fix: populate DefaultBranch for incoming webhooks

Incoming webhooks bypass ParsePayload, which means the event's
DefaultBranch field is never set. When pipelinerun_provenance is
set to 'default_branch' on the Repository CR, GetTektonDir() uses
the empty DefaultBranch as the git tree revision, causing a 404
from the GitHub API.

Fix this by fetching the repository's default branch from the
GitHub API in GetCommitInfo() when the event type is 'incoming'
and DefaultBranch is empty.

Also fix a pre-existing test gap: the 'error' test case in
TestGithubGetCommitInfo was missing wantErr, and the test was not
checking for err == nil on the success path.

Fixes: #2646

Author: vdemeester

pkg/provider/github/github.go

@@ -463,6 +463,21 @@ func (v *Provider) GetCommitInfo(ctx context.Context, runevent *info.Event) erro
 		}
 	}
 
+	// For incoming webhooks, DefaultBranch is not populated from the event
+	// payload (since there is no webhook payload to parse). Fetch it from the
+	// GitHub API so that pipelinerun_provenance: default_branch works correctly.
+	// For other event types (push, pull_request), DefaultBranch is already set
+	// by ParsePayload from the webhook payload's repository.default_branch field.
+	if runevent.DefaultBranch == "" && runevent.EventType == "incoming" {
+		ghRepo, _, err := wrapAPI(v, "get_repo", func() (*github.Repository, *github.Response, error) {
+			return v.Client().Repositories.Get(ctx, runevent.Organization, runevent.Repository)
+		})
+		if err != nil {
+			return err
+		}
+		runevent.DefaultBranch = ghRepo.GetDefaultBranch()
+	}
+
 	return nil
 }
 

pkg/provider/github/github_test.go

@@ -932,6 +932,31 @@ func TestGithubGetCommitInfo(t *testing.T) {
 			message:        "chore: deps\n\n[tkn skip]",
 			wantHasSkipCmd: true,
 		},
+		{
+			name: "incoming webhook populates DefaultBranch",
+			event: &info.Event{
+				Organization: "owner",
+				Repository:   "repository",
+				SHA:          "shacommitinfo",
+				EventType:    "incoming",
+			},
+			shaurl:   "https://git.provider/commit/info",
+			shatitle: "My beautiful pony",
+			message:  "My beautiful pony",
+		},
+		{
+			name: "DefaultBranch already set is preserved",
+			event: &info.Event{
+				Organization:  "owner",
+				Repository:    "repository",
+				SHA:           "shacommitinfo",
+				DefaultBranch: "develop",
+				EventType:     "incoming",
+			},
+			shaurl:   "https://git.provider/commit/info",
+			shatitle: "My beautiful pony",
+			message:  "My beautiful pony",
+		},
 		{
 			name: "error",
 			event: &info.Event{
@@ -940,6 +965,7 @@ func TestGithubGetCommitInfo(t *testing.T) {
 				SHA:          "shacommitinfo",
 			},
 			apiReply: "hello moto",
+			wantErr:  "invalid character",
 		},
 		{
 			name:     "noclient",
@@ -952,6 +978,12 @@ func TestGithubGetCommitInfo(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			fakeclient, mux, _, teardown := ghtesthelper.SetupGH()
 			defer teardown()
+			// Mock the repo endpoint so GetCommitInfo can resolve DefaultBranch
+			// when it is not already set on the event (e.g. incoming webhooks).
+			mux.HandleFunc(fmt.Sprintf("/repos/%s/%s",
+				tt.event.Organization, tt.event.Repository), func(rw http.ResponseWriter, _ *http.Request) {
+				fmt.Fprint(rw, `{"default_branch": "main"}`)
+			})
 			mux.HandleFunc(fmt.Sprintf("/repos/%s/%s/git/commits/%s",
 				tt.event.Organization, tt.event.Repository, tt.event.SHA), func(rw http.ResponseWriter, _ *http.Request) {
 				if tt.apiReply != "" {
@@ -1022,6 +1054,7 @@ func TestGithubGetCommitInfo(t *testing.T) {
 				assert.ErrorContains(t, err, tt.wantErr)
 				return
 			}
+			assert.NilError(t, err)
 			assert.Equal(t, tt.shatitle, tt.event.SHATitle)
 			assert.Equal(t, tt.shaurl, tt.event.SHAURL)
 
@@ -1040,6 +1073,16 @@ func TestGithubGetCommitInfo(t *testing.T) {
 				assert.DeepEqual(t, expectedCommitterDate, tt.event.SHACommitterDate)
 			}
 			assert.Equal(t, tt.wantHasSkipCmd, tt.event.HasSkipCommand)
+
+			// For incoming events, verify DefaultBranch is populated
+			if tt.event.EventType == "incoming" {
+				if tt.event.DefaultBranch == "develop" {
+					// If it was already set, it should be preserved
+					assert.Equal(t, "develop", tt.event.DefaultBranch, "DefaultBranch should be preserved")
+				} else {
+					assert.Equal(t, "main", tt.event.DefaultBranch, "DefaultBranch should be populated from API")
+				}
+			}
 		})
 	}
 }