Add optional durable runtime NTP server configuration - #225
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Introduce an optional `:servers_file` config key that durably persists the
runtime NTP server list. When configured, `NervesTime.set_ntp_servers/1`
writes the list to the file and it is reloaded on the next boot, taking
precedence over `:servers`. This helps devices that receive NTP servers
dynamically (provisioning, DHCP/ConnMan metadata, user configuration) keep
that choice across reboots.
* On startup, load `:servers_file` when configured and present, falling
back to `:servers` otherwise.
* `set_ntp_servers/1` persists the server list when `:servers_file` is set
and returns `{:error, reason}` if persistence fails.
* An empty list is persisted as an explicit "NTP disabled" marker so that
choice also survives a reboot.
Behavior is unchanged unless `:servers_file` is configured.
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Summary
Adds an optional
:servers_fileconfig key for durably persisting the runtime NTP server list.Today,
NervesTime.set_ntp_servers/1updates the runningntpdconfiguration, but the new server list only lives in GenServer state. After a reboot,NervesTimefalls back toconfig :nerves_time, :servers. This makes it hard to support devices that receive their NTP servers dynamically (provisioning, legacy network migration, DHCP/ConnMan metadata, or user configuration).When
:servers_fileis configured, the list set at runtime is written to that file and reloaded on the next boot, taking precedence over:servers:Behavior
:servers_filewhen configured and present.:serverswhen no persisted file exists.set_ntp_servers/1persists the server list when:servers_fileis configured.[](NTP off) rather than falling back to:servers.set_ntp_servers/1now returns:ok | {:error, term()}) and logged. The running configuration is updated regardless.Behavior is unchanged unless
:servers_fileis configured.Implementation
NervesTime.ServerFilemodule encapsulates load/save. File format is one server per line; blank lines and#comments are ignored. Intermediate directories are created on save.NervesTime.Ntpd.init/1loads fromServerFilefirst, falling back to:servers(then the built-in pool default).set_ntp_servers/1persists before restartingntpdand returns the persistence result.Tests
test/server_file_test.exs— unit tests for load/save round-trip, directory creation, empty-list ("NTP disabled") semantics, comment/blank-line parsing, no-op when unconfigured, and write-failure error + log.test/nerves_time_test.exs— integration tests for startup precedence, fallback to:servers, persist-and-reload across an application restart, and disable-via-empty-list across a restart.All tests pass;
mix format,mix credo --strict, andmix dialyzerare clean.