[Proposal] TFBuild.PullRequest API

I wrote a build script today that had to check SYSTEM_PULLREQUEST_PULLREQUESTID to see if the current build was a pull request and to form a version string.

This seems like it might be a nice way to bring the TFBuild API to parity with the AppVeyor API. I'm willing to contribute the PR! 😃

Proposal

Source: https://docs.microsoft.com/vsts/build-release/concepts/definitions/build/variables#systempullrequestisfork

namespace Cake.Common.Build.TFBuild.Data
{
    public sealed class TFBuildEnvironmentInfo : TFInfo
    {
+       /// <summary>
+       /// Gets TF pull request info, or <see langword="null"/> if the current build
+       /// is not for a pull request.
+       /// </summary>
+       public TFBuildPullRequestInfo PullRequest { get; }
    }

+   public sealed class TFBuildPullRequestInfo : TFInfo
+   {
+       public bool IsFork => GetEnvironmentBool("SYSTEM_PULLREQUEST_ISFORK");
+
+       public int Id => GetEnvironmentInteger("SYSTEM_PULLREQUEST_PULLREQUESTID");
+
+       public int Number => GetEnvironmentInteger("SYSTEM_PULLREQUEST_PULLREQUESTNUMBER");
+
+       public string SourceBranch => GetEnvironmentInteger("SYSTEM_PULLREQUEST_SOURCEBRANCH");
+
+       public string SourceRepositoryUri => GetEnvironmentInteger("SYSTEM_PULLREQUEST_SOURCEREPOSITORYURI");
+
+       public string TargetBranch => GetEnvironmentInteger("SYSTEM_PULLREQUEST_TARGETBRANCH");
+   }
}

Questions

1. You have IsPullRequest for AppVeyor. Which is preferable?

   • TFBuild.Environment.PullRequest == null
   • TFBuild.Environment.IsPullRequest
   • TFBuild.Environment.PullRequest.IsPullRequest

2. To decide whether PullRequest == null or IsPullRequest should be true, we could check whether BUILD_REASON is PullRequest. However, if possible, it might be most flexible to check for the presence or absence of SYSTEM_PULLREQUEST_PULLREQUESTID.