Crate tor_config

tor-config

Tools for configuration management in Arti

Overview

This crate is part of Arti, a project to implement Tor in Rust.

It provides types for handling configuration values, and general machinery for configuration management.

Configuration in Arti

The configuration for the arti command line program, and other programs which embed Arti reusing the configuration machinery, works as follows:

1. We use tor_config::ConfigurationSources to enumerate the various places where configuration information needs to come from, and configure how they are to be read. arti uses ConfigurationSources::from_cmdline.

2. ConfigurationSources::load actually reads all of these sources, parses them (eg, as TOML files), and returns a [config::Config]. This is a tree-structured dynamically typed data structure, mirroring the input configuration structure, largely unvalidated, and containing everything in the input config sources.

3. We call one of the tor_config::resolve family. This maps the input configuration data to concrete ConfigBuilder s for the configuration consumers within the program. (For arti, that’s TorClientConfigBuilder and ArtiBuilder). This mapping is done using the Deserialize implementations on the Builders. resolve then calls the build() method on each of these parts of the configuration which applies defaults and validates the resulting configuration.

   It is important to call resolve once for all the configuration consumers, so that it sees a unified view of which config settings in the input were unrecognized, and therefore may need to be reported to the user. See the example in the load module documentation.

4. The resulting configuration objects (eg, TorClientConfig, ArtiConfig) are provided to the code that must use them (eg, to make a TorClient).

See the tor_config::load module-level documentation. for an example.

Facilities and approaches for particular situations

Lists

When the configuration contains a list of items which the user is likely to want to add entries to piecemeal, modify, filter, and so on, use the list builder helper facilities in the list_builder module.

Configuration items which are conditionally compiled

If the user requests, via the configuration, a feature which is compiled out (due to the non-selection of cargo features), it is usually right to have the code simply ignore it.

This can be achieved by applying the appropriate #[cfg] to configuration fields and structs. The result is that if the user does specify the relevant options, Arti will generate an “unknown configuration item” warning. (In the future it might be nice to provide a message saying what feature was missing.)

Config items which must be detected and rejected even when compiled out

For example, if Arti is compiled without bridge support, a configuration specifying use of bridges should result in failure, rather than a direct connection.

In those cases, you should unconditionally include the configuration fields which must be detected and rejected.

Then provide alternative “when-compiled-out” versions of the types for those fields. (If the field is a list which, when enabled, uses list_builder, provide alternative “when-compiled-out” versions of the entry types.)

The built form of the configuration (Field or Entry in the case of a list), should be a #[non_exhaustive] empty enum. It should implement all the same standard traits as the compiled-in version. So everything will compile. But, since it is an uninhabited type, no such value can ever actually appear.

The builder form (FieldBuilder or EntryBuilder) should be an empty #[non_exhaustive] struct. It should have a trivial Deserialize impl which always returns successfully, and a derived Serialize impl (and the usual traits). This will allow configurations which attempt to specify such a value to be recognised.

To get this to compile, naturally, the builder will have to have a .build() method. This should return ConfigBuildError::Invalid. (it can’t return the uninhabited built type, obviously.) The configuration resolution arrangements are set up to call this, and will report the error.

For an example, see crates/tor-guardmgr/src/bridge_disabled.rs.

---

License: MIT OR Apache-2.0

Re-exports

• pub use cmdline::CmdLine;
• pub use list_builder::MultilineListBuilder;
• pub use list_builder::MultilineListBuilderError;
• pub use load::resolve;
• pub use load::resolve_ignore_warnings;
• pub use load::resolve_return_results;
• pub use sources::ConfigurationSource;
• pub use sources::ConfigurationSources;
• pub use config as config_crate;
• pub use educe;
• pub use serde;

Modules

• cmdline
  Implement a configuration source based on command-line arguments.
• list_builder
  Lists in builders
• load
  Processing a config::Config into a validated configuration
• sources
  ConfigurationSources: Helper for handling configuration files

Macros

• convert_helper_via_multi_line_list_builder
  Implement TryFrom<MultilineListBuilder> and Into<MultilineListBuilder> for $Builder.
• define_list_builder_accessors
  Define accessor methods for a configuration item which is a list
• define_list_builder_helper
  Define a list builder struct for use with define_list_builder_accessors
• derive_adhoc_template_Flattenable
  Derives Flattenable for a struct
• impl_standard_builder
  Defines standard impls for a struct with a Builder, incl Default
• macro_first_nonempty
  Helper for assisting with macro “argument” defaulting
• paste

Structs

• CfgPath
  A path in a configuration file: tilde expansion is performed, along with expansion of certain variables.
• ConfigError
  Wrapper for [config::ConfigError] with a more helpful error message.
• Flatten
  Helper for flattening deserialisation, compatible with serde_ignored
• InvalidBoolOrAuto
  Boolean or "auto" configuration is invalid
• Listen
  Specification of (possibly) something to listen on (eg, a port, or some addresses/ports)
• ListenUnsupported
  Listen configuration specified something not supported by application code
• MutCfg
  A mutable configuration object.

Enums

• BoolOrAuto
  Boolean, but with additional "auto" option
• CfgPathError
  An error that has occurred while expanding a path.
• ConfigBuildError
  An error related to an option passed to Arti via a configuration builder.
• InvalidListen
  Listen configuration is invalid
• PaddingLevel
  Padding enablement - rough amount of padding requested
• Reconfigure
  Rules for reconfiguring a running Arti instance.
• ReconfigureError
  An error caused when attempting to reconfigure an existing Arti client, or one of its modules.

Traits

• Flattenable
  Types that can be used with Flatten
• Itertools
  An Iterator blanket implementation that provides extra adaptors and methods.

Functions

• resolve_alternative_specs
  Helper for resolving a config item which can be specified in multiple ways
• resolve_option
  Resolves an Option<Option<T>> (in a builder) into an Option<T>
• resolve_option_general
  Resolves an Option<Option<T>> (in a builder) into an Option<T>, more generally