commit 0e4e96b9bf56d3fdbd8db7b4cf5b66b4a34bf4d3 Author: Nick Mathewson <nickm@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 @@ + +@page configuration Configuration options and persistent state + +@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 +@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 +@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. -