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#27 in Configuration

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Tools for configuration management in Arti


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 ConfigurationTree. 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


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


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