feat(gitprovider/gitlab): Support for squash merge and fast forward

Signed-off-by: GitHub <noreply@github.com>

Author: aslafy-z

pkg/gitprovider/gitlab/gitlab.go

@@ -260,12 +260,39 @@ func (p *provider) GetCommitURL(repoURL string, sha string) (string, error) {
 }
 
 func convertGitlabMR(glMR gitlab.BasicMergeRequest) gitprovider.PullRequest {
+	var merged = glMR.State == "merged"
+	var mergeCommit string
+	if merged {
+		switch {
+		case glMR.MergeCommitSHA != "":
+			// Regular merge (with or without squash):
+			// GitLab gives us the merge commit SHA.
+			mergeCommit = glMR.MergeCommitSHA
+		case glMR.SquashCommitSHA != "":
+			// Fast-forward with squash:
+			// No merge commit exists, but a squash commit SHA may be available.
+			//
+			// Note: GitLab does not reliably populate squash_commit_sha in all
+			// relevant API/webhook payloads yet. We still prefer it when present,
+			// and keep this branch to future-proof against incoming GitLab changes
+			// that aim to backfill and correctly populate squash_commit_sha.
+			//
+			// References:
+			// - https://gitlab.com/gitlab-org/gitlab/-/issues/415449
+			// - https://gitlab.com/gitlab-org/gitlab/-/issues/294257
+			mergeCommit = glMR.SquashCommitSHA
+		default:
+			// Fast-forward without squash:
+			// No merge commit exists; the source HEAD is now on the target branch.
+			mergeCommit = glMR.SHA
+		}
+	}
 	return gitprovider.PullRequest{
 		Number:         glMR.IID,
 		URL:            glMR.WebURL,
 		Open:           isMROpen(glMR),
-		Merged:         glMR.State == "merged",
-		MergeCommitSHA: glMR.MergeCommitSHA,
+		Merged:         merged,
+		MergeCommitSHA: mergeCommit,
 		Object:         glMR,
 		HeadSHA:        glMR.SHA,
 		CreatedAt:      glMR.CreatedAt,

pkg/gitprovider/gitlab/gitlab_test.go

@@ -384,6 +384,77 @@ func TestMergePullRequest(t *testing.T) {
 	}
 }
 
+func TestConvertGitlabMR_MergeCommitSelection(t *testing.T) {
+	testCases := []struct {
+		name                string
+		glMR                gitlab.BasicMergeRequest
+		expectedMerged      bool
+		expectedMergeCommit string
+	}{
+		{
+			name: "merged - prefers MergeCommitSHA when present",
+			glMR: gitlab.BasicMergeRequest{
+				IID:             1,
+				State:           "merged",
+				MergeCommitSHA:  "merge_sha",
+				SquashCommitSHA: "squash_sha",
+				SHA:             "head_sha",
+				WebURL:          "https://gitlab.com/group/project/-/merge_requests/1",
+			},
+			expectedMerged:      true,
+			expectedMergeCommit: "merge_sha",
+		},
+		{
+			name: "merged - falls back to SquashCommitSHA when MergeCommitSHA is empty",
+			glMR: gitlab.BasicMergeRequest{
+				IID:             2,
+				State:           "merged",
+				SquashCommitSHA: "squash_sha",
+				SHA:             "head_sha",
+				WebURL:          "https://gitlab.com/group/project/-/merge_requests/2",
+			},
+			expectedMerged:      true,
+			expectedMergeCommit: "squash_sha",
+		},
+		{
+			name: "merged - falls back to SHA when both MergeCommitSHA and SquashCommitSHA are empty",
+			glMR: gitlab.BasicMergeRequest{
+				IID:    3,
+				State:  "merged",
+				SHA:    "head_sha",
+				WebURL: "https://gitlab.com/group/project/-/merge_requests/3",
+			},
+			expectedMerged:      true,
+			expectedMergeCommit: "head_sha",
+		},
+		{
+			name: "not merged - merge commit is empty regardless of fields",
+			glMR: gitlab.BasicMergeRequest{
+				IID:             4,
+				State:           "opened",
+				MergeCommitSHA:  "merge_sha",
+				SquashCommitSHA: "squash_sha",
+				SHA:             "head_sha",
+				WebURL:          "https://gitlab.com/group/project/-/merge_requests/4",
+			},
+			expectedMerged:      false,
+			expectedMergeCommit: "",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			pr := convertGitlabMR(tc.glMR)
+
+			require.Equal(t, tc.expectedMerged, pr.Merged)
+			require.Equal(t, tc.expectedMergeCommit, pr.MergeCommitSHA)
+			require.Equal(t, tc.glMR.IID, pr.Number)
+			require.Equal(t, tc.glMR.WebURL, pr.URL)
+			require.Equal(t, tc.glMR.SHA, pr.HeadSHA)
+		})
+	}
+}
+
 func TestParseGitLabURL(t *testing.T) {
 	const expectedProjectName = "akuity/kargo"
 	testCases := []struct {