patch 8.1.0228: dropping files is ignored while Vim is busy

Problem:    Dropping files is ignored while Vim is busy.
Solution:   Postpone the effect of dropping files until it's safe.

src/ex_docmd.c

@@ -7859,57 +7859,37 @@ ex_shell(exarg_T *eap UNUSED)
     do_shell(NULL, 0);
 }
 
-#if defined(HAVE_DROP_FILE) \
-	|| (defined(FEAT_GUI_GTK) && defined(FEAT_DND)) \
-	|| defined(FEAT_GUI_MSWIN) \
-	|| defined(FEAT_GUI_MAC) \
-	|| defined(PROTO)
+#if defined(HAVE_DROP_FILE) || defined(PROTO)
 
-/*
- * Handle a file drop. The code is here because a drop is *nearly* like an
- * :args command, but not quite (we have a list of exact filenames, so we
- * don't want to (a) parse a command line, or (b) expand wildcards. So the
- * code is very similar to :args and hence needs access to a lot of the static
- * functions in this file.
- *
- * The list should be allocated using alloc(), as should each item in the
- * list. This function takes over responsibility for freeing the list.
- *
- * XXX The list is made into the argument list. This is freed using
- * FreeWild(), which does a series of vim_free() calls.
- */
-    void
-handle_drop(
-    int		filec,		/* the number of files dropped */
-    char_u	**filev,	/* the list of files dropped */
-    int		split)		/* force splitting the window */
+static int drop_busy = FALSE;
+static int drop_filec;
+static char_u **drop_filev = NULL;
+static int drop_split;
+static void (*drop_callback)(void *);
+static void *drop_cookie;
+
+    static void
+handle_drop_internal(void)
 {
     exarg_T	ea;
     int		save_msg_scroll = msg_scroll;
 
-    /* Postpone this while editing the command line. */
-    if (text_locked())
-	return;
-    if (curbuf_locked())
-	return;
-
-    /* When the screen is being updated we should not change buffers and
-     * windows structures, it may cause freed memory to be used. */
-    if (updating_screen)
-	return;
+    // Setting the argument list may cause screen updates and being called
+    // recursively.  Avoid that by setting drop_busy.
+    drop_busy = TRUE;
 
     /* Check whether the current buffer is changed. If so, we will need
      * to split the current window or data could be lost.
      * We don't need to check if the 'hidden' option is set, as in this
      * case the buffer won't be lost.
      */
-    if (!buf_hide(curbuf) && !split)
+    if (!buf_hide(curbuf) && !drop_split)
     {
 	++emsg_off;
-	split = check_changed(curbuf, CCGD_AW);
+	drop_split = check_changed(curbuf, CCGD_AW);
 	--emsg_off;
     }
-    if (split)
+    if (drop_split)
     {
 	if (win_split(0, 0) == FAIL)
 	    return;
@@ -7924,7 +7904,7 @@ handle_drop(
     /*
      * Set up the new argument list.
      */
-    alist_set(ALIST(curwin), filec, filev, FALSE, NULL, 0);
+    alist_set(ALIST(curwin), drop_filec, drop_filev, FALSE, NULL, 0);
 
     /*
      * Move to the first file.
@@ -7942,6 +7922,78 @@ handle_drop(
      * unexpectedly.  The screen will be redrawn by the caller, thus
      * msg_scroll being set by displaying a message is irrelevant. */
     msg_scroll = save_msg_scroll;
+
+    if (drop_callback != NULL)
+	drop_callback(drop_cookie);
+
+    drop_filev = NULL;
+    drop_busy = FALSE;
+}
+
+/*
+ * Handle a file drop. The code is here because a drop is *nearly* like an
+ * :args command, but not quite (we have a list of exact filenames, so we
+ * don't want to (a) parse a command line, or (b) expand wildcards. So the
+ * code is very similar to :args and hence needs access to a lot of the static
+ * functions in this file.
+ *
+ * The "filev" list must have been allocated using alloc(), as should each item
+ * in the list. This function takes over responsibility for freeing the "filev"
+ * list.
+ */
+    void
+handle_drop(
+    int		filec,		// the number of files dropped
+    char_u	**filev,	// the list of files dropped
+    int		split,		// force splitting the window
+    void	(*callback)(void *), // to be called after setting the argument
+				     // list
+    void	*cookie)	// argument for "callback" (allocated)
+{
+    // Cannot handle recursive drops, finish the pending one.
+    if (drop_busy)
+    {
+	FreeWild(filec, filev);
+	vim_free(cookie);
+	return;
+    }
+
+    // When calling handle_drop() more than once in a row we only use the last
+    // one.
+    if (drop_filev != NULL)
+    {
+	FreeWild(drop_filec, drop_filev);
+	vim_free(drop_cookie);
+    }
+
+    drop_filec = filec;
+    drop_filev = filev;
+    drop_split = split;
+    drop_callback = callback;
+    drop_cookie = cookie;
+
+    // Postpone this when:
+    // - editing the command line
+    // - not possible to change the current buffer
+    // - updating the screen
+    // As it may change buffers and window structures that are in use and cause
+    // freed memory to be used.
+    if (text_locked() || curbuf_locked() || updating_screen)
+	return;
+
+    handle_drop_internal();
+}
+
+/*
+ * To be called when text is unlocked, curbuf is unlocked or updating_screen is
+ * reset: Handle a postponed drop.
+ */
+    void
+handle_any_postponed_drop(void)
+{
+    if (!drop_busy && drop_filev != NULL
+		     && !text_locked() && !curbuf_locked() && !updating_screen)
+	handle_drop_internal();
 }
 #endif