From 64cfcdd19e754a998ebc4e81861ab8512ee92c24 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mislav=20Marohni=C4=87?= <mislav.marohnic@gmail.com>
Date: Sun, 1 Mar 2020 10:52:11 +0100
Subject: [PATCH] Use `git symbolic-ref` to read the branch name for HEAD

This affects reading the current branch as well as reading the default
branch for a remote. Because we're no longer trying to read the ref
manually from the filesystem, this approach works with git worktrees.
---
 git/git.go          | 38 ++++++++++----------------------------
 github/localrepo.go | 14 ++++++++------
 2 files changed, 18 insertions(+), 34 deletions(-)

diff --git a/git/git.go b/git/git.go
index 7921e77e3..88848b8ce 100644
--- a/git/git.go
+++ b/git/git.go
@@ -2,7 +2,6 @@ package git
 
 import (
 	"fmt"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strings"
@@ -105,32 +104,6 @@ func HasFile(segments ...string) bool {
 	return false
 }
 
-func BranchAtRef(paths ...string) (name string, err error) {
-	dir, err := Dir()
-	if err != nil {
-		return
-	}
-
-	segments := []string{dir}
-	segments = append(segments, paths...)
-	path := filepath.Join(segments...)
-	b, err := ioutil.ReadFile(path)
-	if err != nil {
-		return
-	}
-
-	n := string(b)
-	refPrefix := "ref: "
-	if strings.HasPrefix(n, refPrefix) {
-		name = strings.TrimPrefix(n, refPrefix)
-		name = strings.TrimSpace(name)
-	} else {
-		err = fmt.Errorf("No branch info in %s: %s", path, n)
-	}
-
-	return
-}
-
 func Editor() (string, error) {
 	varCmd := gitCmd("var", "GIT_EDITOR")
 	varCmd.Stderr = nil
@@ -143,9 +116,18 @@ func Editor() (string, error) {
 }
 
 func Head() (string, error) {
-	return BranchAtRef("HEAD")
+	return SymbolicRef("HEAD")
+}
+
+// SymbolicRef reads a branch name from a ref such as "HEAD"
+func SymbolicRef(ref string) (string, error) {
+	refCmd := gitCmd("symbolic-ref", ref)
+	refCmd.Stderr = nil
+	output, err := refCmd.Output()
+	return firstLine(output), err
 }
 
+// SymbolicFullName reads a branch name from a ref such as "@{upstream}"
 func SymbolicFullName(name string) (string, error) {
 	parseCmd := gitCmd("rev-parse", "--symbolic-full-name", name)
 	parseCmd.Stderr = nil
diff --git a/github/localrepo.go b/github/localrepo.go
index bee18717a..ad9e318b9 100644
--- a/github/localrepo.go
+++ b/github/localrepo.go
@@ -113,14 +113,16 @@ func (r *GitHubRepo) MasterBranch() *Branch {
 }
 
 func (r *GitHubRepo) DefaultBranch(remote *Remote) *Branch {
-	var name string
-	if remote != nil {
-		name, _ = git.BranchAtRef("refs", "remotes", remote.Name, "HEAD")
+	b := Branch{
+		Repo: r,
+		Name: "refs/heads/master",
 	}
-	if name == "" {
-		name = "refs/heads/master"
+	if remote != nil {
+		if name, err := git.SymbolicRef(fmt.Sprintf("refs/remotes/%s/HEAD", remote.Name)); err == nil {
+			b.Name = name
+		}
 	}
-	return &Branch{r, name}
+	return &b
 }
 
 func (r *GitHubRepo) RemoteBranchAndProject(owner string, preferUpstream bool) (branch *Branch, project *Project, err error) {