Save the lockfile through a temp file and a rename

An interrupted save could leave a truncated lock.json.

lua/arti-pack/lock.lua

@@ -33,15 +33,23 @@ function M:load()
   return self
 end
 
+-- Write then rename, so an interrupted save cannot truncate the live lockfile.
 function M:save()
   vim.fn.mkdir(dirname(self.path), 'p')
   local payload = { version = 1, plugins = self.entries }
-  local fd, err = io.open(self.path, 'w')
+  local tmp = string.format('%s.%d.tmp', self.path, vim.uv and vim.uv.os_getpid() or vim.loop.os_getpid())
+  local fd, err = io.open(tmp, 'w')
   if not fd then
     return false, err
   end
   fd:write(vim.json.encode(payload))
   fd:close()
+
+  local ok, rename_err = os.rename(tmp, self.path)
+  if not ok then
+    os.remove(tmp)
+    return false, rename_err
+  end
   return true
 end
 

tests/lock_spec.lua

@@ -42,6 +42,19 @@ t.describe('lock', function()
     t.eq({}, Lock.new(path):load():names())
   end)
 
+  t.it('leaves no temporary file beside the state file', function()
+    local dir = tmpdir()
+    local lock = Lock.new(dir .. '/lock.json')
+    lock:set('plenary.nvim', { tag = 'v0.1.4' })
+    t.truthy(lock:save())
+
+    local names = {}
+    for _, entry in ipairs(vim.fn.readdir(dir)) do
+      table.insert(names, entry)
+    end
+    t.eq({ 'lock.json' }, names)
+  end)
+
   t.it('lists names in sorted order', function()
     local lock = Lock.new(tmpdir() .. '/lock.json')
     lock:set('zebra', { tag = 'v1' })