Change Notes

Overview

This document describes an infrastructural feature called Structured Data plugins. See the DarwinLog.md doc for a description of one such plugin that makes use of this feature.

StructuredDataPlugin

StructuredDataPlugin instances have the following characteristics:

Each plugin instance is bound to a single Process instance.
Each StructuredData feature has a type name that identifies the feature. For instance, the type name for the DarwinLog feature is “DarwinLog”. This feature type name is used in various places.
The process monitor reports the list of supported StructuredData features advertised by the process monitor. Process goes through the list of supported feature type names, and asks each known StructuredDataPlugin if it can handle the feature. The first plugin that supports the feature is mapped to that Process instance for that feature. Plugins are only mapped when the process monitor advertises that a feature is supported.
The feature may send asynchronous messages in StructuredData format to the Process instance. Process instances route the asynchronous structured data messages to the plugin mapped to that feature type, if one exists.
Plugins can request that the Process instance forward on configuration data to the process monitor if the plugin needs/wants to configure the feature. Plugins may call the new Process method
```
virtual Error
ConfigureStructuredData(ConstString type_name,
                        const StructuredData::ObjectSP &config_sp)
```
where type_name is the feature name and config_sp points to the configuration structured data, which may be nullptr.
Plugins for features present in a process are notified when modules are loaded into the Process instance via this StructuredDataPlugin method:
```
virtual void
ModulesDidLoad(Process &process, ModuleList &module_list);
```
Plugins may optionally broadcast their received structured data as an LLDB process-level event via the following new Process call:
```
void
BroadcastStructuredData(const StructuredData::ObjectSP &object_sp,
                        const lldb::StructuredDataPluginSP &plugin_sp);
```
IDE clients might use this feature to receive information about the process as it is running to monitor memory usage, CPU usage, and logging.
Internally, the event type created is an instance of EventDataStructuredData.
In the case where a plugin chooses to broadcast a received StructuredData event, the command-line LLDB Debugger instance listens for them. The Debugger instance then gives the plugin an opportunity to display info to either the debugger output or error stream at a time that is safe to write to them. The plugin can choose to display something appropriate regarding the structured data that time.
Plugins can provide a ProcessLaunchInfo filter method when the plugin is registered. If such a filter method is provided, then when a process is about to be launched for debugging, the filter callback is invoked, given both the launch info and the target. The plugin may then alter the launch info if needed to better support the feature of the plugin.
The plugin is entirely independent of the type of Process-derived class that it is working with. The only requirements from the process monitor are the following feature-agnostic elements:
- Provide a way to discover features supported by the process monitor for the current process.
- Specify the list of supported feature type names to Process. The process monitor does this by calling the following new method on Process:
```
void
MapSupportedStructuredDataPlugins(const StructuredData::Array
                                  &supported_type_names)
```
  The supported_type_names specifies an array of string entries, where each entry specifies the name of a StructuredData feature.
- Provide a way to forward on configuration data for a feature type to the process monitor. This is the manner by which LLDB can configure a feature, perhaps based on settings or commands from the user. The following virtual method on Process (described earlier) does the job:
```
virtual Error
ConfigureStructuredData(ConstString type_name,
                        const StructuredData::ObjectSP &config_sp)
```
- Listen for asynchronous structured data packets from the process monitor, and forward them on to Process via this new Process member method:
```
bool
RouteAsyncStructuredData(const StructuredData::ObjectSP object_sp)
```
StructuredData producers must send their top-level data as a Dictionary type, with a key called ‘type’ specifying a string value, where the value is equal to the StructuredData feature/type name previously advertised. Everything else about the content of the dictionary is entirely up to the feature.
StructuredDataPlugin commands show up under plugin structured-data plugin-name.
StructuredDataPlugin settings show up under plugin.structured-data.{plugin-name}.