Add support for GitHub Actions workflow by Pull Request

ci.go

@@ -8,6 +8,11 @@ import (
 	"strings"
 )
 
+var (
+	// https://help.github.com/en/actions/reference/events-that-trigger-workflows#pull-request-event-pull_request
+	githubActionsPRRefRegexp = regexp.MustCompile(`refs/pull/\d+/merge`)
+)
+
 // CI represents a common information obtained from all CI platforms
 type CI struct {
 	PR  PullRequest
@@ -144,5 +149,17 @@ func githubActions() (ci CI, err error) {
 		os.Getenv("GITHUB_RUN_ID"),
 	)
 	ci.PR.Revision = os.Getenv("GITHUB_SHA")
+	ci.PR.Number = 0
+
+	if githubActionsPRRefRegexp.MatchString(os.Getenv("GITHUB_REF")) {
+		s := strings.Split(os.Getenv("GITHUB_REF"), "/")[2]
+		pr, err := strconv.Atoi(s)
+		if err != nil {
+			return ci, err
+		}
+
+		ci.PR.Number = pr
+	}
+
 	return ci, err
 }

ci_test.go

@@ -746,6 +746,22 @@ func TestGitHubActions(t *testing.T) {
 			},
 			ok: true,
 		},
+		{
+			fn: func() {
+				os.Setenv("GITHUB_SHA", "abcdefg")
+				os.Setenv("GITHUB_REPOSITORY", "mercari/tfnotify")
+				os.Setenv("GITHUB_RUN_ID", "12345")
+				os.Setenv("GITHUB_REF", "refs/pull/123/merge")
+			},
+			ci: CI{
+				PR: PullRequest{
+					Revision: "abcdefg",
+					Number:   123,
+				},
+				URL: "https://github.com/mercari/tfnotify/actions/runs/12345",
+			},
+			ok: true,
+		},
 	}
 
 	for _, testCase := range testCases {