distro: add ReadOSReleaseFromTree

This new method supports reading os-release from an arbitrary tree,
and it also correctly handles multiple os-release locations (see
os-release(5)). It's public so we can reuse it in multiple places.

pkg/distro/host.go

@@ -3,8 +3,10 @@ package distro
 import (
 	"bufio"
 	"errors"
+	"fmt"
 	"io"
 	"os"
+	"path"
 	"strings"
 )
 
@@ -48,3 +50,25 @@ func readOSRelease(r io.Reader) (map[string]string, error) {
 
 	return osrelease, nil
 }
+
+// ReadOSReleaseFromTree reads the os-release file from the given root directory.
+//
+// According to os-release(5), the os-release file should be located in either /etc/os-release or /usr/lib/os-release,
+// so both locations are tried, with the former taking precedence.
+func ReadOSReleaseFromTree(root string) (map[string]string, error) {
+	locations := []string{
+		"etc/os-release",
+		"usr/lib/os-release",
+	}
+	var errs []string
+	for _, location := range locations {
+		f, err := os.Open(path.Join(root, location))
+		if err == nil {
+			defer f.Close()
+			return readOSRelease(f)
+		}
+		errs = append(errs, fmt.Sprintf("cannot read %s: %v", location, err))
+	}
+
+	return nil, fmt.Errorf("failed to read os-release:\n%s", strings.Join(errs, "\n"))
+}

pkg/distro/host_test.go

@@ -1,9 +1,13 @@
 package distro
 
 import (
+	"os"
+	"path"
 	"reflect"
 	"strings"
 	"testing"
+
+	"github.com/stretchr/testify/require"
 )
 
 func TestOSRelease(t *testing.T) {
@@ -52,3 +56,36 @@ VARIANT_ID=workstation`,
 		}
 	}
 }
+
+func TestReadOSReleaseFromTree(t *testing.T) {
+	tree := t.TempDir()
+
+	// initialize dirs
+	require.NoError(t, os.MkdirAll(path.Join(tree, "usr/lib"), 0755))
+	require.NoError(t, os.MkdirAll(path.Join(tree, "etc"), 0755))
+
+	// firstly, let's write a simple /usr/lib/os-release
+	require.NoError(t,
+		os.WriteFile(path.Join(tree, "usr/lib/os-release"), []byte("ID=toucan\n"), 0600),
+	)
+
+	osRelease, err := ReadOSReleaseFromTree(tree)
+	require.NoError(t, err)
+	require.Equal(t, "toucan", osRelease["ID"])
+
+	// secondly, let's override it with /etc/os-release
+	require.NoError(t,
+		os.WriteFile(path.Join(tree, "etc/os-release"), []byte("ID=kingfisher\n"), 0600),
+	)
+
+	osRelease, err = ReadOSReleaseFromTree(tree)
+	require.NoError(t, err)
+	require.Equal(t, "kingfisher", osRelease["ID"])
+}
+
+func TestReadOSReleaseFromTreeUnhappy(t *testing.T) {
+	tree := t.TempDir()
+
+	_, err := ReadOSReleaseFromTree(tree)
+	require.ErrorContains(t, err, "failed to read os-release")
+}