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Inspect validator puppet architecture


The Validator program sends FIDL commands to control a "puppet" program, which invokes library functionality to modify some state that the Validator then evaluates. For more information about the Inspect Validator, see the README.

The Puppet includes these parts:

  • Serving a FIDL protocol.
  • Unpacking the protocol and making library calls.
  • Building an integration test that includes the Puppet and Validator programs.

This doc focuses on the Inspect Validator Rust Puppet located at //src/diagnostics/validator/inspect/lib/rust/src/main.rs.

FIDL design

The FIDL protocol for Inspect Validator is defined in //src/diagnostics/validator/inspect/fidl/validate.test.fidl. The FIDL protocol corresponds closely to the functions in the Inspect library API which defines actions to be applied to any Inspect API implementation. The FIDL API is written to correspond to the Rust API.

(Note: Inspect APIs are allowed to differ from the Rust API; such APIs may require puppet code architecture modifications.)

Serving FIDL

The main() function performs boilerplate to serve a single FIDL client through run_driver_service(), which receives either Initialize or Act events from the FIDL stream. Act events are unpacked by the Actor object which maintains the state necessary to control the Inspect library.

Actor and the Inspect library

Actor contains an Inspector (the Inspect library's entry-point object), a hashmap of nodes, and a hashmap of properties. It implements one function, act(), which contains a giant match statement ("switch" or "case" in other languages) to invoke each action that the library implements. Puppets can report Unimplemented for actions the library doesn't support.

After the Validator invokes each action, it will test the library's effect on the VMO. The library should handle propagating the effects of actions so that the Validator can see them.

The hashmaps of nodes and properties store values that are returned by the Inspect library. Since Rust is an RAII language that cleans up automatically when reference to memory is lost, failing to store a node or property would cause immediate deletion of that node or property. Also, storing properties allows updating their values in response to FIDL commands.

Testing and the build system

The Validator and Puppet combination should make a hermetic integration test.

Dependencies and names

Validator's BUILD.gn file defines a validator_bin target, which is used by the Rust puppet's BUILD.gn file as a dependency to the test_package() named inspect_validator_test_rust which is the test that exercises the Rust puppet.

The Rust puppet itself is built as a standard rustc_binary. That build rule produces two names, inspect_validator_rust_puppet_bin, which is included in the deps of the test_package() rule, and inspect_validator_rust_puppet, which is included in the binaries of the test_package().

The validator_bin target from the Validator's Build.gn file has a name of validator, which is referred to in the tests of the test_package().


Putting inspect_validator_test_rust in the deps of group("tests") in its BUILD.gn makes it easy to include inspect_validator/lib/rust:tests in the deps of group("tests") of src/diagnostics/BUILD.gn. This will be picked up by the build system and cause the Inspect Validator Rust Puppet test to be run in CQ and CI.

Meta .cmx files

There are the following CMX files in //src/diagnostics/validator/inspect/lib/rust/meta:

  • inspect-validator-rust-puppet.cmx

    Lets the puppet binary run and use the logger. It's referred to in the meta section of test_package("inspect_validator_test_rust").

  • validator.cmx

    This CMX file is implicitly referred to by the tests: name: "validator" that you specified in test_package().

    • sandbox: services specifies the services that the Validator needs to run.
    • program: args supplies command-line arguments to the Validator, including the complete URL of the Rust puppet.
    • program: binary confirms that you want to run the tests: name: "validator".

Running Validator

For information on how to run the Validator against a puppet, see Inspect Validator.