refactor(git): convert blocking subprocess calls to async

lua/git/diff.lua

@@ -59,47 +59,62 @@ local function attach_index_writer(buf, worktree, path)
     })
 end
 
+---Run `git show <revspec>` asynchronously and call `callback(lines?)` on the
+---main loop. `lines` is nil on failure (the error is logged).
 ---@param worktree string
 ---@param revspec string  anything `git show` accepts (e.g. `HEAD:foo`, `:foo`, blob SHA)
----@return string[]
-local function read_show(worktree, revspec)
-    local result = vim.system(
+---@param callback fun(lines: string[]?)
+local function read_show_async(worktree, revspec, callback)
+    vim.system(
         { "git", "show", revspec },
-        { cwd = worktree, text = true }
+        { cwd = worktree, text = true },
+        vim.schedule_wrap(function(result)
+            if result.code ~= 0 then
+                log.error(
+                    "git show %s failed: %s",
+                    revspec,
+                    vim.trim(result.stderr or "")
+                )
+                callback(nil)
+                return
+            end
+            callback(util.split_lines(result.stdout or ""))
+        end)
     )
-        :wait()
-    if result.code ~= 0 then
-        log.error(
-            "git show %s failed: %s",
-            revspec,
-            vim.trim(result.stderr or "")
-        )
-        return {}
-    end
-    return util.split_lines(result.stdout or "")
 end
 
----Internal builder: run `git show <revspec>`, drop the result into a fresh
----scratch buffer, and (when `is_index` is true) wire up the BufWriteCmd
----that writes back to the git index for `index_path`.
+---Internal builder: create a scratch buffer immediately and asynchronously
+---fill it with the content of `git show <revspec>`. Returning the buffer
+---synchronously lets callers wire up windows / `:diffthis` right away; the
+---diff updates when the content arrives. The buffer starts non-modifiable
+---and the index `BufWriteCmd` is only attached after a successful load, so
+---a premature `:w` can't blow away the index entry with empty content.
 ---@param worktree string
 ---@param revspec string
 ---@param is_index boolean
 ---@param index_path string?  required when is_index is true
 ---@return integer
 local function build_show_buf(worktree, revspec, is_index, index_path)
-    local lines = read_show(worktree, revspec)
     local buf = vim.api.nvim_create_buf(false, true)
-    vim.api.nvim_buf_set_lines(buf, 0, -1, false, lines)
-    vim.bo[buf].buftype = is_index and "acwrite" or "nofile"
+    vim.bo[buf].buftype = "nofile"
     vim.bo[buf].bufhidden = "wipe"
     vim.bo[buf].swapfile = false
-    if is_index then
-        attach_index_writer(buf, worktree, assert(index_path))
-    else
-        vim.bo[buf].modifiable = false
-    end
+    vim.bo[buf].modifiable = false
     vim.bo[buf].modified = false
+    read_show_async(worktree, revspec, function(lines)
+        if not vim.api.nvim_buf_is_valid(buf) then
+            return
+        end
+        vim.bo[buf].modifiable = true
+        vim.api.nvim_buf_set_lines(buf, 0, -1, false, lines or {})
+        if is_index then
+            vim.bo[buf].buftype = "acwrite"
+            attach_index_writer(buf, worktree, assert(index_path))
+        else
+            vim.bo[buf].modifiable = false
+        end
+        vim.bo[buf].modified = false
+    end)
     return buf
 end
 
@@ -192,12 +207,14 @@ function M.split(opts)
     local label = is_index and "index" or opts.ref
     M.set_buf_name_and_filetype(other, "git://" .. label .. "/" .. rel)
 
-    local split_cmd = opts.vertical and "leftabove vertical sbuffer "
-        or "leftabove sbuffer "
-    vim.cmd(split_cmd .. other)
-    vim.cmd("diffthis")
-    vim.cmd("wincmd p")
-    vim.cmd("diffthis")
+    local cur_win = vim.api.nvim_get_current_win()
+    local other_win = vim.api.nvim_open_win(other, true, {
+        split = opts.vertical and "left" or "above",
+        win = cur_win,
+    })
+    vim.wo[other_win].diff = true
+    vim.api.nvim_set_current_win(cur_win)
+    vim.wo[cur_win].diff = true
 end
 
 return M