feat: add GitLab squash merge support for automerge

- Add merge method support to GitLab client for squash merging
- Parse merge method from PullRequestOptions in DiscardReviews and MergePull
- Map 'squash' merge method to GitLab API squash parameter
- Add comprehensive test coverage for merge method handling
- Follow established GitLab client test patterns with json.NewDecoder
- Update documentation to reflect GitLab squash merge support

Fixes #5415

Signed-off-by: geoff-kruss <[email protected]>

Author: geoff-kruss

runatlantis.io/docs/using-atlantis.md

@@ -179,7 +179,7 @@ atlantis apply -w staging
 * `-p project` Apply the plan for this project. Refers to the name of the project configured in the repo's [`atlantis.yaml` file](repo-level-atlantis-yaml.md). Cannot be used at same time as `-d` or `-w`.
 * `-w workspace` Apply the plan for this [Terraform workspace](https://developer.hashicorp.com/terraform/language/state/workspaces). Ignore this if Terraform workspaces are unused.
 * `--auto-merge-disabled` Disable [automerge](automerging.md) for this apply command.
-* `--auto-merge-method method` Specify which [merge method](automerging.md#how-to-set-the-merge-method-for-automerge) use for the apply command if [automerge](automerging.md) is enabled. Implemented only for GitHub.
+* `--auto-merge-method method` Specify which [merge method](automerging.md#how-to-set-the-merge-method-for-automerge) to use for the apply command if [automerge](automerging.md) is enabled. Supported for GitHub and GitLab VCS providers.
 * `--verbose` Append Atlantis log to comment.
 
 ### Additional Terraform flags

server/events/models/models.go

@@ -186,13 +186,13 @@ type PullRequest struct {
 	BaseRepo Repo
 }
 
-// PullRequestOptions is used to set optional paralmeters for PullRequest
+// PullRequestOptions is used to set optional parameters for PullRequest
 type PullRequestOptions struct {
 	// When DeleteSourceBranchOnMerge flag is set to true VCS deletes the source branch after the PR is merged
 	// Applied by GitLab & AzureDevops
 	DeleteSourceBranchOnMerge bool
-	// MergeMethod specifies the merge method for the VCS
-	// Implemented only for Github
+	// MergeMethod specifies the merge method for the VCS (merge, squash, rebase)
+	// Supported by GitHub and GitLab VCS providers
 	MergeMethod string
 }
 

server/events/vcs/gitlab_client.go

@@ -612,12 +612,19 @@ func (g *GitlabClient) MergePull(logger logging.SimpleLogging, pull models.PullR
 		g.WaitForSuccessPipeline(logger, context.Background(), pull)
 	}
 
+	// Handle merge method
+	// Gitlab currently only supports squash as a merge method
+	// via a dedicated field
+	// https://docs.gitlab.com/api/merge_requests/#merge-a-merge-request
+	squash := pullOptions.MergeMethod == "squash"
+	
 	_, resp, err = g.Client.MergeRequests.AcceptMergeRequest(
 		pull.BaseRepo.FullName,
 		pull.Num,
 		&gitlab.AcceptMergeRequestOptions{
 			MergeCommitMessage:       &commitMsg,
 			ShouldRemoveSourceBranch: &pullOptions.DeleteSourceBranchOnMerge,
+			Squash:                   &squash,
 		})
 	if resp != nil {
 		logger.Debug("PUT /projects/%s/merge_requests/%d/merge returned: %d", pull.BaseRepo.FullName, pull.Num, resp.StatusCode)

server/events/vcs/gitlab_client_test.go

@@ -300,6 +300,103 @@ func TestGitlabClient_MergePull(t *testing.T) {
 	}
 }
 
+func TestGitlabClient_MergePullSquash(t *testing.T) {
+	logger := logging.NewNoopLogger(t)
+	mergeSuccess, err := os.ReadFile("testdata/github-pull-request.json")
+	Ok(t, err)
+
+	pipelineSuccess, err := os.ReadFile("testdata/gitlab-pipeline-success.json")
+	Ok(t, err)
+
+	projectSuccess, err := os.ReadFile("testdata/gitlab-project-success.json")
+	Ok(t, err)
+
+	cases := []struct {
+		description string
+		mergeMethod string
+		expSquash   bool
+	}{
+		{
+			"squash merge method",
+			"squash",
+			true,
+		},
+		{
+			"merge method (default)",
+			"merge", 
+			false,
+		},
+		{
+			"empty merge method",
+			"",
+			false,
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.description, func(t *testing.T) {
+			var mergeRequestBody map[string]interface{}
+			testServer := httptest.NewServer(
+				http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+					switch r.RequestURI {
+					// The first request should hit this URL.
+					case "/api/v4/projects/runatlantis%2Fatlantis/merge_requests/1/merge":
+						// Capture the request body to verify squash parameter
+						json.NewDecoder(r.Body).Decode(&mergeRequestBody) // nolint: errcheck
+						w.WriteHeader(http.StatusOK)
+						w.Write(mergeSuccess) // nolint: errcheck
+					case "/api/v4/projects/runatlantis%2Fatlantis/merge_requests/1":
+						w.WriteHeader(http.StatusOK)
+						w.Write(pipelineSuccess) // nolint: errcheck
+					case "/api/v4/projects/4580910":
+						w.WriteHeader(http.StatusOK)
+						w.Write(projectSuccess) // nolint: errcheck
+					case "/api/v4/":
+						// Rate limiter requests.
+						w.WriteHeader(http.StatusOK)
+					default:
+						t.Errorf("got unexpected request at %q", r.RequestURI)
+						http.Error(w, "not found", http.StatusNotFound)
+					}
+				}))
+
+			internalClient, err := gitlab.NewClient("token", gitlab.WithBaseURL(testServer.URL))
+			Ok(t, err)
+			client := &GitlabClient{
+				Client:  internalClient,
+				Version: nil,
+			}
+
+			err = client.MergePull(
+				logger,
+				models.PullRequest{
+					Num: 1,
+					BaseRepo: models.Repo{
+						FullName: "runatlantis/atlantis",
+						Owner:    "runatlantis",
+						Name:     "atlantis",
+					},
+				}, models.PullRequestOptions{
+					DeleteSourceBranchOnMerge: false,
+					MergeMethod:               c.mergeMethod,
+				})
+			Ok(t, err)
+
+			// Verify that the squash parameter was sent correctly
+			if c.expSquash {
+				squashValue, exists := mergeRequestBody["squash"]
+				Assert(t, exists, "Expected squash parameter in request body")
+				Assert(t, squashValue == true, "Expected squash:true in request body, got: %v", squashValue)
+			} else {
+				// When not squashing, the squash parameter should be false or not present
+				if squashValue, exists := mergeRequestBody["squash"]; exists {
+					Assert(t, squashValue == false, "Expected squash:false in request body, got: %v", squashValue)
+				}
+			}
+		})
+	}
+}
+
 func TestGitlabClient_UpdateStatus(t *testing.T) {
 	logger := logging.NewNoopLogger(t)