Import "State Retention" page from old web site

Add Ton's "State Retention" RFC to the list of "Proposals" (even though
it has been implemented and documented¹ by now).

¹ https://www.nagios-plugins.org/doc/faq/private-c-api.html#state-information

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+title: State Retention
+parent: Documentation
+---
+
+# State Retention Routines
+
+_Ton Voon, June 16, 2010_
+
+The aim is to create a set of library routines that can be used for saving
+state information between invocations of a plugin.  This way, it is possible
+to calculate the rate of change and provide threshold calculations on this,
+rather than just the current state.
+
+This is based on a patch submitted by Alain Williams,
+Nagios::Plugin::Differences by Jose Luis Martinez and comments on the mailing
+list (see [references](#references)).
+
+Lots of discussion between Holger and I ended up with this.
+
+## Terms
+
+**Location**
+:   Use `./configure` `--sharedstatedir` to define, default `$PREFIX/var`.
+    Override with `NAGIOS_PLUGIN_STATE_DIRECTORY` envvar at runtime if set.
+    Add plugin name to end.
+
+**Key**
+:   Is used as the filename of the store.  Default to `state.dat`.  Recommend
+    that this is set to the string returned by `np_state_generate_key()`, to
+    be unique per plugin call.  Key can only consist of alphanumerics and
+    underscore.
+
+## Format
+
+Example format:
+
+    # NP state file
+    1 [file format version]
+    {data version}
+    {time}
+    {data}
+
+## Structs
+
+### np\_state\_key
+
+    char *name
+    char *plugin_name
+    int data_version
+    char *_filename
+
+### np\_state\_data
+
+    time_t time
+    void *data
+    int length (of binary data)
+
+## Calls
+
+### np\_state\_generate\_key(argv)
+
+Returns a string to use as a `key_name`, based on an MD5 hash of `argv`, thus
+hopefully a unique key per service/plugin invocation.  Use the
+[Extra-Opts][extra-opts] parse of `argv`, so that uniqueness in parameters are
+reflected there.
+
+### np\_state\_init(plugin\_name, key\_name, data\_version)
+
+Sets variables.  Generates filename.  Returns `np_state_key`.  Die with
+`UNKNOWN` if exception.
+
+### np\_state\_read(np\_state\_key)
+
+Returns `np_state_data`.  Will return `NULL` if no data is available (first
+run).  If key currently exists, read data.  If state file format version is
+not expected, return as if no data.  Get state data version number and compare
+to expected.  If numerically lower, then return as no previous state.  Die
+with `UNKNOWN` if exceptional error.
+
+### np\_state\_write\_string(np\_state\_key, time, string)
+
+If `time==NULL`, use current time.  Create state file, with state file format
+version, default text.  Write version, time, and data.  Avoid locking problems
+- use `mv` to write and then swap.  Possible loss of state data if two things
+writing to same key at same time.
+
+### np\_state\_write\_binary(np\_state\_key, time, start, length)
+
+Same as `np_state_write_string()`, but writes binary data.
+
+### np\_state\_data\_cleanup(np\_state\_data)
+
+Cleanup.
+
+### np\_state\_key\_cleanup(np\_state\_key)
+
+Cleanup.
+
+## Notes
+
+- All opens and close within these functions, retaining atomicity.
+- Libtap tests for library.
+- Update [Development Guidelines][guidelines] with library usage.
+- This has problems if a remote host is checked from different Nagios
+  instances.
+- Binary data may not restore on a program compiled with different options
+  from the program that saved it; e.g., 32 or 64 bit.
+- Binary data may include a structure containing a pointer.  Pointer values
+  may not be used in the reading program - i.e., you need to overwrite the
+  value with something `malloc()`ed in the current run of the program.
+- State files could be left lying around.  We recommend you run a regular job
+  to remove unmodified state files older than 1 week.
+
+## References
+
+- <https://www.nagios-plugins.org/archive/devel/2009-September/007767.html>  
+  for the initial patch.
+- <https://www.nagios-plugins.org/archive/devel/2009-September/thread.html#7773>  
+  for a conversation about the patch.
+- <https://www.nagios-plugins.org/archive/devel/2009-September/007749.html>  
+  for Nagios::Plugin::Differences.
+
+[extra-opts]: doc/extra-opts.html "Extra-Opts"
+[guidelines]: doc/guidelines.html "Nagios Plugin Development Guidelines"
+
+<!--% # vim:set filetype=markdown textwidth=78 joinspaces: # %-->