feat(rfds): added RFD #3, "Plugin Architecture Vision"

This commit adds an RFD defining the vision for Hipcheck's planned
addition of support for third party plugins.

Signed-off-by: Andrew Lilley Brinker <[email protected]>

docs/rfds/0003-plugin-architecture-vision.md

@@ -0,0 +1,162 @@
+
+# Plugin Architecture Vision
+
+Currently, all analyses and data sources in Hipcheck are part of Hipcheck
+itself, built into the `hc` binary. This places some constraints on the
+evolution and growth of Hipcheck's capabilities, including:
+
+- __Upstreaming__: Outside contributors who want to add capabilities to
+  Hipcheck have to either make upstream contributions to the project or
+  maintain a fork which includes their changes and must remain synced with
+  upstream.
+- __Intellectual Property Constraint__: Data sources and analyses built
+  into Hipcheck must be open source, because Hipcheck itself is open
+  source.
+- __Language Choice__: Data sources and analyses must be written in Rust,
+  because Hipcheck is written in Rust.
+- __Coordination Cost__: Improvements to data sources and analyses must
+  be coordinated with the Hipcheck project, incurring coordination cost
+  and raising the barrier to contribution.
+
+To address these issues, we'd like to expose a plugin mechanism for
+Hipcheck, so third-parties can add new data sources and new analyses
+without needing to contribute them to Hipcheck directly. These plugins
+can be licensed separately and even remain closed source, they ought to
+be able to be written in languages besides Rust, and for coordination
+the only requirement will be that they comply with the API defined for
+plugins to interoperate with Hipcheck.
+
+## Plugin Contents
+
+The key file for any Hipcheck plugin will be a manifest, which specifies
+the information necessary for Hipcheck to load and run the plugin. This
+manifest ought to include the following information:
+
+- __Publisher__: a string identifying the individual or organization that
+  created the plugin.
+- __Namespace__: a string identifying a namespace owned by the publisher,
+  so publishers can organize their plugins as they see fit.
+- __Name__: a string specifying the name of the plugin.
+- __Version__: a string containing a Semantic Versioning version number.
+- __Dependent Plugins__: The plugins whose data this plugin relies on.
+  - Publisher
+  - Namespace
+  - Name
+  - Version
+- __Kind__: "analysis" or "data"
+- __Interface__: a string indicating what kind of interface ought to be
+  used to interact with this plugin (see the "Interface" section below).
+- __Plugin Interface Version__: a string indicating the version of the
+  Hipcheck plugin interface this plugin is compatible with.
+- __Schema Files__: a map between strings identifying plugin function
+  calls and JSON schema files specifying the schemas of the data
+  returned by those calls.
+- __Entrypoint__: a string with the file path of the file to run to
+  execute the plugin.
+- __Entrypoint Hashes__: a map of hash types to hashes which can be used
+  to validate that the entrypoint file has not been modified from its
+  original source during download.
+
+The specific format of this manifest file (JSON, YAML, KDL, etc.) has
+not yet been decided and is not specified here.
+
+## Interface
+
+One of the open questions for the design of the plugin system is what
+kind of interface ought to be used for Hipcheck to interact with the
+plugins. Currently, we see three possibilities:
+
+- __WebAssembly (WASM)__: Plugins would be compiled WebAssembly files,
+  run by a WebAssembly runtime embedded within Hipcheck.
+- __Inter-Process Communication (IPC)__: Plugins would be run as
+  independent processes, and Hipcheck would use platform-specific IPC
+  mechanisms to communicate with those processes.
+- __C Foreign Function Interface (FFI)__: Plugins would be compiled into
+  ABI-compatible dynamically-loadable libraries, which would be loaded by
+  Hipcheck and then called within the Hipcheck process.
+
+Each of these options represents likely trade-offs in security, stability
+of the communication interface, cross-platform support, performance, and
+more.
+
+One of the follow-on tasks will be to experiment with these options to
+better understand their tradeoffs and limitations. Hipcheck may end up
+supporting more than one option, depending on the outcome of that
+investigation.
+
+## Security
+
+Since Hipcheck, with plugins, would be running potentially untrusted code,
+some precautions ought to be in place to protect users against potentially
+insecure or malicious plugins.
+
+These mechanisms ought to include:
+
+- Checking against hashes provided by users in their configuration file
+  which specifies what plugins to download.
+- Within plugins, checking against hashes for the entrypoint files before
+  executing them.
+- To the greatest extent possible, sandboxing plugins to limit their
+  ability to access the data of other plugins outside of normal Hipcheck-
+  provided API calls, including memory sandboxing and file system
+  sandboxing.
+
+While these will be mitigations, Hipcheck can't feasibly ensure total
+security against malicious plugins, and will still recommend to users that
+they assess plugins before trusting them.
+
+## Hipcheck / Plugin Interaction Flow
+
+The Hipcheck / plugin interaction will be initiated by the user providing
+a configuration file which specifies the plugins it requires to run, and
+the analyses to run from those plugins.
+
+Plugins may themselves be configurable, and part of the flow will include
+asking plugins themselves to validate that the configuration they've been
+provided is usable.
+
+The full flow would run as follows:
+
+- __Configuration Schema Check__: Hipcheck first checks if the configuration
+  file itself meets the schema, and reports errors if not.
+- __Plugin Download__: Hipcheck downloads any requested plugins which are not
+  already downloaded.
+- __Plugin Download Validation__: Hipcheck checks the plugins against their
+  user-provided hashes, and produces errors and halts if the hashes do not
+  match.
+- __Analysis Availability Check__: Hipcheck checks if the analyses requested
+  can all be found (can find the plugin by that producer, with that analysis
+  and that version number), and reports errors if not.
+- __Plugin Configuration Check__: Hipcheck checks if any plugin configuration
+  is correct by passing it to the plugin for validation and reports errors if
+  not.
+- __Data Source Plugin Check__: Hipcheck finds out the data needed from data
+  plugins for all the requested analyses, as reported by them based on their
+  configurations, and reports errors if those data sources can't be found.
+- __Analysis Execution__: Hipcheck then runs the analyses requested by the
+  user, coordinating their execution along with data collection to produce
+  answers as quickly as possible and with minimal data duplication. As it runs
+  each plugin, it will also collect "hints" from plugins to indicate additional
+  information users may want to investigate.
+- __Score Reporting__: Finally, when the final score is reported, it's reported
+  alongside a hash produced from the configuration to limit the degree to which
+  people try to compare scores made from different configurations. This hash is
+  done with the help of plugins, by hashing their fully-initialized
+  configuration value, including any default values.
+
+## User Experience
+
+We do not want the presence of plugins to degrade the user experience. We still
+want to provide rich guidance to users on how to take action based on the
+results of their analyses, and supporting this high quality user experience
+will require interoperation with plugins.
+
+A key mechanism will be the ability for plugins to produce "hints" alongside
+their analysis results, which Hipcheck can provide to users to inform any
+further investigation they wish to perform.
+
+Plugins' configuration will also be used to contextualize scores, as scores in
+Hipcheck are strongly dependent on the configuration of the analyses being run,
+and should not be compared to each other without that context. Scores are an
+expression of _consistency with policy_, and not of an objective universal
+reality.