open_buffer(): Do BufEnter for directories.

Abuse NOTDONE to give some nuance to the return value of readfile(), so
that open_buffer() can distinguish between "failed, lol" and "failed
because the path is a directory".

Before this change, Vim *already* creates a new buffer when a directory
is edited. So there is no reason it should not raise BufEnter, that was
an implementation detail of ye olde readfile().

Most of the changes in this commit merely preserve the old semantics.
The "implicit" change that we actually are interested in, is this line
in `open_buffer()`, where `retval` being non-FAIL allows EVENT_BUFENTER
to be applied:

    apply_autocmds_retval(EVENT_BUFENTER, NULL, NULL, FALSE, curbuf, &retval);

References https://github.com/vim/vim/issues/1353

src/nvim/buffer.c

@@ -193,12 +193,15 @@ open_buffer (
        * it can be changed there. */
       if (!readonlymode && !bufempty())
         changed();
-      else if (retval != FAIL)
+      else if (retval == OK) {
         unchanged(curbuf, FALSE);
+      }
+      if (retval == OK) {
         apply_autocmds_retval(EVENT_STDINREADPOST, NULL, NULL, FALSE,
                               curbuf, &retval);
       }
     }
+  }
 
   /* if first time loading this buffer, init b_chartab[] */
   if (curbuf->b_flags & BF_NEVERLOADED) {
@@ -219,7 +222,7 @@ open_buffer (
       || (aborting() && vim_strchr(p_cpo, CPO_INTMOD) != NULL)
       )
     changed();
-  else if (retval != FAIL && !read_stdin)
+  else if (retval == OK && !read_stdin)
     unchanged(curbuf, FALSE);
   save_file_ff(curbuf);                 /* keep this fileformat */
 

src/nvim/ex_cmds.c

@@ -1193,8 +1193,8 @@ static void do_filter(
 
   if (do_out) {
     if (otmp != NULL) {
-      if (readfile(otmp, NULL, line2, (linenr_T)0, (linenr_T)MAXLNUM,
-              eap, READ_FILTER) == FAIL) {
+      if (readfile(otmp, NULL, line2, (linenr_T)0, (linenr_T)MAXLNUM, eap,
+                   READ_FILTER) != OK) {
         if (!aborting()) {
           msg_putchar('\n');
           EMSG2(_(e_notread), otmp);

src/nvim/ex_docmd.c

@@ -6910,9 +6910,10 @@ static void ex_read(exarg_T *eap)
           eap->line2, (linenr_T)0, (linenr_T)MAXLNUM, eap, 0);
 
     }
-    if (i == FAIL) {
-      if (!aborting())
+    if (i != OK) {
+      if (!aborting()) {
         EMSG2(_(e_notopen), eap->arg);
+      }
     } else {
       if (empty && exmode_active) {
         /* Delete the empty line that remains.  Historically ex does

src/nvim/fileio.c

@@ -249,7 +249,7 @@ void filemess(buf_T *buf, char_u *name, char_u *s, int attr)
  * READ_DUMMY	read into a dummy buffer (to check if file contents changed)
  * READ_KEEP_UNDO  don't clear undo info or read it from a file
  *
- * return FAIL for failure, OK otherwise
+ * return FAIL for failure, NOTDONE for directory (failure), or OK
  */
 int 
 readfile (
@@ -258,7 +258,7 @@ readfile (
     linenr_T from,
     linenr_T lines_to_skip,
     linenr_T lines_to_read,
-    exarg_T *eap,                       /* can be NULL! */
+    exarg_T *eap,                       // can be NULL!
     int flags
 )
 {
@@ -444,13 +444,14 @@ readfile (
         // ... or a character special file named /dev/fd/<n>
 # endif
         ) {
-      if (S_ISDIR(perm))
+      if (S_ISDIR(perm)) {
         filemess(curbuf, fname, (char_u *)_("is a directory"), 0);
-      else
+      } else {
         filemess(curbuf, fname, (char_u *)_("is not a file"), 0);
+      }
       msg_end();
       msg_scroll = msg_save;
-      return FAIL;
+      return S_ISDIR(perm) ? NOTDONE : FAIL;
     }
 #endif
   }
@@ -5108,7 +5109,7 @@ void buf_reload(buf_T *buf, int orig_mode)
     keep_filetype = TRUE;                     /* don't detect 'filetype' */
     if (readfile(buf->b_ffname, buf->b_fname, (linenr_T)0,
             (linenr_T)0,
-            (linenr_T)MAXLNUM, &ea, flags) == FAIL) {
+            (linenr_T)MAXLNUM, &ea, flags) != OK) {
       if (!aborting())
         EMSG2(_("E321: Could not reload \"%s\""), buf->b_fname);
       if (savebuf != NULL && buf_valid(savebuf) && buf == curbuf) {

src/nvim/memline.c

@@ -1064,7 +1064,7 @@ void ml_recover(void)
               line_count = pp->pb_pointer[idx].pe_line_count;
               if (readfile(curbuf->b_ffname, NULL, lnum,
                       pp->pb_pointer[idx].pe_old_lnum - 1,
-                      line_count, NULL, 0) == FAIL)
+                      line_count, NULL, 0) != OK)
                 cannot_open = TRUE;
               else
                 lnum += line_count;