feat(sync): backend interface + CLI refactor (#42)

* refactor(sync): extract backend interface, adapt gcal module

Problem: :Pending sync hardcodes Google Calendar — M.sync() does
pcall(require, 'pending.sync.gcal') and calls gcal.sync() directly.
The config has a flat gcal field. This prevents adding new sync backends
without modifying init.lua.

Solution: Define a backend interface contract (name, auth, sync, health
fields), refactor :Pending sync to dispatch via require('pending.sync.'
.. backend_name), add sync table to config with legacy gcal migration,
rename gcal.authorize to gcal.auth, add gcal.health for checkhealth,
and add tab completion for backend names and actions.

* docs(sync): update vimdoc for backend interface

Problem: Vimdoc documents :Pending sync as a bare command that pushes
to Google Calendar, with no mention of backends or the sync table config.

Solution: Update :Pending sync section to show {backend} [{action}]
syntax with examples, add SYNC BACKENDS section documenting the interface
contract, update config example to use sync.gcal, document legacy gcal
migration, and update health check description.

* test(sync): add backend dispatch tests

Problem: No test coverage for sync dispatch logic, config migration,
or gcal module interface conformance.

Solution: Add spec/sync_spec.lua with tests for: bare sync errors,
empty backend errors, unknown backend errors, unknown action errors,
default-to-sync routing, explicit sync/auth routing, legacy gcal config
migration, explicit sync.gcal precedence, and gcal module interface
fields (name, auth, sync, health).

doc/pending.txt

@@ -47,7 +47,7 @@ REQUIREMENTS                                           *pending-requirements*
 
 - Neovim 0.10+
 - No external dependencies for local use
-- `curl` and `openssl` are required for Google Calendar sync
+- `curl` and `openssl` are required for the `gcal` sync backend
 
 ==============================================================================
 INSTALL                                                      *pending-install*
@@ -250,10 +250,23 @@ COMMANDS                                                    *pending-commands*
     Open the list with |:copen| to navigate to each task's category.
 
                                                                *:Pending-sync*
-:Pending sync
-    Push pending tasks that have a due date to Google Calendar as all-day
-    events. Requires |pending-gcal| to be configured. See |pending-gcal| for
-    full details on what gets created, updated, and deleted.
+:Pending sync {backend} [{action}]
+    Run a sync action against a named backend. {backend} is required — bare
+    `:Pending sync` prints a usage message. {action} defaults to `sync`
+    when omitted. Each backend lives at `lua/pending/sync/<name>.lua`.
+
+    Examples: >vim
+        :Pending sync gcal          " runs gcal.sync()
+        :Pending sync gcal auth     " runs gcal.auth()
+        :Pending sync gcal sync     " explicit sync (same as bare)
+<
+
+    Tab completion after `:Pending sync ` lists discovered backends.
+    Tab completion after `:Pending sync gcal ` lists available actions.
+
+    Built-in backends: ~
+
+    `gcal`    Google Calendar one-way push. See |pending-gcal|.
 
                                                                *:Pending-undo*
 :Pending undo
@@ -440,9 +453,11 @@ loads: >lua
         next_task = ']t',
         prev_task = '[t',
       },
-      gcal = {
-        calendar = 'Tasks',
-        credentials_path = '/path/to/client_secret.json',
+      sync = {
+        gcal = {
+          calendar = 'Tasks',
+          credentials_path = '/path/to/client_secret.json',
+        },
       },
     }
 <
@@ -508,10 +523,16 @@ Fields: ~
                              vim.g.pending = { debug = true }
 <
 
+    {sync}               (table, default: {})          *pending.SyncConfig*
+                         Sync backend configuration. Each key is a backend
+                         name and the value is the backend-specific config
+                         table. Currently only `gcal` is built-in.
+
     {gcal}               (table, default: nil)
-                         Google Calendar sync configuration. See
-                         |pending.GcalConfig|. Omit this field entirely to
-                         disable Google Calendar sync.
+                         Legacy shorthand for `sync.gcal`. If `gcal` is set
+                         but `sync.gcal` is not, the value is migrated
+                         automatically. New configs should use `sync.gcal`
+                         instead. See |pending.GcalConfig|.
 
 ==============================================================================
 LUA API                                                          *pending-api*
@@ -632,13 +653,18 @@ not pulled back into pending.nvim.
 
 Configuration: >lua
     vim.g.pending = {
-      gcal = {
-        calendar = 'Tasks',
-        credentials_path = '/path/to/client_secret.json',
+      sync = {
+        gcal = {
+          calendar = 'Tasks',
+          credentials_path = '/path/to/client_secret.json',
+        },
       },
     }
 <
 
+The legacy `gcal` top-level key is still accepted and migrated automatically.
+New configurations should use `sync.gcal`.
+
                                                           *pending.GcalConfig*
 Fields: ~
     {calendar}           (string, default: 'Pendings')
@@ -654,7 +680,7 @@ Fields: ~
                          that Google provides or as a bare credentials object.
 
 OAuth flow: ~
-On the first `:Pending sync` call the plugin detects that no refresh token
+On the first `:Pending sync gcal` call the plugin detects that no refresh token
 exists and opens the Google authorization URL in the browser using
 |vim.ui.open()|. A temporary local HTTP server listens on port 18392 for the
 OAuth redirect. The PKCE (Proof Key for Code Exchange) flow is used —
@@ -664,7 +690,7 @@ authorization code is exchanged for tokens and the refresh token is stored at
 use the stored refresh token and refresh the access token automatically when
 it is about to expire.
 
-`:Pending sync` behavior: ~
+`:Pending sync gcal` behavior: ~
 For each task in the store:
 - A pending task with a due date and no existing event: a new all-day event is
   created and the event ID is stored in the task's `_extra` table.
@@ -677,6 +703,30 @@ For each task in the store:
 A summary notification is shown after sync: `created: N, updated: N,
 deleted: N`.
 
+==============================================================================
+SYNC BACKENDS                                           *pending-sync-backend*
+
+Sync backends are Lua modules under `lua/pending/sync/<name>.lua`. Each
+module returns a table conforming to the backend interface: >lua
+
+    ---@class pending.SyncBackend
+    ---@field name string
+    ---@field auth fun(): nil
+    ---@field sync fun(): nil
+    ---@field health? fun(): nil
+<
+
+Required fields: ~
+    {name}    Backend identifier (matches the filename).
+    {sync}    Main sync action. Called by `:Pending sync <name>`.
+    {auth}    Authorization flow. Called by `:Pending sync <name> auth`.
+
+Optional fields: ~
+    {health}  Called by `:checkhealth pending` to report backend-specific
+              diagnostics (e.g. checking for external tools).
+
+Backend-specific configuration goes under `sync.<name>` in |pending-config|.
+
 ==============================================================================
 HIGHLIGHT GROUPS                                          *pending-highlights*
 
@@ -728,8 +778,8 @@ Checks performed: ~
 - Whether the data directory exists (warning if not yet created)
 - Whether the data file exists and can be parsed; reports total task count
 - Validates recurrence specs on stored tasks
-- Whether `curl` is available (required for Google Calendar sync)
-- Whether `openssl` is available (required for OAuth PKCE)
+- Discovers sync backends under `lua/pending/sync/` and runs each backend's
+  `health()` function if it exists (e.g. gcal checks for `curl` and `openssl`)
 
 ==============================================================================
 DATA FORMAT                                                    *pending-data*

lua/pending/config.lua

@@ -2,6 +2,9 @@
 ---@field calendar? string
 ---@field credentials_path? string
 
+---@class pending.SyncConfig
+---@field gcal? pending.GcalConfig
+
 ---@class pending.Keymaps
 ---@field close? string|false
 ---@field toggle? string|false
@@ -32,6 +35,7 @@
 ---@field drawer_height? integer
 ---@field debug? boolean
 ---@field keymaps pending.Keymaps
+---@field sync? pending.SyncConfig
 ---@field gcal? pending.GcalConfig
 
 ---@class pending.config
@@ -65,6 +69,7 @@ local defaults = {
     next_task = ']t',
     prev_task = '[t',
   },
+  sync = {},
 }
 
 ---@type pending.Config?
@@ -77,6 +82,10 @@ function M.get()
   end
   local user = vim.g.pending or {}
   _resolved = vim.tbl_deep_extend('force', defaults, user)
+  if _resolved.gcal and not (_resolved.sync and _resolved.sync.gcal) then
+    _resolved.sync = _resolved.sync or {}
+    _resolved.sync.gcal = _resolved.gcal
+  end
   return _resolved
 end
 

lua/pending/health.lua

@@ -47,16 +47,18 @@ function M.check()
     vim.health.info('No data file yet (will be created on first save)')
   end
 
-  if vim.fn.executable('curl') == 1 then
-    vim.health.ok('curl found (required for Google Calendar sync)')
+  local sync_paths = vim.fn.globpath(vim.o.runtimepath, 'lua/pending/sync/*.lua', false, true)
+  if #sync_paths == 0 then
+    vim.health.info('No sync backends found')
   else
-    vim.health.warn('curl not found (needed for Google Calendar sync)')
-  end
-
-  if vim.fn.executable('openssl') == 1 then
-    vim.health.ok('openssl found (required for OAuth PKCE)')
-  else
-    vim.health.warn('openssl not found (needed for Google Calendar OAuth)')
+    for _, path in ipairs(sync_paths) do
+      local name = vim.fn.fnamemodify(path, ':t:r')
+      local bok, backend = pcall(require, 'pending.sync.' .. name)
+      if bok and type(backend.health) == 'function' then
+        vim.health.start('pending.nvim: sync/' .. name)
+        backend.health()
+      end
+    end
   end
 end
 

lua/pending/init.lua

@@ -414,14 +414,25 @@ function M.add(text)
   vim.notify('Pending added: ' .. description)
 end
 
+---@param backend_name string
+---@param action? string
 ---@return nil
-function M.sync()
-  local ok, gcal = pcall(require, 'pending.sync.gcal')
-  if not ok then
-    vim.notify('Google Calendar sync module not available.', vim.log.levels.ERROR)
+function M.sync(backend_name, action)
+  if not backend_name or backend_name == '' then
+    vim.notify('Usage: :Pending sync <backend> [action]', vim.log.levels.ERROR)
     return
   end
-  gcal.sync()
+  action = (action and action ~= '') and action or 'sync'
+  local ok, backend = pcall(require, 'pending.sync.' .. backend_name)
+  if not ok then
+    vim.notify('Unknown sync backend: ' .. backend_name, vim.log.levels.ERROR)
+    return
+  end
+  if type(backend[action]) ~= 'function' then
+    vim.notify(backend_name .. " backend has no '" .. action .. "' action", vim.log.levels.ERROR)
+    return
+  end
+  backend[action]()
 end
 
 ---@param days? integer
@@ -700,7 +711,8 @@ function M.command(args)
     local id_str, edit_rest = rest:match('^(%S+)%s*(.*)')
     M.edit(id_str, edit_rest)
   elseif cmd == 'sync' then
-    M.sync()
+    local backend, action = rest:match('^(%S+)%s*(.*)')
+    M.sync(backend, action)
   elseif cmd == 'archive' then
     local d = rest ~= '' and tonumber(rest) or nil
     M.archive(d)

lua/pending/sync/gcal.lua

@@ -3,6 +3,8 @@ local store = require('pending.store')
 
 local M = {}
 
+M.name = 'gcal'
+
 local BASE_URL = 'https://www.googleapis.com/calendar/v3'
 local TOKEN_URL = 'https://oauth2.googleapis.com/token'
 local AUTH_URL = 'https://accounts.google.com/o/oauth2/v2/auth'
@@ -22,7 +24,7 @@ local SCOPE = 'https://www.googleapis.com/auth/calendar'
 ---@return table<string, any>
 local function gcal_config()
   local cfg = config.get()
-  return cfg.gcal or {}
+  return (cfg.sync and cfg.sync.gcal) or cfg.gcal or {}
 end
 
 ---@return string
@@ -199,7 +201,7 @@ local function get_access_token()
   end
   local tokens = load_tokens()
   if not tokens or not tokens.refresh_token then
-    M.authorize()
+    M.auth()
     tokens = load_tokens()
     if not tokens then
       return nil
@@ -218,7 +220,7 @@ local function get_access_token()
   return tokens.access_token
 end
 
-function M.authorize()
+function M.auth()
   local creds = load_credentials()
   if not creds then
     vim.notify(
@@ -514,4 +516,18 @@ function M.sync()
   )
 end
 
+---@return nil
+function M.health()
+  if vim.fn.executable('curl') == 1 then
+    vim.health.ok('curl found (required for gcal sync)')
+  else
+    vim.health.warn('curl not found (needed for gcal sync)')
+  end
+  if vim.fn.executable('openssl') == 1 then
+    vim.health.ok('openssl found (required for gcal OAuth PKCE)')
+  else
+    vim.health.warn('openssl not found (needed for gcal OAuth)')
+  end
+end
+
 return M

plugin/pending.lua

@@ -171,6 +171,34 @@ end, {
     if cmd_line:match('^Pending%s+edit') then
       return complete_edit(arg_lead, cmd_line)
     end
+    if cmd_line:match('^Pending%s+sync') then
+      local after_sync = cmd_line:match('^Pending%s+sync%s+(.*)')
+      if not after_sync then
+        return {}
+      end
+      local parts = {}
+      for part in after_sync:gmatch('%S+') do
+        table.insert(parts, part)
+      end
+      local trailing_space = after_sync:match('%s$')
+      if #parts == 0 or (#parts == 1 and not trailing_space) then
+        local backends = {}
+        local pattern = vim.fn.globpath(vim.o.runtimepath, 'lua/pending/sync/*.lua', false, true)
+        for _, path in ipairs(pattern) do
+          local name = vim.fn.fnamemodify(path, ':t:r')
+          table.insert(backends, name)
+        end
+        table.sort(backends)
+        return filter_candidates(arg_lead, backends)
+      end
+      if #parts == 1 and trailing_space then
+        return filter_candidates(arg_lead, { 'auth', 'sync' })
+      end
+      if #parts >= 2 and not trailing_space then
+        return filter_candidates(arg_lead, { 'auth', 'sync' })
+      end
+      return {}
+    end
     return {}
   end,
 })

spec/sync_spec.lua

@@ -0,0 +1,174 @@
+require('spec.helpers')
+
+local config = require('pending.config')
+local store = require('pending.store')
+
+describe('sync', function()
+  local tmpdir
+  local pending
+
+  before_each(function()
+    tmpdir = vim.fn.tempname()
+    vim.fn.mkdir(tmpdir, 'p')
+    vim.g.pending = { data_path = tmpdir .. '/tasks.json' }
+    config.reset()
+    store.unload()
+    package.loaded['pending'] = nil
+    pending = require('pending')
+  end)
+
+  after_each(function()
+    vim.fn.delete(tmpdir, 'rf')
+    vim.g.pending = nil
+    config.reset()
+    store.unload()
+    package.loaded['pending'] = nil
+  end)
+
+  describe('dispatch', function()
+    it('errors on bare :Pending sync with no backend', function()
+      local msg
+      local orig = vim.notify
+      vim.notify = function(m, level)
+        if level == vim.log.levels.ERROR then
+          msg = m
+        end
+      end
+      pending.sync(nil)
+      vim.notify = orig
+      assert.are.equal('Usage: :Pending sync <backend> [action]', msg)
+    end)
+
+    it('errors on empty backend string', function()
+      local msg
+      local orig = vim.notify
+      vim.notify = function(m, level)
+        if level == vim.log.levels.ERROR then
+          msg = m
+        end
+      end
+      pending.sync('')
+      vim.notify = orig
+      assert.are.equal('Usage: :Pending sync <backend> [action]', msg)
+    end)
+
+    it('errors on unknown backend', function()
+      local msg
+      local orig = vim.notify
+      vim.notify = function(m, level)
+        if level == vim.log.levels.ERROR then
+          msg = m
+        end
+      end
+      pending.sync('notreal')
+      vim.notify = orig
+      assert.are.equal('Unknown sync backend: notreal', msg)
+    end)
+
+    it('errors on unknown action for valid backend', function()
+      local msg
+      local orig = vim.notify
+      vim.notify = function(m, level)
+        if level == vim.log.levels.ERROR then
+          msg = m
+        end
+      end
+      pending.sync('gcal', 'notreal')
+      vim.notify = orig
+      assert.are.equal("gcal backend has no 'notreal' action", msg)
+    end)
+
+    it('defaults to sync action when action is omitted', function()
+      local called = false
+      local gcal = require('pending.sync.gcal')
+      local orig_sync = gcal.sync
+      gcal.sync = function()
+        called = true
+      end
+      pending.sync('gcal')
+      gcal.sync = orig_sync
+      assert.is_true(called)
+    end)
+
+    it('routes explicit sync action', function()
+      local called = false
+      local gcal = require('pending.sync.gcal')
+      local orig_sync = gcal.sync
+      gcal.sync = function()
+        called = true
+      end
+      pending.sync('gcal', 'sync')
+      gcal.sync = orig_sync
+      assert.is_true(called)
+    end)
+
+    it('routes auth action', function()
+      local called = false
+      local gcal = require('pending.sync.gcal')
+      local orig_auth = gcal.auth
+      gcal.auth = function()
+        called = true
+      end
+      pending.sync('gcal', 'auth')
+      gcal.auth = orig_auth
+      assert.is_true(called)
+    end)
+  end)
+
+  describe('config migration', function()
+    it('migrates legacy gcal to sync.gcal', function()
+      config.reset()
+      vim.g.pending = {
+        data_path = tmpdir .. '/tasks.json',
+        gcal = { calendar = 'MyCalendar' },
+      }
+      local cfg = config.get()
+      assert.is_not_nil(cfg.sync)
+      assert.is_not_nil(cfg.sync.gcal)
+      assert.are.equal('MyCalendar', cfg.sync.gcal.calendar)
+    end)
+
+    it('does not overwrite explicit sync.gcal with legacy gcal', function()
+      config.reset()
+      vim.g.pending = {
+        data_path = tmpdir .. '/tasks.json',
+        gcal = { calendar = 'Legacy' },
+        sync = { gcal = { calendar = 'Explicit' } },
+      }
+      local cfg = config.get()
+      assert.are.equal('Explicit', cfg.sync.gcal.calendar)
+    end)
+
+    it('works with sync.gcal and no legacy gcal', function()
+      config.reset()
+      vim.g.pending = {
+        data_path = tmpdir .. '/tasks.json',
+        sync = { gcal = { calendar = 'NewStyle' } },
+      }
+      local cfg = config.get()
+      assert.are.equal('NewStyle', cfg.sync.gcal.calendar)
+    end)
+  end)
+
+  describe('gcal module', function()
+    it('has name field', function()
+      local gcal = require('pending.sync.gcal')
+      assert.are.equal('gcal', gcal.name)
+    end)
+
+    it('has auth function', function()
+      local gcal = require('pending.sync.gcal')
+      assert.are.equal('function', type(gcal.auth))
+    end)
+
+    it('has sync function', function()
+      local gcal = require('pending.sync.gcal')
+      assert.are.equal('function', type(gcal.sync))
+    end)
+
+    it('has health function', function()
+      local gcal = require('pending.sync.gcal')
+      assert.are.equal('function', type(gcal.health))
+    end)
+  end)
+end)