fix(diff): use mmfile_t in linematch

Problem:

Linematch used to use strchr to navigate a string, however strchr does
not supoprt embedded NULs.

Solution:

Use `mmfile_t` instead of `char *` in linematch and introduce `strnchr()`.

Also remove heap allocations from `matching_char_iwhite()`

Fixes: #30505

1 files changed, 4 insertions, 4 deletions

diff --git a/src/nvim/lua/xdiff.c b/src/nvim/lua/xdiff.c
index 8d791a7e74..b9f96abf73 100644
--- a/src/nvim/lua/xdiff.c
+++ b/src/nvim/lua/xdiff.c
@@ -67,11 +67,11 @@ static void get_linematch_results(lua_State *lstate, mmfile_t *ma, mmfile_t *mb,
                                   int count_a, int start_b, int count_b, bool iwhite)
 {
   // get the pointer to char of the start of the diff to pass it to linematch algorithm
-  const char *diff_begin[2] = { ma->ptr, mb->ptr };
-  int diff_length[2] = { count_a, count_b };
+  mmfile_t ma0 = fastforward_buf_to_lnum(*ma, (linenr_T)start_a + 1);
+  mmfile_t mb0 = fastforward_buf_to_lnum(*mb, (linenr_T)start_b + 1);
 
-  fastforward_buf_to_lnum(&diff_begin[0], (linenr_T)start_a + 1);
-  fastforward_buf_to_lnum(&diff_begin[1], (linenr_T)start_b + 1);
+  const mmfile_t *diff_begin[2] = { &ma0, &mb0 };
+  int diff_length[2] = { count_a, count_b };
 
   int *decisions = NULL;
   size_t decisions_length = linematch_nbuffers(diff_begin, diff_length, 2, &decisions, iwhite);