tools/internal/github: support loading PR diffs for merged PRs

Previously, you could only load PSLs for open PRs. This extends the
PR loading logic to also find the correct state for merged PRs.

tools/internal/github/pr.go

@@ -60,52 +60,49 @@ func (c *Client) PSLForPullRequest(ctx context.Context, prNum int) (withoutPR, w
 		return nil, nil, err
 	}
 
-	if state := pr.GetState(); state != "open" {
-		return nil, nil, fmt.Errorf("cannot get PSL for PR %d with status %q", prNum, state)
+	mergeCommit := pr.GetMergeCommitSHA()
+	if mergeCommit == "" {
+		return nil, nil, fmt.Errorf("no merge commit available for PR %d", prNum)
 	}
-	if !pr.GetMergeable() {
-		return nil, nil, fmt.Errorf("cannot get PSL for PR %d, needs rebase", prNum)
-	}
-	trialMergeCommit := pr.GetMergeCommitSHA()
-	if trialMergeCommit == "" {
-		return nil, nil, fmt.Errorf("no trial merge commit available for PR %d", prNum)
-	}
-
-	prHeadCommit := pr.GetHead().GetSHA()
-	if prHeadCommit == "" {
-		return nil, nil, fmt.Errorf("no commit SHA available for head of PR %d", prNum)
-	}
-
-	// We want to return the trial merge commit's PSL as withPR, and
-	// the non-PR parent of that merge as withoutPR. Github only
-	// provides information about the trial merge commit and the PR
-	// head commit in the PR API. It also provides a "base" ref, but
-	// empirical evidence shows this points at some random commit
-	// somewhere and updates based on unclear triggers. IOW, it is
-	// _not_ "master without the PR applied".
-	//
-	// Instead, we have to ask the git API for information about the
-	// trial merge commit, and find the correct withoutPR SHA from
-	// that.
-	commitInfo, _, err := c.apiClient().Git.GetCommit(ctx, c.owner(), c.repo(), trialMergeCommit)
+	commitInfo, _, err := c.apiClient().Git.GetCommit(ctx, c.owner(), c.repo(), mergeCommit)
 	if err != nil {
-		return nil, nil, fmt.Errorf("getting info for trial merge SHA %q: %w", trialMergeCommit, err)
+		return nil, nil, fmt.Errorf("getting info for trial merge SHA %q: %w", mergeCommit, err)
 	}
+
 	var beforeMergeCommit string
-	if numParents := len(commitInfo.Parents); numParents != 2 {
-		return nil, nil, fmt.Errorf("unexpected parent count %d for trial merge commit on PR %d, expected 2 parents", numParents, prNum)
-	}
-	if commitInfo.Parents[0].GetSHA() == prHeadCommit {
-		beforeMergeCommit = commitInfo.Parents[1].GetSHA()
-	} else {
+	if pr.GetMerged() && len(commitInfo.Parents) == 1 {
+		// PR was merged, PSL policy is to use squash-and-merge, so
+		// the pre-PR commit is simply the parent of the merge commit.
 		beforeMergeCommit = commitInfo.Parents[0].GetSHA()
+	} else if !pr.GetMergeable() {
+		// PR isn't merged, and there's a merge conflict that prevents
+		// us from knowing what the pre- and post-merge states are.
+		return nil, nil, fmt.Errorf("cannot get PSL for PR %d, needs rebase", prNum)
+	} else {
+		// PR is open, which means the merge commit is a "trial merge"
+		// that shows what would happen if you merged the PR. The
+		// trial merge commit has 2 parents, one is the PR head commit
+		// and the other is master without the PR's changes.
+		if numParents := len(commitInfo.Parents); numParents != 2 {
+			return nil, nil, fmt.Errorf("unexpected parent count %d for trial merge commit on PR %d, expected 2 parents", numParents, prNum)
+		}
+
+		prHeadCommit := pr.GetHead().GetSHA()
+		if prHeadCommit == "" {
+			return nil, nil, fmt.Errorf("no commit SHA available for head of PR %d", prNum)
+		}
+		if commitInfo.Parents[0].GetSHA() == prHeadCommit {
+			beforeMergeCommit = commitInfo.Parents[1].GetSHA()
+		} else {
+			beforeMergeCommit = commitInfo.Parents[0].GetSHA()
+		}
 	}
 
 	withoutPR, err = c.PSLForHash(ctx, beforeMergeCommit)
 	if err != nil {
 		return nil, nil, err
 	}
-	withPR, err = c.PSLForHash(ctx, trialMergeCommit)
+	withPR, err = c.PSLForHash(ctx, mergeCommit)
 	if err != nil {
 		return nil, nil, err
 	}