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

5 files changed, 20 insertions, 15 deletions

diff --git a/src/nvim/buffer.c b/src/nvim/buffer.c
index d9fdc80c60..5ab96ca67d 100644
--- a/src/nvim/buffer.c
+++ b/src/nvim/buffer.c
@@ -193,10 +193,13 @@ open_buffer (
        * it can be changed there. */
       if (!readonlymode && !bufempty())
         changed();
-      else if (retval != FAIL)
+      else if (retval == OK) {
         unchanged(curbuf, FALSE);
-      apply_autocmds_retval(EVENT_STDINREADPOST, NULL, NULL, FALSE,
-          curbuf, &retval);
+      }
+      if (retval == OK) {
+        apply_autocmds_retval(EVENT_STDINREADPOST, NULL, NULL, FALSE,
+                              curbuf, &retval);
+      }
     }
   }
 
@@ -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 */
 
diff --git a/src/nvim/ex_cmds.c b/src/nvim/ex_cmds.c
index b1404abc05..69eed33736 100644
--- a/src/nvim/ex_cmds.c
+++ b/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);
diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index 19691ccc3a..5ff79bcfef 100644
--- a/src/nvim/ex_docmd.c
+++ b/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
diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index 29114201d2..c075b05b46 100644
--- a/src/nvim/fileio.c
+++ b/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) {
diff --git a/src/nvim/memline.c b/src/nvim/memline.c
index a9a53ebca7..8deae54706 100644
--- a/src/nvim/memline.c
+++ b/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;