providers/system: support .d directory for base config fragments

Fixes coreos#1101
This change allows the two distrosi (i.e. FCOS and RHCOS) to carry
different base.ign files with common elements. Also,this will support
an additional .d directory for each platform ID.

dracut/30ignition/module-setup.sh

@@ -56,8 +56,6 @@ install() {
     # Required on system using SELinux
     inst_multiple -o setfiles
 
-    inst_script "$moddir/ignition-setup-base.sh" \
-        "/usr/sbin/ignition-setup-base"
     inst_script "$moddir/ignition-setup-user.sh" \
         "/usr/sbin/ignition-setup-user"
 
@@ -82,7 +80,6 @@ install() {
             "$systemdsystemunitdir/ignition-$x.target"
     done
 
-    install_ignition_unit ignition-setup-base.service
     install_ignition_unit ignition-setup-user.service
     install_ignition_unit ignition-fetch.service
     install_ignition_unit ignition-fetch-offline.service

internal/exec/engine.go

@@ -88,7 +88,7 @@ func (e Engine) Run(stageName string) error {
 	}
 	baseConfig := emptyConfig
 
-	systemBaseConfig, r, err := system.FetchBaseConfig(e.Logger)
+	systemBaseConfig, r, err := system.FetchBaseConfig(e.Logger, e.PlatformConfig.Name())
 	e.logReport(r)
 	if err != nil && err != providers.ErrNoProvider {
 		e.Logger.Crit("failed to acquire system base config: %v", err)

internal/providers/system/system.go

@@ -15,10 +15,12 @@
 package system
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 
+	latest "github.com/coreos/ignition/v2/config/v3_3_experimental"
 	"github.com/coreos/ignition/v2/config/v3_3_experimental/types"
 	"github.com/coreos/ignition/v2/internal/distro"
 	"github.com/coreos/ignition/v2/internal/log"
@@ -30,12 +32,24 @@ import (
 )
 
 const (
-	baseFilename = "base.ign"
 	userFilename = "user.ign"
 )
 
-func FetchBaseConfig(logger *log.Logger) (types.Config, report.Report, error) {
-	return fetchConfig(logger, baseFilename)
+func FetchBaseConfig(logger *log.Logger, platformName string) (types.Config, report.Report, error) {
+	var fullBaseConfig types.Config
+	baseDConfig, baseDReport, err := fetchBaseDirectoryConfig(logger, "base.d")
+	if err != nil {
+		return types.Config{}, baseDReport, err
+	}
+
+	platformDir := fmt.Sprintf("base.%s.d", platformName)
+	basePlatformDConfig, basePlatformDReport, err := fetchBaseDirectoryConfig(logger, platformDir)
+	if err != nil {
+		logger.Info("no config at %q: %v", platformDir, err)
+	}
+	fullBaseConfig = latest.Merge(baseDConfig, basePlatformDConfig)
+	baseDReport.Merge(basePlatformDReport)
+	return fullBaseConfig, baseDReport, nil
 }
 
 func FetchConfig(f *resource.Fetcher) (types.Config, report.Report, error) {
@@ -56,3 +70,31 @@ func fetchConfig(logger *log.Logger, filename string) (types.Config, report.Repo
 	}
 	return util.ParseConfig(logger, rawConfig)
 }
+
+// fetchBaseDirectoryConfig is a helper function to merge all the base config fragments inside of a particular directory.
+func fetchBaseDirectoryConfig(logger *log.Logger, dir string) (types.Config, report.Report, error) {
+	var baseConfig types.Config
+	var report report.Report
+	path := filepath.Join(distro.SystemConfigDir(), dir)
+	configs, err := ioutil.ReadDir(path)
+	if os.IsNotExist(err) {
+		logger.Info("no config dir at %q", path)
+		return types.Config{}, report, nil
+	} else if err != nil {
+		logger.Err("couldn't read config dir %q: %v", path, err)
+		return types.Config{}, report, err
+	}
+	if len(configs) == 0 {
+		logger.Info("no configs at %q", path)
+		return types.Config{}, report, nil
+	}
+	for _, config := range configs {
+		intermediateConfig, intermediateReport, err := fetchConfig(logger, filepath.Join(dir, config.Name()))
+		if err != nil {
+			return types.Config{}, intermediateReport, err
+		}
+		baseConfig = latest.Merge(baseConfig, intermediateConfig)
+		report.Merge(intermediateReport)
+	}
+	return baseConfig, report, nil
+}