pkg/utils: Support host operating systems without VERSION_ID

The VERSION_ID field in os-release(5) is optional [1].  It's absent on
Arch Linux, which follows a rolling-release model and uses the BUILD_ID
field instead:
  BUILD_ID=rolling

A subsequent commit will add built-in support for Arch Linux.  Hence,
the code to get the default release from the host operating system can
no longer assume the presence of the VERSION_ID field in os-release(5).

Note that the arch-toolbox image is tagged with 'latest', in accordance
with OCI conventions, not 'rolling' [2,3], which is the os-release(5)
BUILD_ID.  Therefore, it will be wise to use 'latest' as the default
release on Arch Linux, to simplify how the default release matches with
the default image's tag.  This means that a os-release(5) field can't be
used for the default release on Arch.

[1] https://www.freedesktop.org/software/systemd/man/os-release.html

[2] Commit 2568528
    containers#861

[3] Commit a4e5861
    containers#1308

containers#1303

src/pkg/utils/utils.go

@@ -39,12 +39,14 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+type GetDefaultReleaseFunc func() (string, error)
 type GetFullyQualifiedImageFunc func(string, string) string
 type ParseReleaseFunc func(string) (string, error)
 
 type Distro struct {
 	ContainerNamePrefix    string
 	ImageBasename          string
+	GetDefaultRelease      GetDefaultReleaseFunc
 	GetFullyQualifiedImage GetFullyQualifiedImageFunc
 	ParseRelease           ParseReleaseFunc
 }
@@ -100,18 +102,21 @@ var (
 		"fedora": {
 			"fedora-toolbox",
 			"fedora-toolbox",
+			getDefaultReleaseFedora,
 			getFullyQualifiedImageFedora,
 			parseReleaseFedora,
 		},
 		"rhel": {
 			"rhel-toolbox",
 			"toolbox",
+			getDefaultReleaseRHEL,
 			getFullyQualifiedImageRHEL,
 			parseReleaseRHEL,
 		},
 		"ubuntu": {
 			"ubuntu-toolbox",
 			"ubuntu-toolbox",
+			getDefaultReleaseUbuntu,
 			getFullyQualifiedImageUbuntu,
 			parseReleaseUbuntu,
 		},
@@ -140,7 +145,7 @@ func init() {
 	hostID, err := GetHostID()
 	if err == nil {
 		if distroObj, supportedDistro := supportedDistros[hostID]; supportedDistro {
-			release, err := getHostVersionID()
+			release, err := getDefaultReleaseForDistro(hostID)
 			if err == nil {
 				containerNamePrefixDefault = distroObj.ContainerNamePrefix
 				distroDefault = hostID
@@ -273,6 +278,52 @@ func getDefaultImageForDistro(distro, release string) string {
 	return image
 }
 
+func getDefaultReleaseForDistro(distro string) (string, error) {
+	if distro == "" {
+		panic("distro not specified")
+	}
+
+	distroObj, supportedDistro := supportedDistros[distro]
+	if !supportedDistro {
+		panicMsg := fmt.Sprintf("failed to find %s in the list of supported distributions", distro)
+		panic(panicMsg)
+	}
+
+	release, err := distroObj.GetDefaultRelease()
+	if err != nil {
+		return "", err
+	}
+
+	return release, nil
+}
+
+func getDefaultReleaseFedora() (string, error) {
+	release, err := getHostVersionID()
+	if err != nil {
+		return "", err
+	}
+
+	return release, nil
+}
+
+func getDefaultReleaseRHEL() (string, error) {
+	release, err := getHostVersionID()
+	if err != nil {
+		return "", err
+	}
+
+	return release, nil
+}
+
+func getDefaultReleaseUbuntu() (string, error) {
+	release, err := getHostVersionID()
+	if err != nil {
+		return "", err
+	}
+
+	return release, nil
+}
+
 func GetEnvOptionsForPreservedVariables() []string {
 	logrus.Debug("Creating list of environment variables to forward")