bib: simplify getContainerSize()

The getContainerSize() was not using some of the modern go helpers.
So let's use `exec.Command().Output()` and introduce a new
`util.OutputErr()` helper that will be able to also show stderr to
the user if the Output() call returns an error.

bib/cmd/bootc-image-builder/main.go

@@ -12,7 +12,10 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/osbuild/bootc-image-builder/bib/internal/setup"
+	"github.com/sirupsen/logrus"
+	"github.com/spf13/cobra"
+	"golang.org/x/exp/slices"
+
 	"github.com/osbuild/images/pkg/arch"
 	"github.com/osbuild/images/pkg/blueprint"
 	"github.com/osbuild/images/pkg/cloud/awscloud"
@@ -21,9 +24,9 @@ import (
 	"github.com/osbuild/images/pkg/manifest"
 	"github.com/osbuild/images/pkg/osbuild"
 	"github.com/osbuild/images/pkg/rpmmd"
-	"github.com/sirupsen/logrus"
-	"github.com/spf13/cobra"
-	"golang.org/x/exp/slices"
+
+	"github.com/osbuild/bootc-image-builder/bib/internal/setup"
+	"github.com/osbuild/bootc-image-builder/bib/internal/util"
 )
 
 //go:embed fedora-eln.json
@@ -113,15 +116,11 @@ func loadConfig(path string) (*BuildConfig, error) {
 
 // getContainerSize returns the size of an already pulled container image in bytes
 func getContainerSize(imgref string) (uint64, error) {
-	var stdout, stderr strings.Builder
-
-	cmd := exec.Command("podman", "image", "inspect", imgref, "--format", "{{.Size}}")
-	cmd.Stdout = &stdout
-	cmd.Stderr = &stderr
-	if err := cmd.Run(); err != nil {
-		return 0, fmt.Errorf("failed inspect image: %w:\n%s", err, stderr.String())
+	output, err := exec.Command("podman", "image", "inspect", imgref, "--format", "{{.Size}}").Output()
+	if err != nil {
+		return 0, fmt.Errorf("failed inspect image: %w", util.OutputErr(err))
 	}
-	size, err := strconv.ParseUint(strings.TrimSpace(stdout.String()), 10, 64)
+	size, err := strconv.ParseUint(strings.TrimSpace(string(output)), 10, 64)
 	if err != nil {
 		return 0, fmt.Errorf("cannot parse image size: %w", err)
 	}

bib/internal/util/util.go

@@ -26,3 +26,12 @@ func RunCmdSync(cmdName string, args ...string) error {
 	}
 	return nil
 }
+
+// OutputErr takes an error from exec.Command().Output() and tries
+// generate an error with stderr details
+func OutputErr(err error) error {
+	if err, ok := err.(*exec.ExitError); ok {
+		return fmt.Errorf("%w, stderr:\n%s", err, err.Stderr)
+	}
+	return err
+}

bib/internal/util/util_test.go

@@ -0,0 +1,21 @@
+package util_test
+
+import (
+	"fmt"
+	"os/exec"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/osbuild/bootc-image-builder/bib/internal/util"
+)
+
+func TestOutputErrPassthrough(t *testing.T) {
+	err := fmt.Errorf("boom")
+	assert.Equal(t, util.OutputErr(err), err)
+}
+
+func TestOutputErrExecError(t *testing.T) {
+	_, err := exec.Command("bash", "-c", ">&2 echo some-stderr; exit 1").Output()
+	assert.Equal(t, "exit status 1, stderr:\nsome-stderr\n", util.OutputErr(err).Error())
+}