Guess current Git branch name even on Detached HEAD.

find.go

import (
  "log"
  "bufio"
	"os"
	"strings"
	"os/exec"
	"bytes"
)

func findCurrentBranch() string {
	if v, exists := os.LookupEnv("GIT_BRANCH"); exists {
		// Respect the value if Jenkins provides.
		return v
	}
	cmd := exec.Command("git", "show", "-s", "-pretty=%d", "HEAD")
	err := cmd.Run()
	if err != nil {
		log.Fatal("Failed to run git command: ", err)
	}
	out, err := cmd.Output()
	if err != nil {
		log.Fatal("Failed to get stdout from git: ", err)
	}
	scanner := bufio.NewScanner(bytes.NewReader(out))
	if !scanner.Scan() {
		log.Fatal("Invalid git show output: ", string(out))
	}
	l1 := scanner.Text()

	// $ git show -s --pretty=%d HEAD
	if strings.HasSuffix(l1, "(HEAD -> ") {
		// Normal result:
		// (HEAD -> generalize-template)
		return l1[9 : len(l1)-1]
	}
	if strings.HasSuffix(l1, "(HEAD, ") {
		// Detached HEAD:
		// (HEAD, origin/pr/24, origin/attributed-offer)
		refs := strings.Split(l1[1:len(l1)-1], ", ")
		for _, ref := range refs {
		  return ref
    }
	}

	return ""
}