patch 8.2.0521: crash when reading a blob fails

Problem:    Crash when reading a blob fails.
Solution:   Avoid keeping a pointer to a freed blob object. (Dominique Pelle,
            closes #5890)  Adjust error messages.

src/filepath.c

@@ -1452,20 +1452,18 @@ f_readfile(typval_T *argvars, typval_T *rettv)
 	    maxline = (long)tv_get_number(&argvars[2]);
     }
 
-    if (blob)
-    {
-	if (rettv_blob_alloc(rettv) == FAIL)
-	    return;
-    }
-    else
-    {
-	if (rettv_list_alloc(rettv) == FAIL)
-	    return;
-    }
+    if ((blob ? rettv_blob_alloc(rettv) : rettv_list_alloc(rettv)) == FAIL)
+	return;
 
     // Always open the file in binary mode, library functions have a mind of
     // their own about CR-LF conversion.
     fname = tv_get_string(&argvars[0]);
+
+    if (mch_isdir(fname))
+    {
+	semsg(_(e_isadir2), fname);
+	return;
+    }
     if (*fname == NUL || (fd = mch_fopen((char *)fname, READBIN)) == NULL)
     {
 	semsg(_(e_notopen), *fname == NUL ? (char_u *)_("<empty>") : fname);
@@ -1476,8 +1474,10 @@ f_readfile(typval_T *argvars, typval_T *rettv)
     {
 	if (read_blob(fd, rettv->vval.v_blob) == FAIL)
 	{
-	    emsg("cannot read file");
+	    semsg(_(e_notread), fname);
+	    // An empty blob is returned on error.
 	    blob_free(rettv->vval.v_blob);
+	    rettv->vval.v_blob = NULL;
 	}
 	fclose(fd);
 	return;

src/testdir/test_blob.vim

@@ -257,6 +257,9 @@ func Test_blob_read_write()
   let br = readfile('Xblob', 'B')
   call assert_equal(b, br)
   call delete('Xblob')
+
+  " This was crashing when calling readfile() with a directory.
+  call assert_fails("call readfile('.', 'B')", 'E17: "." is a directory')
 endfunc
 
 " filter() item in blob

src/version.c

@@ -738,6 +738,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    521,
 /**/
     520,
 /**/