This scan process uses 3 plugins :
-first plugin triggers the scan each time a pull request is created (Pull Request Notifier for Atlassian Stash)
-second plugin creates the prerequisite for SonarQube so it can do the scan (analyze-bitbucket-pull-requests.ps1)
-third plugin report the issues on BitBucket (sonar-stash)

The plugin that triggers the scan is Pull Request Notifier for Atlassian Stash . More informations and documentation about this plugin on
https://github.com/tomasbjerre/pull-request-notifier-for-stash

Pull Request Notifier for Atlassian Stash will trigger a powershell script named analyze-bitbucket-pull-requests.ps1 which can receive as parameters:
-bitBucketUrl = the url to BitBucket
-userName and password are the credentials for BitBucket and by default are environmental variables in windows (BITBUCKET_USER and BITBUCKET_PASSWORD)
-projectKey ,pullRequestBranch ,pullRequestId ,repositoryName are the informations about the pull request that will be scanned
-maximumAllowedIssues = the maximum number of allowed issues . If the number of issues is exceeded no comments will be added to the pull request.The default value is 100
This script downloads the  difference files from the pull-requests and create all the files that SonarQube needs in order for the scan to be done .

The plugin that report the informations to BitBucket is sonar-stash . More informations and documentation about this plugin can be found on https://github.com/AmadeusITGroup/sonar-stash