-- Copyright (c) 2024 Florian Fischer. All rights reserved.
--
-- vis-lockfiles is free software: you can redistribute it and/or modify it under
-- the terms of the GNU General Public License as published by the Free Software
-- Foundation, either version 3 of the License, or (at your option) any later
-- version.
--
-- vis-lockfiles is distributed in the hope that it will be useful, but WITHOUT
-- ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
-- FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
--
-- You should have received a copy of the GNU General Public License along with
-- vis-lockfiles found in the LICENSE file.
-- If not, see <https://www.gnu.org/licenses/>.
-- Helper to execute a command and capture its output
local function capture_cmd(cmd)
  local p = assert(io.popen(cmd, 'r'))
  local s = assert(p:read('*a'))
  local success, _, status = p:close()
  if not success then
    local err = cmd .. ' failed with exit code: ' .. status
    vis:info('vis-lockfiles: ' .. err)
  end
  return s
end

local function lock_file_path(lock_dir, path)
  -- escape '%' first
  local filename = path:gsub('%%', '%%25'):gsub('/', '%%2F')
  return lock_dir .. '/' .. filename .. '.lock'
end

-- determine vis' pid
local vis_pid
do
  local vis_proc_file = io.open('/proc/self/stat', 'r')
  if vis_proc_file then
    vis_pid = vis_proc_file:read('*n')
    vis_proc_file:close()

  else -- fallback if /proc/self/stat is not available
    local out = capture_cmd('sh -c "echo $PPID"')
    vis_pid = tonumber(out)
  end
end
assert(vis_pid)

local function pid_lives(pid)
  local tst = io.open('/proc/' .. pid .. '/stat')
  if tst then
    tst:close()
    return true
  end
  return false
end

-- returns PID owning lock (possibly vis_pid)
local function create_lock_file(lock_dir, lfile_path)
  local lock_pid
  local lock_file = io.open(lfile_path)
  if lock_file then
    lock_pid = lock_file:read('*n')
    lock_file:close()
    if lock_pid == vis_pid or pid_lives(lock_pid) then
      return lock_pid
    end
  end

  -- create the lock file with our pid
  lock_file = io.open(lfile_path, 'w')
  if not lock_file then
    -- retry, this time asserting success; first ensure directory exists
    assert(os.execute('mkdir -p "' .. lock_dir .. '"'))
    lock_file = assert(io.open(lfile_path, 'w'))
  end
  lock_file:write(vis_pid)
  lock_file:close()

  -- detect a possible race
  lock_file = assert(io.open(lfile_path))
  lock_pid = tonumber(lock_file:read('*n'))
  lock_file:close()
  return lock_pid
end

local function delete_lock_file(lfile_path)
  local success, err = os.remove(lfile_path)
  if not success then
    vis:info('vis-lockfiles: ' .. err)
  end
end

return {
  vis_pid = vis_pid,
  capture_cmd = capture_cmd,
  lock_file_path = lock_file_path,
  delete_lock_file = delete_lock_file,
  create_lock_file = create_lock_file,
}