fix(pack): handle lockfile in case of install errors #36064

Problem: If plugin was intended to install but there were errors (like
  if there is a typo in `src`), lockfile still includes its entry.
  This leads to all source of problems (like not correct `get()` output,
  not working `update()`, etc.).

Solution: Explicitly account for plugins that were not installed.
  Alternative solution might be to write a safe
  `lock_set(plug, field, value)` wrapper (which sets field for a correct
  `plugins` entry in the lockfile Lua table) and use it after install
  to detect the change in `version`. However, this always requires
  an extra pass through plugins on every startup, which is suboptimal.
  Optimizing for the "happy path" should be a priority in `add()`.

runtime/lua/vim/pack.lua

@@ -789,6 +789,11 @@ function M.add(specs, opts)
   if #plugs_to_install > 0 then
     git_ensure_exec()
     install_list(plugs_to_install, opts.confirm)
+    for _, p in ipairs(plugs_to_install) do
+      if not p.info.installed then
+        plugin_lock.plugins[p.spec.name] = nil
+      end
+    end
   end
 
   if needs_lock_write then

test/functional/plugin/pack_spec.lua

@@ -541,6 +541,29 @@ describe('vim.pack', function()
       eq(ref_lockfile, get_lock_tbl())
     end)
 
+    it('handles lockfile during install errors', function()
+      local repo_not_exist = 'file://' .. repo_get_path('does-not-exist')
+      pcall_err(exec_lua, function()
+        vim.pack.add({
+          repo_not_exist,
+          { src = repos_src.basic, version = 'not-exist' },
+          { src = repos_src.pluginerr, version = 'main' },
+        })
+      end)
+
+      local pluginerr_hash = git_get_hash('main', 'pluginerr')
+      local ref_lockfile = {
+        -- Should be no entry for `repo_not_exist`
+        plugins = {
+          -- No `rev` because there was no relevant checkout
+          basic = { src = repos_src.basic, version = "'not-exist'" },
+          -- Error during sourcing 'plugin/' should not affect lockfile
+          pluginerr = { rev = pluginerr_hash, src = repos_src.pluginerr, version = "'main'" },
+        },
+      }
+      eq(ref_lockfile, get_lock_tbl())
+    end)
+
     it('installs at proper version', function()
       local out = exec_lua(function()
         vim.pack.add({