buildconfig: support passing build config via stdin

This commit adds support for reading the build config via stdin.
To do that the `-config` switch now supports `-` and with that it
will use `os.Stdin` insteadof readin the file. Because we have no
file extension to auto-detect the format only JSON is supported
in this mode.

This feature will be used by osbuild-composer when it adds bootc
image mode integration.

README.md

@@ -301,6 +301,9 @@ sudo podman run \
     quay.io/centos-bootc/centos-bootc:stream9
 ```
 
+The configuration can also be passed in via stdin when `--config -`
+is used. Only JSON configuration is supported in this mode.
+
 ### Users (`user`, array)
 
 Possible fields:

bib/internal/buildconfig/config.go

@@ -67,17 +67,26 @@ func decodeTomlBuildConfig(r io.Reader, what string) (*BuildConfig, error) {
 	return &conf, nil
 }
 
+var osStdin = os.Stdin
+
 func loadConfig(path string) (*BuildConfig, error) {
-	fp, err := os.Open(path)
-	if err != nil {
-		return nil, err
+	var fp *os.File
+	var err error
+
+	if path == "-" {
+		fp = osStdin
+	} else {
+		fp, err = os.Open(path)
+		if err != nil {
+			return nil, err
+		}
+		defer fp.Close()
 	}
-	defer fp.Close()
 
-	switch filepath.Ext(path) {
-	case ".json":
+	switch {
+	case path == "-", filepath.Ext(path) == ".json":
 		return decodeJsonBuildConfig(fp, path)
-	case ".toml":
+	case filepath.Ext(path) == ".toml":
 		return decodeTomlBuildConfig(fp, path)
 	default:
 		return nil, fmt.Errorf("unsupported file extension for %q", path)

bib/internal/buildconfig/config_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 
 	"github.com/osbuild/images/pkg/blueprint"
 
@@ -168,3 +169,17 @@ minsize = 1000
 		},
 	}, conf)
 }
+
+func TestReadWithFallbackFromStdin(t *testing.T) {
+	fakeUserCnfPath := makeFakeConfig(t, "fake-stdin", fakeConfigJSON)
+	fakeStdinFp, err := os.Open(fakeUserCnfPath)
+	require.NoError(t, err)
+	defer fakeStdinFp.Close()
+
+	restore := buildconfig.MockOsStdin(fakeStdinFp)
+	defer restore()
+
+	cfg, err := buildconfig.ReadWithFallback("-")
+	assert.NoError(t, err)
+	assert.Equal(t, expectedBuildConfig, cfg)
+}

bib/internal/buildconfig/export_test.go

@@ -1,9 +1,21 @@
 package buildconfig
 
+import (
+	"os"
+)
+
 func MockConfigRootDir(newDir string) (restore func()) {
 	saved := configRootDir
 	configRootDir = newDir
 	return func() {
 		configRootDir = saved
 	}
 }
+
+func MockOsStdin(new *os.File) (restore func()) {
+	saved := osStdin
+	osStdin = new
+	return func() {
+		osStdin = saved
+	}
+}