[tor-commits] [tor/master] High-level documentation of configuration in Tor.

[nickm at torproject.org]

commit 0e4e96b9bf56d3fdbd8db7b4cf5b66b4a34bf4d3
Author: Nick Mathewson <nickm at torproject.org>
Date:   Fri Nov 15 10:49:25 2019 -0500

    High-level documentation of configuration in Tor.
    
    Closes ticket 32209.
---
 src/lib/conf/config.md | 126 +++++++++++++++++++++++++++++++++++++++++++++++++
 src/mainpage.md        |   3 +-
 2 files changed, 128 insertions(+), 1 deletion(-)

diff --git a/src/lib/conf/config.md b/src/lib/conf/config.md
new file mode 100644
index 000000000..4dff316dc
--- /dev/null
+++ b/src/lib/conf/config.md
@@ -0,0 +1,126 @@
+
+ at page configuration Configuration options and persistent state
+
+ at tableofcontents
+
+## Introduction
+
+Tor uses a shared, table-driven mechanism to handle its
+configuration (torrc) files and its state files.  Each module can
+declare a set of named fields for these files, and get notified
+whenever the configuration changes, or when the state is about to be
+flushed to disk.
+
+## Declaring options
+
+Most modules will only need to use the macros in confdecl.h to
+declare a configuration or state structure.
+
+You'll write something like this:
+
+    // my_module_config.inc
+    BEGIN_CONF_STRUCT(module_options_t)
+    CONF_VAR(FieldOne, INT, 0, "7")
+    CONF_VAR(FieldTwo, STRING, 0, NULL)
+    END_CONF_STRUCT(module_options_t)
+
+The above example will result in a structure called module_config_t
+with two fields: one an integer called FieldOne and one a string
+called FieldTwo.  The integer gets a default value of 7; the
+string's default value is NULL.
+
+After making a definition file like that, you include it twice: once
+in a header, after saying \#define CONF_CONTEXT STRUCT, and once in
+a C file, after saying \#define CONF_CONTEXT TABLE.  The first time
+defines a module_options_t structure, and the second time defines a
+table that tells the configuration manager how to use it.
+
+Using the table, you declare a `const` config_format_t, which
+associates the fields with a set of functions for validating and
+normalizing them, a list of abbreviations and deprecations, and
+other features.
+
+See confdecl.h and conftypes.h for more information. For example
+usage, see crypto_options.inc or mainloop_state.inc.
+
+## Getting notifications
+
+After using those macros, you must tell the subsystem management
+code about your module's configuration/state.
+
+If you're writing configuration code, you'll need a function that
+receives the configuration object, and acts upon it.  This function
+needs to be safe to call multiple times, since Tor will reconfigure
+its subsystems whenever it, or whenever it restarts in process.
+This function goes in your subsystem's subsys_fns_t.set_options
+field.
+
+If you're writing state code, you'll need a function that receives
+state (subsys_fns_t.set_state), and a function that flushes the
+application state into a state object (subsys_fns_t.flush_state).
+The `set_state` function will be called once (@ref config_once_per
+"1") when Tor is starting, whereas the `flush_state` function will
+be called whenever Tor is about to save the state to disk.
+
+See subsys_fns_t for more information here, and \ref initialization
+for more information about initialization and subsystems in general.
+
+> @anchor config_once_per 1. Technically, state is set once _per startup_.
+> Remember that Tor can be stopped and started multiple times in
+> the same process.  If this happens, then your set_state() function
+> is called once every time Tor starts.
+
+## How it works
+
+The common logic used to handle configuration and state files lives
+in @refdir{lib/confmgt}.  At the highest level, a configuration
+manager object (config_mgr_t) maintains a list of each module's
+configuration objects, and a list of all their fields.  When the
+user specifies a configuration value, the manager finds out how to
+parse it, where to store it, and which configuration object is
+affected.
+
+The top-level configuration module (config.c) and state module
+(statefile.c) use config_mgr_t to create, initialize, set, compare,
+and free a "top level configuration object".  This object contains a
+list of sub-objects: one for each module that participates in the
+configuration/state system.  This top-level code then invokes the
+subsystem manager code (subsysmgr.c) to pass the corresponding
+configuration or state objects to each module that has one.
+
+Note that the top level code does not have easy access to the
+configuration objects used by the sub-modules.  This is by design.
+A module _may_ expose its configuration or state object, if it
+likes, but if it does not, that object should be considered
+module-local.
+
+## Adding new types
+
+Configuration and state fields each have a "type".  These types
+specify how the fields' values are represented in C; how they are
+stored in files; and how they are encoded back and forth.
+
+There is a set of built-in types listed in conftypes.h, but
+higher-level code can define its own types.  To do so, you make an
+instance of var_type_fns_t that describes how to manage your type,
+and an instance of var_type_def_t that wraps your var_type_fns_t
+with a name and optional parameters and flags.
+
+For an example of how a higher-level type is defined, see
+ROUTERSET_type_defn in routerset.c.  Also see the typedef
+`config_decl_ROUTERSET`.  Together, these let the routerset type be
+used with the macros in confdecl.h.
+
+## Legacy configuration and state
+
+As of this writing, most of the configuration and state is still
+handled directly in config.c and statefile.c, and stored in the
+monolithic structures or_options_t and or_state_t respectively.
+
+These top-level structures are accessed with get_options() and
+get_state(), and used throughout much of the code, at the level of
+ at refdir{core} and higher.
+
+With time we hope to refactor this configuration into more
+reasonable pieces, so that they are no longer (effectively) global
+variables used throughout the code.
diff --git a/src/mainpage.md b/src/mainpage.md
index a2c5ec630..c5cb1af23 100644
--- a/src/mainpage.md
+++ b/src/mainpage.md
@@ -41,6 +41,8 @@ Tor repository.
 
 @subpage time_periodic
 
+ at subpage configuration
+
 
 @page intro A high-level overview
 
@@ -140,4 +142,3 @@ more connection types.
 
 A 'Node' (node_t) is a view of a Tor instance's current knowledge and opinions
 about a Tor relay or bridge.
-