vim-patch:9.1.1600: using diff anchors with hidden buffers fails silently (#35218)

Problem:  diff: using diff anchors with hidden buffers fails silently
Solution: Give specific error message for diff anchors when using hidden
          buffers (Yee Cheng Chin).

Diff anchors currently will fail to parse if a buffer used for diff'ing
is hidden. Previously it would just fail as the code assumes it would
not happen normally, but this is actually possible to do if `closeoff`
and `hideoff` are not set in diffopt. Git's default diff tool "vimdiff3"
also takes advantage of this.

This fix this properly would require the `{address}` parser to be
smarter about whether a particular address relies on window position or
not (e.g. the `'.` address requires an active window, but `'a` or `1234`
do not). Since hidden diff buffers seem relatively niche, just provide a
better error message / documentation for now. This could be improved
later if there's a demand for it.

related: vim/vim#17615
closes: vim/vim#17904

cad3b2421d

Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>

runtime/doc/options.txt

@@ -2135,6 +2135,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 	If some of the {address} do not resolve to a line in each buffer (e.g.
 	a pattern search that does not match anything), none of the anchors
 	will be used.
+								*E1562*
+	Diff anchors can only be used when there are no hidden diff buffers.
 
 						*'diffexpr'* *'dex'*
 'diffexpr' 'dex'	string	(default "")

runtime/lua/vim/_meta/options.lua

@@ -1736,6 +1736,8 @@ vim.wo.diff = vim.o.diff
 --- If some of the {address} do not resolve to a line in each buffer (e.g.
 --- a pattern search that does not match anything), none of the anchors
 --- will be used.
+--- 							*E1562*
+--- Diff anchors can only be used when there are no hidden diff buffers.
 ---
 --- @type string
 vim.o.diffanchors = ""

src/nvim/diff.c

@@ -2553,8 +2553,13 @@ static int parse_diffanchors(bool check_only, buf_T *buf, linenr_T *anchors, int
         break;
       }
     }
-    if (bufwin == NULL) {
-      return FAIL;  // should not really happen
+    if (bufwin == NULL && *dia != NUL) {
+      // The buffer is hidden. Currently this is not supported due to the
+      // edge cases of needing to decide if an address is window-specific
+      // or not. We could add more checks in the future so we can detect
+      // whether an address relies on curwin to make this more fleixble.
+      emsg(_(e_diff_anchors_with_hidden_windows));
+      return FAIL;
     }
   }
 

src/nvim/errors.h

@@ -191,6 +191,7 @@ EXTERN const char e_invalid_return_type_from_findfunc[] INIT( = N_("E1514: 'find
 EXTERN const char e_cannot_switch_to_a_closing_buffer[] INIT( = N_("E1546: Cannot switch to a closing buffer"));
 EXTERN const char e_cannot_have_more_than_nr_diff_anchors[] INIT( = N_("E1549: Cannot have more than %d diff anchors"));
 EXTERN const char e_failed_to_find_all_diff_anchors[] INIT( = N_("E1550: Failed to find all diff anchors"));
+EXTERN const char e_diff_anchors_with_hidden_windows[] INIT( = N_("E1562: Diff anchors cannot be used with hidden diff windows"));
 
 EXTERN const char e_trustfile[] INIT(= N_("E5570: Cannot update trust file: %s"));
 

src/nvim/options.lua

@@ -2286,6 +2286,8 @@ local options = {
         If some of the {address} do not resolve to a line in each buffer (e.g.
         a pattern search that does not match anything), none of the anchors
         will be used.
+        							*E1562*
+        Diff anchors can only be used when there are no hidden diff buffers.
       ]=],
       full_name = 'diffanchors',
       list = 'onecomma',

test/old/testdir/test_diffmode.vim

@@ -3231,6 +3231,10 @@ func Test_diffanchors_invalid()
   diffupdate
   call assert_equal('orig_search_pat', @/) " also check we don't pollute the search register
   set diffanchors=1/does_not_exist/
+  call assert_fails('diffupdate', 'E1550:')
+  call assert_equal('orig_search_pat', @/)
+
+  " Hidden buffers are not supported right now
   hide
   call assert_fails('diffupdate', 'E1562:')