fix: revert dev

lua/pending/init.lua

@@ -1141,18 +1141,60 @@ end
 
 ---@class pending.SyncBackend
 ---@field name string
----@field auth fun(): nil
+---@field auth? fun(sub_action?: string): nil
 ---@field push? fun(): nil
 ---@field pull? fun(): nil
 ---@field sync? fun(): nil
 ---@field health? fun(): nil
 
+---@type table<string, pending.SyncBackend>
+local _registered_backends = {}
+
 ---@type string[]?
 local _sync_backends = nil
 
 ---@type table<string, true>?
 local _sync_backend_set = nil
 
+---@param name string
+---@return pending.SyncBackend?
+function M.resolve_backend(name)
+  if _registered_backends[name] then
+    return _registered_backends[name]
+  end
+  local ok, mod = pcall(require, 'pending.sync.' .. name)
+  if ok and type(mod) == 'table' and mod.name then
+    return mod
+  end
+  return nil
+end
+
+---@param backend pending.SyncBackend
+---@return nil
+function M.register_backend(backend)
+  if type(backend) ~= 'table' or type(backend.name) ~= 'string' or backend.name == '' then
+    log.error('register_backend: backend must have a non-empty `name` field')
+    return
+  end
+  local builtin_ok, builtin = pcall(require, 'pending.sync.' .. backend.name)
+  if builtin_ok and type(builtin) == 'table' and builtin.name then
+    log.error('register_backend: backend `' .. backend.name .. '` already exists as a built-in')
+    return
+  end
+  if _registered_backends[backend.name] then
+    log.error('register_backend: backend `' .. backend.name .. '` is already registered')
+    return
+  end
+  _registered_backends[backend.name] = backend
+  _sync_backends = nil
+  _sync_backend_set = nil
+end
+
+---@return table<string, pending.SyncBackend>
+function M.registered_backends()
+  return _registered_backends
+end
+
 ---@return string[], table<string, true>
 local function discover_backends()
   if _sync_backends then
@@ -1169,6 +1211,12 @@ local function discover_backends()
       _sync_backend_set[mod.name] = true
     end
   end
+  for name, _ in pairs(_registered_backends) do
+    if not _sync_backend_set[name] then
+      table.insert(_sync_backends, name)
+      _sync_backend_set[name] = true
+    end
+  end
   table.sort(_sync_backends)
   return _sync_backends, _sync_backend_set
 end
@@ -1177,8 +1225,8 @@ end
 ---@param action? string
 ---@return nil
 local function run_sync(backend_name, action)
-  local ok, backend = pcall(require, 'pending.sync.' .. backend_name)
-  if not ok then
+  local backend = M.resolve_backend(backend_name)
+  if not backend then
     log.error('Unknown sync backend: ' .. backend_name)
     return
   end
@@ -1543,8 +1591,8 @@ function M.auth(args)
   local backends_list = discover_backends()
   local auth_backends = {}
   for _, name in ipairs(backends_list) do
-    local ok, mod = pcall(require, 'pending.sync.' .. name)
-    if ok and type(mod.auth) == 'function' then
+    local mod = M.resolve_backend(name)
+    if mod and type(mod.auth) == 'function' then
       table.insert(auth_backends, { name = name, mod = mod })
     end
   end