[5.0.0] mainline fails when a single commit

[JonCubed]

I've come across this problem quite often for new repos and finally looked into it. When I run GitVersion on a new repo where it uses mainline calculation, GitVersion will crash with an exception

 ERROR [08/08/19 6:09:47:47] An unexpected error occurred:
System.InvalidOperationException: Sequence contains no elements
   at System.Linq.Enumerable.First[TSource](IEnumerable`1 source)
   at GitVersion.VersionCalculation.MainlineVersionCalculator.FindMergeBaseBeforeForwardMerge(GitVersionContext context, Commit baseVersionSource, Branch mainline, Commit& mainlineTip) in D:\a\1\s\src\GitVersionCore\VersionCalculation\MainlineVersionCalculator.cs:line 218   at GitVersion.VersionCalculation.MainlineVersionCalculator.FindMainlineModeVersion(BaseVersion baseVersion, GitVersionContext context) in D:\a\1\s\src\GitVersionCore\VersionCalculation\MainlineVersionCalculator.cs:line 38
   at GitVersion.VersionCalculation.NextVersionCalculator.FindVersion(GitVersionContext context) in D:\a\1\s\src\GitVersionCore\VersionCalculation\NextVersionCalculator.cs:line 47
   at GitVersion.GitVersionFinder.FindVersion(GitVersionContext context) in D:\a\1\s\src\GitVersionCore\GitVersionFinder.cs:line 24
   at GitVersion.ExecuteCore.<>c__DisplayClass7_0.<ExecuteInternal>b__0(IRepository repo) in D:\a\1\s\src\GitVersionCore\ExecuteCore.cs:line 111
   at GitVersion.GitPreparer.WithRepository[TResult](Func`2 action) in D:\a\1\s\src\GitVersionCore\GitPreparer.cs:line 93
   at GitVersion.ExecuteCore.ExecuteGitVersion(String targetUrl, String dynamicRepositoryLocation, Authentication authentication, String targetBranch, Boolean noFetch, String workingDirectory, String commitId, Config overrideConfig, Boolean noCache) in D:\a\1\s\src\GitVersionCore\ExecuteCore.cs:line 55
   at GitVersion.SpecifiedArgumentRunner.Run(Arguments arguments, IFileSystem fileSystem) in D:\a\1\s\src\GitVersionExe\SpecifiedArgumentRunner.cs:line 46
   at GitVersion.Program.VerifyArgumentsAndRun() in D:\a\1\s\src\GitVersionExe\Program.cs:line 111

I've trace this back to the following if block with ln223 being the culprit. mergeBase is the current commit and mainlineCommitLog is an empty list but it expects that there are parents commits however it is the first so there are not.

GitVersion/src/GitVersionCore/VersionCalculation/MainlineVersionCalculator.cs

Lines 220 to 231 in 11ee72a

	// detect forward merge and rewind mainlineTip to before it
	if (mergeBase == context.CurrentCommit && !mainlineCommitLog.Contains(mergeBase))
	{
	var mainlineTipPrevious = mainlineTip.Parents.First();
	var message = $"Detected forward merge at {mainlineTip}; rewinding mainline to previous commit {mainlineTipPrevious}";
	Logger.WriteInfo(message);
	// re-do mergeBase detection before the forward merge
	mergeBase = context.Repository.ObjectDatabase.FindMergeBase(context.CurrentCommit, mainlineTipPrevious);
	mainlineTip = GetEffectiveMainlineTip(mainlineCommitLog, mergeBase, mainlineTipPrevious);
	}

This can be fixed if the if statement is adjusted to the following

if (mergeBase == context.CurrentCommit && !mainlineCommitLog.Contains(mergeBase) && mainlineTip.Parents.Any())

I am uncertain if this is an acceptable fix, but if it is I am happy to make a pr for it

[stale]

This issue has been automatically marked as stale because it has not had recent activity. After 30 days from now, it will be closed if no further activity occurs. Thank you for your contributions.

[asbjornu]

Hi, @JonCubed and sorry for the lack of response leading to your issue being closed by stalebot. We would love a PR that fixes this problem, alongside a RepositoryFixture test that ensures the problem is fixed. 👍

[stale]

This issue has been automatically marked as stale because it has not had recent activity. After 30 days from now, it will be closed if no further activity occurs. Thank you for your contributions.

[asbjornu]

Is this still a problem in 5.2.4?

[asbjornu]

#2318 may help alleviate the exception here and give you the fallback strategy.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. After 30 days from now, it will be closed if no further activity occurs. Thank you for your contributions.