fix(git): check write/root capability before installing git-lfs

pkg/git/installer.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"os"
 
 	"github.com/devsy-org/devsy/pkg/command"
 	"github.com/devsy-org/devsy/pkg/log"
@@ -106,7 +107,13 @@ type pkgManagerStrategy struct {
 
 func (s *pkgManagerStrategy) name() string { return s.manager }
 
-func (s *pkgManagerStrategy) usable() bool { return command.Exists(s.manager) }
+var isRoot = func() bool { return os.Geteuid() == 0 }
+
+// usable requires root: package installs write to system-owned locations
+// (e.g. apt's lock files, dpkg's database).
+func (s *pkgManagerStrategy) usable() bool {
+	return command.Exists(s.manager) && isRoot()
+}
 
 func (s *pkgManagerStrategy) install(ctx context.Context, t tool) error {
 	log.Infof("installing %s with %s", t.pkg, s.manager)

pkg/git/installer_release.go

@@ -84,6 +84,10 @@ func (s *releaseSource) install(ctx context.Context, binary string) error {
 		}
 	}
 
+	if err := ensureDirWritable(installDir); err != nil {
+		return fmt.Errorf("install dir %q is not writable: %w", installDir, err)
+	}
+
 	req := fetchRequest{
 		binary:   binary,
 		url:      s.downloadURL(s.version, asset),
@@ -96,9 +100,6 @@ func (s *releaseSource) install(ctx context.Context, binary string) error {
 	}
 	defer cleanup()
 
-	if err := os.MkdirAll(installDir, 0o750); err != nil {
-		return fmt.Errorf("create install dir %q: %w", installDir, err)
-	}
 	dst := filepath.Join(installDir, req.execName)
 	if err := moveExecutable(src, dst); err != nil {
 		return fmt.Errorf("install %s to %q: %w", binary, dst, err)
@@ -206,3 +207,16 @@ func moveExecutable(src, dst string) error {
 	// #nosec G306,G703 -- dst is internally constructed; an executable must be world-executable
 	return os.WriteFile(dst, data, 0o755)
 }
+
+func ensureDirWritable(dir string) error {
+	if err := os.MkdirAll(dir, 0o750); err != nil {
+		return err
+	}
+	probe, err := os.CreateTemp(dir, ".devsy-write-test-*")
+	if err != nil {
+		return err
+	}
+	name := probe.Name()
+	_ = probe.Close()
+	return os.Remove(name)
+}

pkg/git/installer_test.go

@@ -3,6 +3,8 @@ package git
 import (
 	"context"
 	"fmt"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 
@@ -48,6 +50,40 @@ func TestReleaseStrategyRequiresReleaseSource(t *testing.T) {
 	assert.Assert(t, err != nil)
 }
 
+func TestPkgManagerStrategyUsableRequiresRoot(t *testing.T) {
+	s := &pkgManagerStrategy{manager: "sh"} // "sh" always exists in test environments
+
+	original := isRoot
+	t.Cleanup(func() { isRoot = original })
+
+	isRoot = func() bool { return true }
+	assert.Assert(t, s.usable())
+
+	isRoot = func() bool { return false }
+	assert.Assert(t, !s.usable())
+}
+
+func TestEnsureDirWritable(t *testing.T) {
+	assert.NilError(t, ensureDirWritable(t.TempDir()))
+}
+
+func TestEnsureDirWritableCreatesMissingDir(t *testing.T) {
+	dir := filepath.Join(t.TempDir(), "does", "not", "exist", "yet")
+	assert.NilError(t, ensureDirWritable(dir))
+}
+
+func TestEnsureDirWritableRejectsReadOnlyDir(t *testing.T) {
+	if os.Geteuid() == 0 {
+		t.Skip("permission test not meaningful when running as root")
+	}
+
+	dir := t.TempDir()
+	// #nosec G302 -- intentional: testing restrictive perms
+	assert.NilError(t, os.Chmod(dir, 0o500))
+
+	assert.Assert(t, ensureDirWritable(dir) != nil)
+}
+
 // fakeStrategy is a test installStrategy with configurable behavior.
 type fakeStrategy struct {
 	label     string