Plugin Architecture GSoC 2017 Final Report

Google Summer of Code 2017 - Final report

Project: Plugin architecture for ScanCode

Yash D. Saraf <yashdsaraf@gmail.com>

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This project’s purpose was to create a decoupled plugin architecture for ScanCode such that it can handle plugins at different stages of a scan and can be coupled at runtime. These stages were,

1. Format:

In this stage, the plugins are supposed to run after the scanning is done and post-scan plugins are called. These plugins could be used for

• converting the scanned output to the given format (say csv, json, etc.)

HOWTO

Here, a plugin needs to add an entry in the scancode_output_writers entry point in the following format
'<format> = <module>:<function>'

• <format> is the format name which will be used as the command line option name (e.g csv or json).
• <module> is a python module which implements the output hook specification.
• <function> is the function to which the scan output will be passed if this plugin is called.

The <format> name will be automatically added to the --format command line option and (if called) the scanned data will be passed to the plugin.

2. Post-scan:

In this stage, the plugins are supposed to run after the scanning is done. Some uses for these plugins were

• summarization of scan outputs
  e.g A post-scan plugin for marking is_source to true for directories with ~90% of source files
• simplification of scan outputs
  e.g The --only-findings option to return files or directories with findings for the requested scans. Files and directories without findings are omitted (not considering basic file information as findings)).
  This option already existed, I just ported it to a post-scan plugin.

HOWTO

Here, a plugin needs to add an entry in the scancode_post_scan entry point in the following format
'<name> = <module>:<function>'

• <name> is the command line option name (e.g only-findings).
• <module> is a python module which implements the post_scan hook specification.
• <function> is the function to which the scanned files will be passed if this plugin is called.

The command line option for this plugin will be automatically created using the <function>'s doctring as its help text and (if called) the scanned files will be passed to the plugin.

3. Pre-scan:

In this stage, the plugins are supposed to run before the scan starts. So the potential uses for these types of plugins were to

• ignore files based on a given pattern (glob)
• ignore files based on their info i.e size, type etc.
• extract archives before scanning

HOWTO

Here, a plugin needs to add an entry in the scancode_pre_scan entry point in the following format
'<name> = <module>:<class>'

• <name> is the command line option name (e.g ignore).
• <module> is a python module which implements the pre_scan hook specification.
• <class> is the class which is instantiated and its appropriate method is invoked if this plugin is called. This needs to extend the plugincode.pre_scan.PreScanPlugin class.

The command line option for this plugin will be automatically created using the <class>'s doctring as its help text.
Since there isn't a single spot where pre-scan plugins can be plugged in, more methods to PreScanPlugin class can be added which can represent different hooks, say to add or delete a scan there might be a method called process_scan.
If a plugin's option is passed by the user, then the <class> is instantiated with the user input and its appropriate aforementioned methods are called.

4. Scan (proper):

In this stage, the plugins are supposed to run before the scan starts and after the pre-scan plugins are called. These plugins would have been used for

• adding or deleting scans
• adding dependency scans (whose data could be used in other scans)

No development has been done for this stage, but it will be quite similar to pre-scan.

Other work:

Group cli options in cli help
Here, the goal was to add command line options to pre-defined groups such that they are displayed in their respective groups when scancode -h or scancode --help is called. This helped to better visually represent the command line options and determine more easily what context they belong to.

Add a Resource class to hold all scanned info for a resource Ongoing
Here, the goal was to create a Resource class such that it holds all the scanned data for a resource (i.e a file or a directory).
This class would go on to eventually encapsulate the caching logic entirely. For now, it just holds the info and path of a resource.

What's left?

• Pre-scan plugin for archive extraction
• Scan (proper) plugins
• More complex post-scan plugins
• Support plugins written in languages other than python

Additionally, all my commits can be found here.