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lib/file_lock.ex
defmodule FileLock do
@moduledoc """
Cross-process filesystem lock helpers for arbitrary files. Uses a lock dir
with atomic stale lock takeover.
API is intentionally small:
- acquire_lock(path)
- release_lock(path)
- with_lock(path, fun, opts \\\\ [])
"""
@timeout_ms 10_000
@stale_ms 120_000
@doc """
Execute `fun` while holding a lock for `path`.
Returns:
- `{:ok, result}` when the callback returns normally
- `{:error, :lock_failed}` when the lock cannot be acquired
- `{:callback_error, exception}` when the callback raises
- any `{:error, reason}` tuple returned by the callback itself
"""
@spec with_lock(binary, (-> any), keyword) ::
{:ok, any}
| {:error, :lock_failed}
| {:error, term}
| {:callback_error, Exception.t()}
def with_lock(path, fun, _opts \\ []) do
case acquire_lock(path) do
:ok ->
try do
{:ok, fun.()}
rescue
e -> {:callback_error, e}
after
release_lock(path)
end
{:error, _reason} ->
{:error, :lock_failed}
end
end
@doc """
Release the lock directory created for `path`.
"""
@spec release_lock(path :: binary) :: :ok
def release_lock(path) do
lock_dir = path <> ".lock"
stale_name = "#{lock_dir}.released.#{:erlang.unique_integer([:positive])}"
case File.rename(lock_dir, stale_name) do
:ok ->
File.rm_rf(stale_name)
:ok
{:error, :enoent} ->
:ok
{:error, _} ->
File.rm_rf(lock_dir)
:ok
end
end
@doc """
Acquire a lock directory alongside the file at `path`.
Returns `:ok` on success, `{:error, reason}` on failure.
"""
@spec acquire_lock(path :: binary) :: :ok | {:error, term}
def acquire_lock(path) do
lock_dir = path <> ".lock"
do_acquire_lock(lock_dir, path, System.monotonic_time(:millisecond))
end
defp do_acquire_lock(lock_dir, path, start_time) do
elapsed = System.monotonic_time(:millisecond) - start_time
if elapsed >= @timeout_ms do
{:error, {:timeout, path, elapsed}}
else
case File.mkdir(lock_dir) do
:ok ->
case write_owner_info(lock_dir) do
:ok ->
:ok
{:retry} ->
:timer.sleep(10)
do_acquire_lock(lock_dir, path, System.monotonic_time(:millisecond))
end
{:error, :eexist} ->
age_ms =
case File.stat(lock_dir, time: :posix) do
{:ok, %File.Stat{mtime: mtime}} when is_integer(mtime) ->
now = System.system_time(:second)
max(0, (now - mtime) * 1_000)
_ ->
@stale_ms + 1
end
if age_ms > @stale_ms do
case attempt_stale_takeover(lock_dir) do
:ok ->
:ok
:retry ->
:timer.sleep(50)
do_acquire_lock(lock_dir, path, start_time)
{:error, reason} ->
{:error, reason}
end
else
:timer.sleep(50)
do_acquire_lock(lock_dir, path, start_time)
end
{:error, reason} ->
{:error, {:mkdir_failed, lock_dir, reason}}
end
end
end
@spec attempt_stale_takeover(binary()) :: :ok | :retry | {:error, term}
defp attempt_stale_takeover(lock_dir) do
temp_lock = "#{lock_dir}.tmp.#{:erlang.unique_integer([:positive])}"
case File.mkdir(temp_lock) do
:ok ->
stale_name = "#{lock_dir}.stale.#{:erlang.unique_integer([:positive])}"
case File.rename(lock_dir, stale_name) do
:ok ->
case File.rename(temp_lock, lock_dir) do
:ok ->
write_owner_info(lock_dir)
Services.Globals.Spawn.spawn(fn ->
"#{lock_dir}.stale.*" |> Path.wildcard() |> Enum.each(&File.rm_rf/1)
end)
:ok
{:error, _} ->
File.rm_rf(temp_lock)
:retry
end
{:error, :enoent} ->
File.rm_rf(temp_lock)
:retry
{:error, reason} ->
File.rm_rf(temp_lock)
{:error, {:rename_failed, stale_name, reason}}
end
{:error, _} ->
:retry
end
end
defp write_owner_info(lock_dir) do
owner_file = Path.join(lock_dir, "owner")
owner_info =
[
"pid: #{inspect(self())}",
"at: #{DateTime.utc_now() |> DateTime.to_iso8601()}"
]
|> Enum.join("\n")
case File.write(owner_file, owner_info) do
:ok -> :ok
{:error, :enoent} -> {:retry}
{:error, reason} -> {:error, {:write_failed, owner_file, reason}}
end
end
end