Merge #10589 'os/fs: introduce os_fopen()'

20 files changed, 134 insertions, 54 deletions

diff --git a/src/nvim/diff.c b/src/nvim/diff.c
index f720e702a4..f2b3abb526 100644
--- a/src/nvim/diff.c
+++ b/src/nvim/diff.c
@@ -951,7 +951,7 @@ static int check_external_diff(diffio_T *diffio)
   TriState ok = kFalse;
   for (;;) {
     ok = kFalse;
-    FILE *fd = mch_fopen((char *)diffio->dio_orig.din_fname, "w");
+    FILE *fd = os_fopen((char *)diffio->dio_orig.din_fname, "w");
 
     if (fd == NULL) {
       io_error = true;
@@ -960,7 +960,7 @@ static int check_external_diff(diffio_T *diffio)
         io_error = true;
       }
       fclose(fd);
-      fd = mch_fopen((char *)diffio->dio_new.din_fname, "w");
+      fd = os_fopen((char *)diffio->dio_new.din_fname, "w");
 
       if (fd == NULL) {
         io_error = true;
@@ -971,7 +971,7 @@ static int check_external_diff(diffio_T *diffio)
         fclose(fd);
         fd = NULL;
         if (diff_file(diffio) == OK) {
-          fd = mch_fopen((char *)diffio->dio_diff.dout_fname, "r");
+          fd = os_fopen((char *)diffio->dio_diff.dout_fname, "r");
         }
 
         if (fd == NULL) {
@@ -1505,7 +1505,7 @@ static void diff_read(int idx_orig, int idx_new, diffout_T *dout)
   if (dout->dout_fname == NULL) {
     diffstyle = DIFF_UNIFIED;
   } else {
-    fd = mch_fopen((char *)dout->dout_fname, "r");
+    fd = os_fopen((char *)dout->dout_fname, "r");
     if (fd == NULL) {
       EMSG(_("E98: Cannot read diff output"));
       return;
diff --git a/src/nvim/edit.c b/src/nvim/edit.c
index 6a37c52e3f..6355000175 100644
--- a/src/nvim/edit.c
+++ b/src/nvim/edit.c
@@ -2844,7 +2844,7 @@ static void ins_compl_files(int count, char_u **files, int thesaurus, int flags,
   int add_r;
 
   for (i = 0; i < count && !got_int && !compl_interrupted; i++) {
-    fp = mch_fopen((char *)files[i], "r");      /* open dictionary file */
+    fp = os_fopen((char *)files[i], "r");  // open dictionary file
     if (flags != DICT_EXACT) {
       vim_snprintf((char *)IObuff, IOSIZE,
                    _("Scanning dictionary: %s"), (char *)files[i]);
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 8e79ff59b6..0b943cedc7 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -13616,7 +13616,7 @@ static void f_readfile(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   // Always open the file in binary mode, library functions have a mind of
   // their own about CR-LF conversion.
   const char *const fname = tv_get_string(&argvars[0]);
-  if (*fname == NUL || (fd = mch_fopen(fname, READBIN)) == NULL) {
+  if (*fname == NUL || (fd = os_fopen(fname, READBIN)) == NULL) {
     EMSG2(_(e_notopen), *fname == NUL ? _("<empty>") : fname);
     return;
   }
diff --git a/src/nvim/ex_cmds.c b/src/nvim/ex_cmds.c
index 8e12ad8366..9a2b683bac 100644
--- a/src/nvim/ex_cmds.c
+++ b/src/nvim/ex_cmds.c
@@ -4557,7 +4557,7 @@ void ex_help(exarg_T *eap)
     } else {
       // There is no help window yet.
       // Try to open the file specified by the "helpfile" option.
-      if ((helpfd = mch_fopen((char *)p_hf, READBIN)) == NULL) {
+      if ((helpfd = os_fopen((char *)p_hf, READBIN)) == NULL) {
         smsg(_("Sorry, help file \"%s\" not found"), p_hf);
         goto erret;
       }
@@ -5087,7 +5087,7 @@ void fix_help_buffer(void)
                 continue;
               }
 
-              FILE *const fd = mch_fopen((char *)fnames[fi], "r");
+              FILE *const fd = os_fopen((char *)fnames[fi], "r");
               if (fd == NULL) {
                 continue;
               }
@@ -5223,7 +5223,7 @@ static void helptags_one(char_u *const dir, const char_u *const ext,
     return;
   }
 
-  FILE *const fd_tags = mch_fopen((char *)NameBuff, "w");
+  FILE *const fd_tags = os_fopen((char *)NameBuff, "w");
   if (fd_tags == NULL) {
     EMSG2(_("E152: Cannot open %s for writing"), NameBuff);
     FreeWild(filecount, files);
@@ -5247,7 +5247,7 @@ static void helptags_one(char_u *const dir, const char_u *const ext,
    * Go over all the files and extract the tags.
    */
   for (int fi = 0; fi < filecount && !got_int; fi++) {
-    FILE *const fd = mch_fopen((char *)files[fi], "r");
+    FILE *const fd = os_fopen((char *)files[fi], "r");
     if (fd == NULL) {
       EMSG2(_("E153: Unable to open %s for reading"), files[fi]);
       continue;
diff --git a/src/nvim/ex_cmds2.c b/src/nvim/ex_cmds2.c
index 9e915dfc8b..cd0b05c6c9 100644
--- a/src/nvim/ex_cmds2.c
+++ b/src/nvim/ex_cmds2.c
@@ -990,7 +990,7 @@ void profile_dump(void)
   FILE        *fd;
 
   if (profile_fname != NULL) {
-    fd = mch_fopen((char *)profile_fname, "w");
+    fd = os_fopen((char *)profile_fname, "w");
     if (fd == NULL) {
       EMSG2(_(e_notopen), profile_fname);
     } else {
@@ -1139,7 +1139,7 @@ static void script_dump_profile(FILE *fd)
       fprintf(fd, "\n");
       fprintf(fd, "count  total (s)   self (s)\n");
 
-      sfd = mch_fopen((char *)si->sn_name, "r");
+      sfd = os_fopen((char *)si->sn_name, "r");
       if (sfd == NULL) {
         fprintf(fd, "Cannot open file!\n");
       } else {
@@ -2858,7 +2858,7 @@ static int requires_py_version(char_u *filename)
     lines = 5;
   }
 
-  file = mch_fopen((char *)filename, "r");
+  file = os_fopen((char *)filename, "r");
   if (file != NULL) {
     for (i = 0; i < lines; i++) {
       if (vim_fgets(IObuff, IOSIZE, file)) {
diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index 28c7bc300f..c8edf7173e 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -8106,8 +8106,9 @@ open_exfile (
     return NULL;
   }
 
-  if ((fd = mch_fopen((char *)fname, mode)) == NULL)
+  if ((fd = os_fopen((char *)fname, mode)) == NULL) {
     EMSG2(_("E190: Cannot open \"%s\" for writing"), fname);
+  }
 
   return fd;
 }
diff --git a/src/nvim/hardcopy.c b/src/nvim/hardcopy.c
index 0001062588..3e0e438434 100644
--- a/src/nvim/hardcopy.c
+++ b/src/nvim/hardcopy.c
@@ -1670,7 +1670,7 @@ static int prt_open_resource(struct prt_ps_resource_S *resource)
   FILE        *fd_resource;
   struct prt_dsc_line_S dsc_line;
 
-  fd_resource = mch_fopen((char *)resource->filename, READBIN);
+  fd_resource = os_fopen((char *)resource->filename, READBIN);
   if (fd_resource == NULL) {
     EMSG2(_("E624: Can't open file \"%s\""), resource->filename);
     return FALSE;
@@ -2343,11 +2343,11 @@ int mch_print_init(prt_settings_T *psettings, char_u *jobname, int forceit)
       EMSG(_(e_notmp));
       return FAIL;
     }
-    prt_ps_fd = mch_fopen((char *)prt_ps_file_name, WRITEBIN);
+    prt_ps_fd = os_fopen((char *)prt_ps_file_name, WRITEBIN);
   } else {
     p = expand_env_save(psettings->outfile);
     if (p != NULL) {
-      prt_ps_fd = mch_fopen((char *)p, WRITEBIN);
+      prt_ps_fd = os_fopen((char *)p, WRITEBIN);
       xfree(p);
     }
   }
@@ -2382,7 +2382,7 @@ static int prt_add_resource(struct prt_ps_resource_S *resource)
   char_u resource_buffer[512];
   size_t bytes_read;
 
-  fd_resource = mch_fopen((char *)resource->filename, READBIN);
+  fd_resource = os_fopen((char *)resource->filename, READBIN);
   if (fd_resource == NULL) {
     EMSG2(_("E456: Can't open file \"%s\""), resource->filename);
     return FALSE;
diff --git a/src/nvim/if_cscope.c b/src/nvim/if_cscope.c
index ca2a163c57..0f9984ec38 100644
--- a/src/nvim/if_cscope.c
+++ b/src/nvim/if_cscope.c
@@ -1008,10 +1008,10 @@ static int cs_find_common(char *opt, char *pat, int forceit, int verbose,
     qf_info_T   *qi = NULL;
     win_T       *wp = NULL;
 
-    f = mch_fopen((char *)tmp, "w");
-    if (f == NULL)
+    f = os_fopen((char *)tmp, "w");
+    if (f == NULL) {
       EMSG2(_(e_notopen), tmp);
-    else {
+    } else {
       cs_file_results(f, nummatches);
       fclose(f);
       if (use_ll)           /* Use location list */
diff --git a/src/nvim/macros.h b/src/nvim/macros.h
index 61009528a8..c758e000a9 100644
--- a/src/nvim/macros.h
+++ b/src/nvim/macros.h
@@ -86,8 +86,6 @@
 #define READBIN    "rb"
 #define APPENDBIN  "ab"
 
-#  define mch_fopen(n, p)       fopen((n), (p))
-
 /* mch_open_rw(): invoke os_open() with third argument for user R/W. */
 #if defined(UNIX)  /* open in rw------- mode */
 # define mch_open_rw(n, f)      os_open((n), (f), (mode_t)0600)
diff --git a/src/nvim/main.c b/src/nvim/main.c
index b2f068cb51..9c342e62c0 100644
--- a/src/nvim/main.c
+++ b/src/nvim/main.c
@@ -1174,8 +1174,8 @@ scripterror:
             if (scriptout != NULL) {
               goto scripterror;
             }
-            if ((scriptout = mch_fopen(argv[0],
-                    c == 'w' ? APPENDBIN : WRITEBIN)) == NULL) {
+            if ((scriptout = os_fopen(argv[0], c == 'w' ? APPENDBIN : WRITEBIN))
+                == NULL) {
               mch_errmsg(_("Cannot open for script output: \""));
               mch_errmsg(argv[0]);
               mch_errmsg("\"\n");
@@ -1263,8 +1263,9 @@ static void init_params(mparm_T *paramp, int argc, char **argv)
 static void init_startuptime(mparm_T *paramp)
 {
   for (int i = 1; i < paramp->argc; i++) {
-    if (STRICMP(paramp->argv[i], "--startuptime") == 0 && i + 1 < paramp->argc) {
-      time_fd = mch_fopen(paramp->argv[i + 1], "a");
+    if (STRICMP(paramp->argv[i], "--startuptime") == 0
+        && i + 1 < paramp->argc) {
+      time_fd = os_fopen(paramp->argv[i + 1], "a");
       time_start("--- NVIM STARTING ---");
       break;
     }
diff --git a/src/nvim/message.c b/src/nvim/message.c
index df130565e0..12da48347e 100644
--- a/src/nvim/message.c
+++ b/src/nvim/message.c
@@ -2992,7 +2992,7 @@ int verbose_open(void)
     /* Only give the error message once. */
     verbose_did_open = TRUE;
 
-    verbose_fd = mch_fopen((char *)p_vfile, "a");
+    verbose_fd = os_fopen((char *)p_vfile, "a");
     if (verbose_fd == NULL) {
       EMSG2(_(e_notopen), p_vfile);
       return FAIL;
diff --git a/src/nvim/misc1.c b/src/nvim/misc1.c
index 463c29405b..db0d56b5fd 100644
--- a/src/nvim/misc1.c
+++ b/src/nvim/misc1.c
@@ -2836,7 +2836,7 @@ char_u *get_cmd_output(char_u *cmd, char_u *infile, ShellOpts flags,
   xfree(command);
 
   // read the names from the file into memory
-  FILE *fd = mch_fopen((char *)tempname, READBIN);
+  FILE *fd = os_fopen((char *)tempname, READBIN);
 
   if (fd == NULL) {
     EMSG2(_(e_notopen), tempname);
diff --git a/src/nvim/os/fs.c b/src/nvim/os/fs.c
index 0ded36942e..dcb3ef7c4a 100644
--- a/src/nvim/os/fs.c
+++ b/src/nvim/os/fs.c
@@ -404,10 +404,11 @@ end:
 /// calls (read, write, lseek, fcntl, etc.). If the operation fails, a libuv
 /// error code is returned, and no file is created or modified.
 ///
+/// @param path Filename
 /// @param flags Bitwise OR of flags defined in <fcntl.h>
 /// @param mode Permissions for the newly-created file (IGNORED if 'flags' is
 ///        not `O_CREAT` or `O_TMPFILE`), subject to the current umask
-/// @return file descriptor, or libuv error code on failure
+/// @return file descriptor, or negative error code on failure
 int os_open(const char *path, int flags, int mode)
 {
   if (path == NULL) {  // uv_fs_open asserts on NULL. #7561
@@ -418,6 +419,68 @@ int os_open(const char *path, int flags, int mode)
   return r;
 }
 
+/// Compatibility wrapper conforming to fopen(3).
+///
+/// Windows: works with UTF-16 filepaths by delegating to libuv (os_open).
+///
+/// Future: remove this, migrate callers to os/fileio.c ?
+///         But file_open_fd does not support O_RDWR yet.
+///
+/// @param path  Filename
+/// @param flags  String flags, one of { r w a r+ w+ a+ rb wb ab }
+/// @return FILE pointer, or NULL on error.
+FILE *os_fopen(const char *path, const char *flags)
+{
+  assert(flags != NULL && strlen(flags) > 0 && strlen(flags) <= 2);
+  int iflags = 0;
+  // Per table in fopen(3) manpage.
+  if (flags[1] == '\0' || flags[1] == 'b') {
+    switch (flags[0]) {
+      case 'r':
+        iflags = O_RDONLY;
+        break;
+      case 'w':
+        iflags = O_WRONLY | O_CREAT | O_TRUNC;
+        break;
+      case 'a':
+        iflags = O_WRONLY | O_CREAT | O_APPEND;
+        break;
+      default:
+        abort();
+    }
+#ifdef WIN32
+    if (flags[1] == 'b') {
+      iflags |= O_BINARY;
+    }
+#endif
+  } else {
+    // char 0 must be one of ('r','w','a').
+    // char 1 is always '+' ('b' is handled above).
+    assert(flags[1] == '+');
+    switch (flags[0]) {
+      case 'r':
+        iflags = O_RDWR;
+        break;
+      case 'w':
+        iflags = O_RDWR | O_CREAT | O_TRUNC;
+        break;
+      case 'a':
+        iflags = O_RDWR | O_CREAT | O_APPEND;
+        break;
+      default:
+        abort();
+    }
+  }
+  // Per open(2) manpage.
+  assert((iflags|O_RDONLY) || (iflags|O_WRONLY) || (iflags|O_RDWR));
+  // Per fopen(3) manpage: default to 0666, it will be umask-adjusted.
+  int fd = os_open(path, iflags, 0666);
+  if (fd < 0) {
+    return NULL;
+  }
+  return fdopen(fd, flags);
+}
+
 /// Sets file descriptor `fd` to close-on-exec.
 //
 // @return -1 if failed to set, 0 otherwise.
@@ -829,12 +892,12 @@ int os_mkdir_recurse(const char *const dir, int32_t mode,
   // We're done when it's "/" or "c:/".
   const size_t dirlen = strlen(dir);
   char *const curdir = xmemdupz(dir, dirlen);
-  char *const past_head = (char *) get_past_head((char_u *) curdir);
+  char *const past_head = (char *)get_past_head((char_u *)curdir);
   char *e = curdir + dirlen;
   const char *const real_end = e;
   const char past_head_save = *past_head;
-  while (!os_isdir((char_u *) curdir)) {
-    e = (char *) path_tail_with_sep((char_u *) curdir);
+  while (!os_isdir((char_u *)curdir)) {
+    e = (char *)path_tail_with_sep((char_u *)curdir);
     if (e <= past_head) {
       *past_head = NUL;
       break;
@@ -986,7 +1049,7 @@ bool os_fileinfo_fd(int file_descriptor, FileInfo *file_info)
 ///
 /// @return `true` if the two FileInfos represent the same file.
 bool os_fileinfo_id_equal(const FileInfo *file_info_1,
-                           const FileInfo *file_info_2)
+                          const FileInfo *file_info_2)
   FUNC_ATTR_NONNULL_ALL
 {
   return file_info_1->stat.st_ino == file_info_2->stat.st_ino
diff --git a/src/nvim/quickfix.c b/src/nvim/quickfix.c
index e030d66e41..cc4a2a90b9 100644
--- a/src/nvim/quickfix.c
+++ b/src/nvim/quickfix.c
@@ -863,7 +863,7 @@ qf_init_ext(
   fields.errmsg = xmalloc(fields.errmsglen);
   fields.pattern = xmalloc(CMDBUFFSIZE + 1);
 
-  if (efile != NULL && (state.fd = mch_fopen((char *)efile, "r")) == NULL) {
+  if (efile != NULL && (state.fd = os_fopen((char *)efile, "r")) == NULL) {
     EMSG2(_(e_openerrf), efile);
     goto qf_init_end;
   }
@@ -5495,7 +5495,7 @@ void ex_helpgrep(exarg_T *eap)
                                + STRLEN(fnames[fi]) - 3, 3) == 0)) {
             continue;
           }
-          fd = mch_fopen((char *)fnames[fi], "r");
+          fd = os_fopen((char *)fnames[fi], "r");
           if (fd != NULL) {
             lnum = 1;
             while (!vim_fgets(IObuff, IOSIZE, fd) && !got_int) {
diff --git a/src/nvim/search.c b/src/nvim/search.c
index ed5934cec2..c1770fd80d 100644
--- a/src/nvim/search.c
+++ b/src/nvim/search.c
@@ -4575,10 +4575,9 @@ find_pattern_in_path(
           xfree(files);
           files = bigger;
         }
-        if ((files[depth + 1].fp = mch_fopen((char *)new_fname, "r"))
-            == NULL)
+        if ((files[depth + 1].fp = os_fopen((char *)new_fname, "r")) == NULL) {
           xfree(new_fname);
-        else {
+        } else {
           if (++depth == old_files) {
             // Something wrong. We will forget one of our already visited files
             // now.
diff --git a/src/nvim/spell.c b/src/nvim/spell.c
index 66eb4ac3c6..893a13b9f0 100644
--- a/src/nvim/spell.c
+++ b/src/nvim/spell.c
@@ -3262,7 +3262,7 @@ static void spell_suggest_file(suginfo_T *su, char_u *fname)
   char_u cword[MAXWLEN];
 
   // Open the file.
-  fd = mch_fopen((char *)fname, "r");
+  fd = os_fopen((char *)fname, "r");
   if (fd == NULL) {
     EMSG2(_(e_notopen), fname);
     return;
diff --git a/src/nvim/spellfile.c b/src/nvim/spellfile.c
index 5f5f74cf2e..405e390589 100644
--- a/src/nvim/spellfile.c
+++ b/src/nvim/spellfile.c
@@ -586,7 +586,7 @@ spell_load_file (
   int c = 0;
   int res;
 
-  fd = mch_fopen((char *)fname, "r");
+  fd = os_fopen((char *)fname, "r");
   if (fd == NULL) {
     if (!silent)
       EMSG2(_(e_notopen), fname);
@@ -885,9 +885,10 @@ void suggest_load_files(void)
         continue;
       }
       STRCPY(dotp, ".sug");
-      fd = mch_fopen((char *)slang->sl_fname, "r");
-      if (fd == NULL)
+      fd = os_fopen((char *)slang->sl_fname, "r");
+      if (fd == NULL) {
         goto nextone;
+      }
 
       // <SUGHEADER>: <fileID> <versionnr> <timestamp>
       for (i = 0; i < VIMSUGMAGICL; ++i)
@@ -1984,7 +1985,7 @@ static afffile_T *spell_read_aff(spellinfo_T *spin, char_u *fname)
   char_u      *sofoto = NULL;           // SOFOTO value
 
   // Open the file.
-  fd = mch_fopen((char *)fname, "r");
+  fd = os_fopen((char *)fname, "r");
   if (fd == NULL) {
     EMSG2(_(e_notopen), fname);
     return NULL;
@@ -3019,7 +3020,7 @@ static int spell_read_dic(spellinfo_T *spin, char_u *fname, afffile_T *affile)
   int duplicate = 0;
 
   // Open the file.
-  fd = mch_fopen((char *)fname, "r");
+  fd = os_fopen((char *)fname, "r");
   if (fd == NULL) {
     EMSG2(_(e_notopen), fname);
     return FAIL;
@@ -3539,7 +3540,7 @@ static int spell_read_wordfile(spellinfo_T *spin, char_u *fname)
   int regionmask;
 
   // Open the file.
-  fd = mch_fopen((char *)fname, "r");
+  fd = os_fopen((char *)fname, "r");
   if (fd == NULL) {
     EMSG2(_(e_notopen), fname);
     return FAIL;
@@ -4185,7 +4186,7 @@ static int write_vim_spell(spellinfo_T *spin, char_u *fname)
   int retval = OK;
   int regionmask;
 
-  FILE *fd = mch_fopen((char *)fname, "w");
+  FILE *fd = os_fopen((char *)fname, "w");
   if (fd == NULL) {
     EMSG2(_(e_notopen), fname);
     return FAIL;
@@ -4986,7 +4987,7 @@ static int offset2bytes(int nr, char_u *buf)
 static void sug_write(spellinfo_T *spin, char_u *fname)
 {
   // Create the file.  Note that an existing file is silently overwritten!
-  FILE *fd = mch_fopen((char *)fname, "w");
+  FILE *fd = os_fopen((char *)fname, "w");
   if (fd == NULL) {
     EMSG2(_(e_notopen), fname);
     return;
@@ -5365,7 +5366,7 @@ spell_add_word (
   if (bad || undo) {
     // When the word appears as good word we need to remove that one,
     // since its flags sort before the one with WF_BANNED.
-    fd = mch_fopen((char *)fname, "r");
+    fd = os_fopen((char *)fname, "r");
     if (fd != NULL) {
       while (!vim_fgets(line, MAXWLEN * 2, fd)) {
         fpos = fpos_next;
@@ -5376,7 +5377,7 @@ spell_add_word (
           // the start of the line.  Mixing reading and writing
           // doesn't work for all systems, close the file first.
           fclose(fd);
-          fd = mch_fopen((char *)fname, "r+");
+          fd = os_fopen((char *)fname, "r+");
           if (fd == NULL) {
             break;
           }
@@ -5399,7 +5400,7 @@ spell_add_word (
   }
 
   if (!undo) {
-    fd = mch_fopen((char *)fname, "a");
+    fd = os_fopen((char *)fname, "a");
     if (fd == NULL && new_spf) {
       char_u *p;
 
@@ -5415,7 +5416,7 @@ spell_add_word (
         *p = NUL;
         os_mkdir((char *)fname, 0755);
         *p = c;
-        fd = mch_fopen((char *)fname, "a");
+        fd = os_fopen((char *)fname, "a");
       }
     }
 
diff --git a/src/nvim/tag.c b/src/nvim/tag.c
index 6e883a1c3d..504a099df2 100644
--- a/src/nvim/tag.c
+++ b/src/nvim/tag.c
@@ -1303,8 +1303,9 @@ find_tags (
           }
         }
 
-        if ((fp = mch_fopen((char *)tag_fname, "r")) == NULL)
+        if ((fp = os_fopen((char *)tag_fname, "r")) == NULL) {
           continue;
+        }
 
         if (p_verbose >= 5) {
           verbose_enter();
diff --git a/src/nvim/undo.c b/src/nvim/undo.c
index 116bdd02b8..c9b0d96866 100644
--- a/src/nvim/undo.c
+++ b/src/nvim/undo.c
@@ -1299,7 +1299,7 @@ void u_read_undo(char *name, char_u *hash, char_u *orig_name)
     verbose_leave();
   }
 
-  FILE *fp = mch_fopen(file_name, "r");
+  FILE *fp = os_fopen(file_name, "r");
   if (fp == NULL) {
     if (name != NULL || p_verbose > 0) {
       EMSG2(_("E822: Cannot open undo file for reading: %s"), file_name);
diff --git a/test/functional/core/fileio_spec.lua b/test/functional/core/fileio_spec.lua
index c74eb3bb02..e6bce85b8a 100644
--- a/test/functional/core/fileio_spec.lua
+++ b/test/functional/core/fileio_spec.lua
@@ -22,6 +22,7 @@ describe('fileio', function()
     os.remove('Xtest_startup_file1')
     os.remove('Xtest_startup_file1~')
     os.remove('Xtest_startup_file2')
+    os.remove('Xtest_тест.md')
     rmdir('Xtest_startup_swapdir')
   end)
 
@@ -85,7 +86,22 @@ describe('fileio', function()
 
     eq('foobar', foobar_contents);
     eq('foo', bar_contents);
+  end)
 
+  it('readfile() on multibyte filename #10586', function()
+    clear()
+    local text = {
+      'line1',
+      '  ...line2...  ',
+      '',
+      'line3!',
+      'тест yay тест.',
+      '',
+    }
+    local fname = 'Xtest_тест.md'
+    funcs.writefile(text, fname, 's')
+    table.insert(text, '')
+    eq(text, funcs.readfile(fname, 'b'))
   end)
 end)