Fix bug when rerunning the GitHub action

Prior to this commit rerunning the GitHub action would always lead
to an error.  This was because the GITHUB_REF environment variable
exposed by the Github Action does not contain the pull request number
when rerunning a job (see [1]).

Fixed by now retrieving the pull request number from the Github event
JSON (instead of via the GITHUB_REF), via the GITHUB_EVENT_PATH
environment variable.

[1] actions/checkout#58 (comment)

internal/github/pullrequest/action.go

@@ -2,14 +2,19 @@ package pullrequest
 
 import (
 	"encoding/json"
+	"io/ioutil"
 	"os"
-	"strconv"
+	"path/filepath"
 	"strings"
 )
 
 type action struct {
 }
 
+type event struct {
+	PullRequestNumber int `json:"number"`
+}
+
 func (action action) repositoryOwner() string {
 	return strings.Split(os.Getenv("GITHUB_REPOSITORY"), "/")[0]
 }
@@ -19,11 +24,14 @@ func (action action) repositoryName() string {
 }
 
 func (action action) pullRequestNumber() int {
-	s := strings.Split(os.Getenv("GITHUB_REF"), "/")[2]
-	pullRequestNumber, err := strconv.Atoi(s)
-	panicIfError((err))
-
-	return pullRequestNumber
+	var event event
+
+	githubEventJSONFile, err := os.Open(filepath.Clean(os.Getenv("GITHUB_EVENT_PATH")))
+	panicIfError(err)
+	defer githubEventJSONFile.Close() //nolint
+	byteValue, _ := ioutil.ReadAll(githubEventJSONFile)
+	panicIfError(json.Unmarshal(byteValue, &event))
+	return event.PullRequestNumber
 }
 
 func (action action) token() string {

magefile_test.go

@@ -3,20 +3,24 @@ package test
 import (
 	"bytes"
 	"fmt"
+	"io/ioutil"
 	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/magefile/mage/mage"
 )
 
 const (
 	EnvGitHubRepository        = "GITHUB_REPOSITORY"
-	EnvGitHubRef               = "GITHUB_REF"
+	EnvGitHubEventPath         = "GITHUB_EVENT_PATH"
 	EnvGitHubActionInputLabels = "GITHUB_ACTION_INPUT_LABELS"
 	GitHubTestRepo             = "agilepathway/test-label-checker-consumer"
 	PRWithNoLabels             = 1 // https://github.com/agilepathway/test-label-checker-consumer/pull/1
 	PRWithOneSpecifiedLabel    = 2 // https://github.com/agilepathway/test-label-checker-consumer/pull/2
 	PRWithTwoSpecifiedLabels   = 3 // https://github.com/agilepathway/test-label-checker-consumer/pull/3
+	GitHubEventJsonDir         = "testdata"
+	GitHubEventJsonFilename    = "github_event.json"
 )
 
 func TestPullRequestWithOneSpecifiedLabelShouldSucceed(t *testing.T) {
@@ -49,15 +53,18 @@ func TestPullRequestWithTwoSpecifiedLabelsShouldFail(t *testing.T) {
 }
 
 func TestMain(m *testing.M) {
-	os.Setenv(EnvGitHubRepository, GitHubTestRepo) //nolint
+	os.Mkdir(GitHubEventJsonDir, os.ModePerm)
+	os.Setenv(EnvGitHubRepository, GitHubTestRepo)                                            //nolint
+	os.Setenv(EnvGitHubEventPath, filepath.Join(GitHubEventJsonDir, GitHubEventJsonFilename)) //nolint
 	os.Exit(testMainWrapper(m))
 }
 
 func testMainWrapper(m *testing.M) int {
 	//nolint
 	defer func() {
+		os.RemoveAll(GitHubEventJsonDir)
 		os.Unsetenv(EnvGitHubRepository)
-		os.Unsetenv(EnvGitHubRef)
+		os.Unsetenv(EnvGitHubEventPath)
 		os.Unsetenv(EnvGitHubActionInputLabels)
 	}()
 
@@ -73,7 +80,9 @@ func checkLabels() (int, *bytes.Buffer, *bytes.Buffer) {
 }
 
 func setPullRequestNumber(prNumber int) {
-	os.Setenv(EnvGitHubRef, fmt.Sprintf("refs/pull/%d/merge", prNumber)) //nolint
+	githubEventJSON := []byte(fmt.Sprintf("{ \"number\": %d }", prNumber))
+	gp := filepath.Join(GitHubEventJsonDir, GitHubEventJsonFilename)
+	ioutil.WriteFile(gp, githubEventJSON, os.ModePerm)
 }
 
 func specifySemVerLabels() {