patch 8.1.0988: deleting location list buffer breaks location list window

Problem:    Deleting a location list buffer breaks location list window
            functionality.
Solution:   (Yegappan Lakshmanan, closes #4056)

src/quickfix.c

@@ -3980,6 +3980,27 @@ qf_goto_cwindow(qf_info_T *qi, int resize, int sz, int vertsplit)
     return OK;
 }
 
+/*
+ * Set options for the buffer in the quickfix or location list window.
+ */
+    static void
+qf_set_cwindow_options(void)
+{
+    // switch off 'swapfile'
+    set_option_value((char_u *)"swf", 0L, NULL, OPT_LOCAL);
+    set_option_value((char_u *)"bt", 0L, (char_u *)"quickfix",
+	    OPT_LOCAL);
+    set_option_value((char_u *)"bh", 0L, (char_u *)"hide", OPT_LOCAL);
+    RESET_BINDING(curwin);
+#ifdef FEAT_DIFF
+    curwin->w_p_diff = FALSE;
+#endif
+#ifdef FEAT_FOLDING
+    set_option_value((char_u *)"fdm", 0L, (char_u *)"manual",
+	    OPT_LOCAL);
+#endif
+}
+
 /*
  * Open a new quickfix or location list window, load the quickfix buffer and
  * set the appropriate options for the window.
@@ -4032,23 +4053,16 @@ qf_open_new_cwindow(qf_info_T *qi, int height)
 	// Create a new quickfix buffer
 	(void)do_ecmd(0, NULL, NULL, NULL, ECMD_ONE, ECMD_HIDE, oldwin);
 
-	// switch off 'swapfile'
-	set_option_value((char_u *)"swf", 0L, NULL, OPT_LOCAL);
-	set_option_value((char_u *)"bt", 0L, (char_u *)"quickfix",
-		OPT_LOCAL);
-	set_option_value((char_u *)"bh", 0L, (char_u *)"hide", OPT_LOCAL);
-	RESET_BINDING(curwin);
-#ifdef FEAT_DIFF
-	curwin->w_p_diff = FALSE;
-#endif
-#ifdef FEAT_FOLDING
-	set_option_value((char_u *)"fdm", 0L, (char_u *)"manual",
-		OPT_LOCAL);
-#endif
 	// save the number of the new buffer
 	qi->qf_bufnr = curbuf->b_fnum;
     }
 
+    // Set the options for the quickfix buffer/window (if not already done)
+    // Do this even if the quickfix buffer was already present, as an autocmd
+    // might have previously deleted (:bdelete) the quickfix buffer.
+    if (curbuf->b_p_bt[0] != 'q')
+	qf_set_cwindow_options();
+
     // Only set the height when still in the same tab page and there is no
     // window to the side.
     if (curtab == prevtab && curwin->w_width == Columns)

src/testdir/test_quickfix.vim

@@ -3933,6 +3933,16 @@ func Xqfbuf_test(cchar)
     call assert_match(qfbnum . '  h-  "\[Location List]"', execute('ls'))
     call assert_true(bufloaded(qfbnum))
 
+    " After deleting a location list buffer using ":bdelete", opening the
+    " location list window should mark the buffer as a location list buffer.
+    exe "bdelete " . qfbnum
+    lopen
+    call assert_equal("quickfix", &buftype)
+    call assert_equal(1, getwininfo(win_getid(winnr()))[0].loclist)
+    call assert_equal(wid, getloclist(0, {'filewinid' : 0}).filewinid)
+    call assert_false(&swapfile)
+    lclose
+
     " When the location list is cleared for the window, the buffer should be
     " removed
     call setloclist(0, [], 'f')

src/version.c

@@ -779,6 +779,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    988,
 /**/
     987,
 /**/