diagnosticsd daemon

Please see ../README.md for general information.

IPC mechanisms

This daemon uses three IPC mechanisms:

• gRPC - for talking to the diagnostics_processor daemon.
• Mojo - for talking to the browser.
• D-Bus - for receiving Mojo bootstrap requests from the browser.

Class structure

`DiagnosticsdDaemon`
 ^
 |
 v
`DiagnosticsdCore`
 ^
 |
 |   // gRPC-related members:
 +-> `DiagnosticsdGrpcService`
 |       (handles incoming gRPC requests)
 +-> `AsyncGrpcServer<grpc_api::Diagnosticsd>`
 |       (connects `DiagnosticsdGrpcService` with the actual gRPC
 |        pipe)
 +-> `AsyncGrpcClient<grpc_api::DiagnosticsProcessor>`
 |       (sends outgoing gRPC requests through the actual gRPC pipe)
 |
 |   // Mojo-related members:
 +-> `DiagnosticsdMojoService`
 |       (handles incoming Mojo requests and sends outgoing ones)
 +-> `mojo::Binding<mojom::DiagnosticsdService>`
 |       (connects `DiagnosticsdMojoService` with the actual Mojo
 |        pipe)
 |
 |   // D-Bus-related members:
 +-> `DiagnosticsdDBusService`
 |       (handles incoming D-Bus requests)
 +-> `brillo::dbus_utils::DBusObject`
         (connects `DiagnosticsdDBusService` with the actual D-Bus
          pipe)

Classes are generally organized such that they don't know about their owners or siblings in this graph. Instead, these classes are parameterized with delegate(s), which implement these cross-class calls. This allows to test each individual piece of logic separately.