Revert "Backport merged commit SHA instead of individual commits (#313)"

bot/internal/bot/backport.go

@@ -46,7 +46,7 @@ func (b *Bot) Backport(ctx context.Context) error {
 		return trace.BadParameter("automatic backports are only supported for internal contributors")
 	}
 
-	pull, err := b.c.GitHub.GetPullRequest(ctx,
+	pull, err := b.c.GitHub.GetPullRequestWithCommits(ctx,
 		b.c.Environment.Organization,
 		b.c.Environment.Repository,
 		b.c.Environment.Number)
@@ -155,7 +155,7 @@ func (b *Bot) Backport(ctx context.Context) error {
 // BackportLocal executes dry run backport workflow locally. No git commands
 // are actually executed, just printed in the console.
 func (b *Bot) BackportLocal(ctx context.Context, branch string) error {
-	pull, err := b.c.GitHub.GetPullRequest(ctx,
+	pull, err := b.c.GitHub.GetPullRequestWithCommits(ctx,
 		b.c.Environment.Organization,
 		b.c.Environment.Repository,
 		b.c.Environment.Number)
@@ -236,15 +236,16 @@ func (b *Bot) createBackportBranch(ctx context.Context, organization string, rep
 		return trace.Wrap(err)
 	}
 
-	// Cherry-pick the _merged_ commit on the base branch instead of
-	// the individual commits from the PR to the backport branch.
-	if err := git("cherry-pick", pull.MergeCommitSHA); err != nil {
-		// If cherry-pick fails with conflict, abort it, otherwise we
-		// won't be able to switch branch for the next backport.
-		if errAbrt := git("cherry-pick", "--abort"); errAbrt != nil {
-			return trace.NewAggregate(err, errAbrt)
+	// Cherry-pick all commits from the PR to the backport branch.
+	for _, commit := range pull.Commits {
+		if err := git("cherry-pick", commit); err != nil {
+			// If cherry-pick fails with conflict, abort it, otherwise we
+			// won't be able to switch branch for the next backport.
+			if errAbrt := git("cherry-pick", "--abort"); errAbrt != nil {
+				return trace.NewAggregate(err, errAbrt)
+			}
+			return trace.Wrap(err)
 		}
-		return trace.Wrap(err)
 	}
 
 	// Push the backport branch to Github.

bot/internal/bot/bot.go

@@ -50,6 +50,9 @@ type Client interface {
 	// GetPullRequest returns a specific Pull Request.
 	GetPullRequest(ctx context.Context, organization string, repository string, number int) (github.PullRequest, error)
 
+	// GetPullRequestWithCommits returns a specific Pull Request with commits.
+	GetPullRequestWithCommits(ctx context.Context, organization string, repository string, number int) (github.PullRequest, error)
+
 	// CreatePullRequest will create a Pull Request.
 	CreatePullRequest(ctx context.Context, organization string, repository string, title string, head string, base string, body string, draft bool) (int, error)
 

bot/internal/github/github.go

@@ -196,14 +196,11 @@ type PullRequest struct {
 	UnsafeLabels []string
 	// Fork determines if the pull request is from a fork.
 	Fork bool
-	// MergeCommitSHA changes depending on the state of the pull request. Before merging a pull request,
-	// the MergeCommitSHA holds the SHA of the test merge commit. After merging a pull request,
-	// the MergeCommitSHA changes depending on how you merged the pull request:
+	// Commits is a list of commit SHAs for the pull request.
 	//
-	// If merged as a merge commit, MergeCommitSHA represents the SHA of the merge commit.
-	// If merged via a squash, MergeCommitSHA represents the SHA of the squashed commit on the base branch.
-	// If rebased, MergeCommitSHA represents the commit that the base branch was updated to.
-	MergeCommitSHA string
+	// It is only populated if the pull request was fetched using
+	// GetPullRequestWithCommits method.
+	Commits []string
 }
 
 // Branch is a git Branch.
@@ -283,6 +280,27 @@ func (f PullRequestFiles) SourceFiles() (files PullRequestFiles) {
 	return files
 }
 
+// GetPullRequestWithCommits returns the specified pull request with commits.
+func (c *Client) GetPullRequestWithCommits(ctx context.Context, organization string, repository string, number int) (PullRequest, error) {
+	pull, err := c.GetPullRequest(ctx, organization, repository, number)
+	if err != nil {
+		return PullRequest{}, trace.Wrap(err)
+	}
+
+	commits, _, err := c.client.PullRequests.ListCommits(ctx, organization, repository, number, &go_github.ListOptions{})
+	if err != nil {
+		return PullRequest{}, trace.Wrap(err)
+	}
+
+	for _, commit := range commits {
+		if len(commit.Parents) <= 1 { // Skip merge commits.
+			pull.Commits = append(pull.Commits, *commit.SHA)
+		}
+	}
+
+	return pull, nil
+}
+
 // GetPullRequest returns a specific Pull Request.
 func (c *Client) GetPullRequest(ctx context.Context, organization string, repository string, number int) (PullRequest, error) {
 	pull, _, err := c.client.PullRequests.Get(ctx,
@@ -311,11 +329,10 @@ func (c *Client) GetPullRequest(ctx context.Context, organization string, reposi
 			Ref: pull.GetHead().GetRef(),
 			SHA: pull.GetHead().GetSHA(),
 		},
-		UnsafeTitle:    pull.GetTitle(),
-		UnsafeBody:     pull.GetBody(),
-		UnsafeLabels:   labels,
-		Fork:           pull.GetHead().GetRepo().GetFork(),
-		MergeCommitSHA: pull.GetMergeCommitSHA(),
+		UnsafeTitle:  pull.GetTitle(),
+		UnsafeBody:   pull.GetBody(),
+		UnsafeLabels: labels,
+		Fork:         pull.GetHead().GetRepo().GetFork(),
 	}, nil
 }
 
@@ -358,11 +375,10 @@ func (c *Client) ListPullRequests(ctx context.Context, organization string, repo
 					Ref: pull.GetHead().GetRef(),
 					SHA: pull.GetHead().GetSHA(),
 				},
-				UnsafeTitle:    pull.GetTitle(),
-				UnsafeBody:     pull.GetBody(),
-				UnsafeLabels:   labels,
-				Fork:           pull.GetHead().GetRepo().GetFork(),
-				MergeCommitSHA: pull.GetMergeCommitSHA(),
+				UnsafeTitle:  pull.GetTitle(),
+				UnsafeBody:   pull.GetBody(),
+				UnsafeLabels: labels,
+				Fork:         pull.GetHead().GetRepo().GetFork(),
 			})
 		}
 		if resp.NextPage == 0 {