githubclt: check mergeable_state field when evaluating if a PR is upt…

…odate

The githubclt.PRIsUptodate() can return the wrong result
(#28).

It evaluates if a PR is uptodate with the base branch via information from the
github REST pulls API. It compares the base.sha field of the PR message with the
HEAD SHA that is retrieved for the base branch.

The base.sha returned from the pulls API is the SHA of the base-branch from when
the PR was created.
It is not related to the commits in the PR branch, it is not the most recent
ancestor of both branches.

That means if e.g. a PR is created with a branch based on commit main-HEAD^, the
base.sha in the pulls message will be main-HEAD instead.
The Check will return that the PR is uptodate despite it is not.

To improve the behavior PRIsUptodate() now also checks if the mergeable_state
has the value "behind", which means that the branch is not update.

This does not fix the issue, it only makes it a bit more unlikely to occur.
mergeable_state can have another value like UNSTABLE and the PR might be
outofdate.

internal/githubclt/client.go

@@ -110,6 +110,7 @@ func (clt *Client) CombinedStatus(ctx context.Context, owner, repo, ref string)
 // Additionally it returns the SHA of the head commit for which the status was
 // checked.
 // If the PR is closed PullRequestIsClosedError is returned.
+// Known Bugs: https://github.com/simplesurance/goordinator/issues/28
 func (clt *Client) PRIsUptodate(ctx context.Context, owner, repo string, pullRequestNumber int) (isUptodate bool, headSHA string, err error) {
 	pr, _, err := clt.clt.PullRequests.Get(ctx, owner, repo, pullRequestNumber)
 	if err != nil {
@@ -120,6 +121,15 @@ func (clt *Client) PRIsUptodate(ctx context.Context, owner, repo string, pullReq
 		return false, "", ErrPullRequestIsClosed
 	}
 
+	prHead := pr.GetHead()
+	if prHead == nil {
+		return false, "", errors.New("got pull request object with empty head")
+	}
+
+	if pr.GetMergeableState() == "behind" {
+		return false, prHead.GetSHA(), nil
+	}
+
 	base := pr.GetBase()
 	if base == nil {
 		return false, "", errors.New("got pull request object with empty base")
@@ -133,12 +143,7 @@ func (clt *Client) PRIsUptodate(ctx context.Context, owner, repo string, pullReq
 		return false, "", fmt.Errorf("could not retrieve head SHA of base branch %q: %w", baseBranch, err)
 	}
 
-	head := pr.GetHead()
-	if head == nil {
-		return false, "", errors.New("got pull request object with empty head")
-	}
-
-	return PRbaseSHA == baseBranchHEADSHA, head.GetSHA(), nil
+	return PRbaseSHA == baseBranchHEADSHA, prHead.GetSHA(), nil
 }
 
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