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lib/settings/file_lock.ex
defmodule Settings.FileLock do
@moduledoc """
Cross-process filesystem lock helpers for settings.json.
"""
@doc """
Acquire a lock directory alongside the JSON file at `path`.
Blocks up to `opts[:timeout_ms]` (default 10_000ms), treats locks older than
`opts[:stale_ms]` (default 120_000ms) as stale and reclaims them.
"""
@spec acquire_lock!(path :: binary, opts :: keyword) :: :ok
def acquire_lock!(path, opts \\ []) do
lock_dir = path <> ".lock"
timeout = Keyword.get(opts, :timeout_ms, 10_000)
stale = Keyword.get(opts, :stale_ms, 120_000)
start_time = System.monotonic_time(:millisecond)
do_acquire = fn fun ->
case File.mkdir(lock_dir) do
:ok ->
owner_info =
[
"pid: #{inspect(self())}",
"at: #{DateTime.utc_now() |> DateTime.to_iso8601()}"
]
|> Enum.join("\n")
File.write!(Path.join(lock_dir, "owner"), owner_info)
:ok
{:error, :eexist} ->
age_ms =
case File.stat(lock_dir) do
{:ok, %File.Stat{mtime: mtime}} ->
{{y, m, d}, {hh, mm, ss}} = mtime
file_secs = :calendar.datetime_to_gregorian_seconds({{y, m, d}, {hh, mm, ss}})
now_secs = System.os_time(:second)
(now_secs - file_secs) * 1_000
_ ->
stale + 1
end
if age_ms > stale do
File.rm_rf!(lock_dir)
fun.(fun)
else
elapsed = System.monotonic_time(:millisecond) - start_time
if elapsed >= timeout do
raise "Timeout acquiring file lock for #{path}\n" <>
"waited #{elapsed}ms (timeout is #{timeout}ms)"
else
:timer.sleep(50)
fun.(fun)
end
end
{:error, reason} ->
raise "Unable to create lock directory #{lock_dir}: #{inspect(reason)}"
end
end
do_acquire.(do_acquire)
end
@doc """
Release the lock directory created for `path`.
"""
@spec release_lock!(path :: binary) :: :ok
def release_lock!(path) do
lock_dir = path <> ".lock"
File.rm_rf!(lock_dir)
:ok
end
end