This refactors sequencer_add_exec_commands() to work on a todo_list to
avoid redundant reads and writes to the disk.

An obvious way to do this would be to insert the `exec' command between
the other commands, and reparse it once this is done.  This is not what
is done here.  Instead, the command is appended to the buffer once, and
a new list of items is created.  Items from the old list are copied to
it, and new `exec' items are appended when necessary.  This eliminates
the need to reparse the todo list, but this also means its buffer cannot
be directly written to the disk, hence todo_list_write_to_disk().

sequencer_add_exec_commands() still reads the todo list from the disk,
as it is needed by rebase -p.  todo_list_add_exec_commands() works on a
todo_list structure, and reparses it at the end.

complete_action() still uses sequencer_add_exec_commands() for now.
This will be changed in a future commit.

Signed-off-by: Alban Gruin <alban.gr...@gmail.com>
---
 sequencer.c | 69 +++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 49 insertions(+), 20 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index e12860c047..12a3efeca8 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -4216,6 +4216,50 @@ int sequencer_make_script(FILE *out, int argc, const 
char **argv,
        return 0;
 }
 
+static void todo_list_add_exec_commands(struct todo_list *todo_list,
+                                       const char *commands)
+{
+       struct strbuf *buf = &todo_list->buf;
+       const char *old_buf = buf->buf;
+       size_t commands_len = strlen(commands + strlen("exec ")) - 1;
+       int i, first = 1, nr = 0, alloc = 0;
+       struct todo_item *items = NULL,
+               base_item = {TODO_EXEC, NULL, 0, 0, commands_len, 0};
+
+       strbuf_addstr(buf, commands);
+       base_item.offset_in_buf = buf->len - commands_len - 1;
+       base_item.arg = buf->buf + base_item.offset_in_buf;
+
+       /*
+        * Insert <commands> after every pick. Here, fixup/squash chains
+        * are considered part of the pick, so we insert the commands *after*
+        * those chains if there are any.
+        */
+       for (i = 0; i < todo_list->nr; i++) {
+               enum todo_command command = todo_list->items[i].command;
+               if (todo_list->items[i].arg)
+                       todo_list->items[i].arg = todo_list->items[i].arg - 
old_buf + buf->buf;
+
+               if (command == TODO_PICK && !first) {
+                       ALLOC_GROW(items, nr + 1, alloc);
+                       memcpy(items + nr++, &base_item, sizeof(struct 
todo_item));
+               }
+
+               ALLOC_GROW(items, nr + 1, alloc);
+               memcpy(items + nr++, todo_list->items + i, sizeof(struct 
todo_item));
+               first = 0;
+       }
+
+       /* insert or append final <commands> */
+       ALLOC_GROW(items, nr + 1, alloc);
+       memcpy(items + nr++, &base_item, sizeof(struct todo_item));
+
+       FREE_AND_NULL(todo_list->items);
+       todo_list->items = items;
+       todo_list->nr = nr;
+       todo_list->alloc = alloc;
+}
+
 /*
  * Add commands after pick and (series of) squash/fixup commands
  * in the todo list.
@@ -4224,10 +4268,7 @@ int sequencer_add_exec_commands(const char *commands)
 {
        const char *todo_file = rebase_path_todo();
        struct todo_list todo_list = TODO_LIST_INIT;
-       struct todo_item *item;
-       struct strbuf *buf = &todo_list.buf;
-       size_t offset = 0, commands_len = strlen(commands);
-       int i, first;
+       int res;
 
        if (strbuf_read_file(&todo_list.buf, todo_file, 0) < 0)
                return error(_("could not read '%s'."), todo_file);
@@ -4237,23 +4278,11 @@ int sequencer_add_exec_commands(const char *commands)
                return error(_("unusable todo list: '%s'"), todo_file);
        }
 
-       first = 1;
-       /* insert <commands> before every pick except the first one */
-       for (item = todo_list.items, i = 0; i < todo_list.nr; i++, item++) {
-               if (item->command == TODO_PICK && !first) {
-                       strbuf_insert(buf, item->offset_in_buf + offset,
-                                     commands, commands_len);
-                       offset += commands_len;
-               }
-               first = 0;
-       }
-
-       /* append final <commands> */
-       strbuf_add(buf, commands, commands_len);
-
-       i = write_message(buf->buf, buf->len, todo_file, 0);
+       todo_list_add_exec_commands(&todo_list, commands);
+       res = todo_list_write_to_file(&todo_list, todo_file, NULL, NULL, 0, 0, 
-1, 0);
        todo_list_release(&todo_list);
-       return i;
+
+       return res;
 }
 
 static void todo_list_to_strbuf(struct todo_list *todo_list, struct strbuf 
*buf,
-- 
2.19.1

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