chore: try to auto-detect default branch name

cmd/terramate/cli/project.go

@@ -185,14 +185,23 @@ func (p *project) setDefaults() error {
 		gitOpt.DefaultBranchBaseRef = defaultBranchBaseRef
 	}
 
-	if gitOpt.DefaultBranch == "" {
-		gitOpt.DefaultBranch = defaultBranch
-	}
-
 	if gitOpt.DefaultRemote == "" {
 		gitOpt.DefaultRemote = defaultRemote
 	}
 
+	if gitOpt.DefaultBranch == "" {
+		autoDefaultBranch := ""
+		if p.git.wrapper != nil {
+			autoDefaultBranch = p.git.wrapper.RemoteDefaultBranch(gitOpt.DefaultRemote)
+		}
+
+		if autoDefaultBranch != "" {
+			gitOpt.DefaultBranch = autoDefaultBranch
+		} else {
+			gitOpt.DefaultBranch = defaultBranch
+		}
+	}
+
 	return nil
 }
 

cmd/terramate/e2etests/core/general_test.go

@@ -308,7 +308,7 @@ func TestMainAfterOriginMainMustUseDefaultBaseRef(t *testing.T) {
 	AssertRunResult(t, ts.ListChangedStacks(), wantRes)
 }
 
-func TestFailsOnChangeDetectionIfRepoDoesntHaveOriginMain(t *testing.T) {
+func TestFailsOnChangeDetectionIfRepoDoesntHaveOrigin(t *testing.T) {
 	t.Parallel()
 
 	rootdir := test.TempDir(t)
@@ -344,10 +344,6 @@ func TestFailsOnChangeDetectionIfRepoDoesntHaveOriginMain(t *testing.T) {
 
 	git.SetupRemote("notorigin", "main", "main")
 	assertFails("repository must have a configured")
-
-	git.CheckoutNew("not-main")
-	git.SetupRemote("origin", "not-main", "main")
-	assertFails("has no default branch ")
 }
 
 func TestNoArgsProvidesBasicHelp(t *testing.T) {
@@ -368,6 +364,17 @@ func TestFailsIfDefaultRemoteDoesntHaveDefaultBranch(t *testing.T) {
 	})
 
 	cli := NewCLI(t, s.RootDir())
+
+	test.WriteFile(t, s.RootDir(), "terramate.tm.hcl", `
+terramate {
+	config {
+		git {
+			default_branch = "wrong"
+		}
+	}
+}
+`)
+
 	AssertRunResult(t,
 		cli.ListChangedStacks(),
 		RunExpected{
@@ -414,6 +421,20 @@ terramate {
 	AssertRun(t, cli.ListChangedStacks())
 }
 
+func TestDefaultBranchDetection(t *testing.T) {
+	t.Parallel()
+
+	s := sandbox.NewWithGitConfig(t, sandbox.GitConfig{
+		DefaultRemoteName:       "origin",
+		DefaultRemoteBranchName: "master",
+		LocalBranchName:         "feat",
+	})
+
+	cli := newCLI(t, s.RootDir())
+
+	assertRun(t, cli.listChangedStacks())
+}
+
 func TestE2ETerramateLogsWarningIfRootConfigIsNotAtProjectRoot(t *testing.T) {
 	t.Parallel()
 

git/git.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"regexp"
 	"sort"
 	"strconv"
 	"strings"
@@ -733,6 +734,41 @@ func (git *Git) GetConfigValue(key string) (string, error) {
 		return "", err
 	}
 	return strings.TrimSpace(s), nil
+
+}
+
+// RemoteDefaultBranch returns the default branch for the given remote,
+// or an empty string if detection failed.
+func (git *Git) RemoteDefaultBranch(remote string) string {
+	logger := log.With().
+		Str("action", "Git.RemoteDefaultBranch()").
+		Str("remote", remote).
+		Logger()
+
+	// First try to get default branch from local information
+	if out, err := git.exec("symbolic-ref", fmt.Sprintf("refs/remotes/%s/HEAD", remote), "--short"); err == nil {
+		if strings.HasPrefix(out, remote+"/") {
+			return strings.TrimPrefix(out, remote+"/")
+		}
+		logger.Debug().Msgf("failed to parse remote branch name from symbolic-ref: '%s'", out)
+	} else {
+		logger.Debug().Err(err).Msg("failed to get name of default branch locally")
+	}
+
+	// Otherwise try to query remote
+	if out, err := git.exec("ls-remote", "--symref", remote, "HEAD"); err == nil {
+		r := regexp.MustCompile(`^ref: refs/heads/(.+)\tHEAD`)
+		matches := r.FindStringSubmatch(out)
+		if len(matches) == 2 {
+			return matches[1]
+		}
+		logger.Debug().Msgf("failed to parse remote branch name from ls-remote: '%s'", out)
+	} else {
+		logger.Debug().Err(err).Msg("failed to get name of default branch remotely")
+	}
+
+	logger.Debug().Msg("failed to get name of default branch")
+	return ""
 }
 
 func (git *Git) exec(command string, args ...string) (string, error) {

git/git_test.go

@@ -1,7 +1,7 @@
 // Copyright 2023 Terramate GmbH
 // SPDX-License-Identifier: MPL-2.0
 
-//go:build linux || darwin
+///go:build linux
 
 package git_test
 
@@ -447,6 +447,31 @@ func TestGetConfigValue(t *testing.T) {
 	assert.Error(t, err, "git config: non-existing key")
 }
 
+func TestGetRemoteDefaultBranch(t *testing.T) {
+	const (
+		remote        = "origin"
+		defaultBranch = "main"
+	)
+
+	repodir := mkOneCommitRepo(t)
+	g := test.NewGitWrapper(t, repodir, []string{})
+
+	remoteDir := test.EmptyRepo(t, true)
+
+	assert.NoError(t, g.RemoteAdd(remote, remoteDir))
+	assert.NoError(t, g.Push(remote, defaultBranch))
+
+	name := g.RemoteDefaultBranch(remote)
+	assert.EqualStrings(t, name, defaultBranch, "default branch with init")
+
+	clonedir := t.TempDir()
+	g = test.NewGitWrapper(t, clonedir, []string{})
+	assert.NoError(t, g.Clone(remoteDir, clonedir))
+
+	name = g.RemoteDefaultBranch(remote)
+	assert.EqualStrings(t, name, defaultBranch, "default branch with clone")
+}
+
 const defaultBranch = "main"
 
 func mkOneCommitRepo(t *testing.T) string {