
+    if (len >= 8 && vim_strnicmp_asc(path + 4, "unc/", 4) == 0) {
+      info->root_off = 8;
+      info->type = kPathDeviceUNC;
+    } else {
+      info->root_off = 4;
+      info->type = kPathDevice;
+    }
+    return true;
+  }
+#endif
+  return true;
+}
+
 /// Get the file information for a given path without following links
 ///
 /// @param path Path to the file.

src/nvim/os/fs_defs.h

@@ -2,9 +2,24 @@
 
 #include <uv.h>
 
-/// Struct which encapsulates stat information.
+/// Currently supports Windows and is extensible.
+/// @see https://learn.microsoft.com/en-us/dotnet/standard/io/file-path-formats
+typedef enum {
+  kPathUnknown = 0,
+  kPathDOS,         ///< C:/foo/bar
+  kPathUNC,         ///< //server/share/foo/bar
+  kPathDevice,      ///< //?/C:/foo/bar or //?/Volume{xxx}/foo/bar
+  kPathDeviceUNC,   ///< //?/UNC/server/share/foo/bar
+} PathType;
+
+/// Struct which encapsulates file stat and path information.
 typedef struct {
   uv_stat_t stat;  ///< @private
+
+  /// Offset of the root component after any path type prefix. Currently
+  /// only used for Widnows device paths. e.g. "//?/" and "//?/UNC/"
+  size_t root_off;
+  PathType type;
 } FileInfo;
 
 /// Struct which encapsulates inode/dev_id information.

test/functional/core/startup_spec.lua

@@ -118,6 +118,49 @@ describe('startup', function()
     command('filetype detect')
     eq('filetype detection:ON  plugin:OFF  indent:OFF', exec_capture('filetype'))
   end)
+
+  it('opens swapfile when using device path #31606', function()
+    t.skip(not is_os('win'), 'N/A: Windows feature')
+    local cwd = vim.fs.normalize(vim.uv.cwd())
+    local drive, path = cwd:sub(1, 1), cwd:sub(3)
+    local swapdir = ('%s/Xtest_swap_dir'):format(cwd)
+    local dos_path = ('%s/file.txt'):format(cwd)
+    local device_quest_path = ('//?/%s'):format(dos_path)
+    clear({
+      args = {
+        '--embed',
+        '--headless',
+        '--cmd',
+        ('set directory=%s//'):format(swapdir),
+        '--',
+        device_quest_path,
+      },
+      merge = false,
+    })
+    local escaped_name = ('%s/%s.swp'):format(swapdir, dos_path:gsub('[:/]', '%%'))
+    eq(escaped_name, fn.swapname('%'))
+
+    command('bw!')
+    api.nvim_buf_set_name(0, dos_path)
+    eq(escaped_name, fn.swapname('%'))
+
+    command('bw!')
+    local device_dot_path = ('//./%s'):format(dos_path)
+    api.nvim_buf_set_name(0, device_dot_path)
+    eq(escaped_name, fn.swapname('%'))
+
+    command('bw!')
+    local device_unc_path = ('//?/UNC/localhost/%s$%s/file.txt'):format(drive, path)
+    api.nvim_buf_set_name(0, device_unc_path)
+    eq(('%s/%%%s.swp'):format(swapdir, device_unc_path:sub(8):gsub('/', '%%')), fn.swapname('%'))
+
+    command('bw!')
+    -- Volume GUID form, like \\?\Volume{d4857377-2ac4-45d8-a4cb-9b2e447fd02e}\
+    local guid = vim.fs.normalize(vim.trim(vim.fn.system(('mountvol %s: /L'):format(drive))))
+    local device_guid_path = ('%s%s/file.txt'):format(guid, path:sub(2))
+    api.nvim_buf_set_name(0, device_guid_path)
+    eq(escaped_name, fn.swapname('%'))
+  end)
 end)
 
 describe('startup', function()