allow setting cache dir via env var

Signed-off-by: Avi Deitcher <avi@deitcher.net>

src/cmd/linuxkit/build.go

@@ -51,9 +51,10 @@ func build(args []string) {
 	buildPull := buildCmd.Bool("pull", false, "Always pull images")
 	buildDocker := buildCmd.Bool("docker", false, "Check for images in docker before linuxkit cache")
 	buildDecompressKernel := buildCmd.Bool("decompress-kernel", false, "Decompress the Linux kernel (default false)")
-	buildCacheDir := buildCmd.String("cache", defaultLinuxkitCache(), "Directory for caching and finding cached image")
 	buildCmd.Var(&buildFormats, "format", "Formats to create [ "+strings.Join(outputTypes, " ")+" ]")
 	buildArch := buildCmd.String("arch", runtime.GOARCH, "target architecture for which to build")
+	cacheDir := flagOverEnvVarOverDefaultString{def: defaultLinuxkitCache(), envVar: envVarCacheDir}
+	buildCmd.Var(&cacheDir, "cache", fmt.Sprintf("Directory for caching and finding cached image, overrides env var %s", envVarCacheDir))
 
 	if err := buildCmd.Parse(args); err != nil {
 		log.Fatal("Unable to parse args")
@@ -98,8 +99,6 @@ func build(args []string) {
 		}
 	}
 
-	cacheDir := *buildCacheDir
-
 	if len(buildFormats) == 1 && moby.Streamable(buildFormats[0]) {
 		if *buildOutputFile == "" {
 			*buildOutputFile = filepath.Join(*buildDir, name+"."+buildFormats[0])
@@ -108,7 +107,7 @@ func build(args []string) {
 			*buildDir = ""
 		}
 	} else {
-		err := moby.ValidateFormats(buildFormats, cacheDir)
+		err := moby.ValidateFormats(buildFormats, cacheDir.String())
 		if err != nil {
 			log.Errorf("Error parsing formats: %v", err)
 			buildCmd.Usage()
@@ -204,7 +203,7 @@ func build(args []string) {
 	if moby.Streamable(buildFormats[0]) {
 		tp = buildFormats[0]
 	}
-	err = moby.Build(m, w, moby.BuildOpts{Pull: *buildPull, BuilderType: tp, DecompressKernel: *buildDecompressKernel, CacheDir: cacheDir, DockerCache: *buildDocker, Arch: *buildArch})
+	err = moby.Build(m, w, moby.BuildOpts{Pull: *buildPull, BuilderType: tp, DecompressKernel: *buildDecompressKernel, CacheDir: cacheDir.String(), DockerCache: *buildDocker, Arch: *buildArch})
 	if err != nil {
 		log.Fatalf("%v", err)
 	}
@@ -216,7 +215,7 @@ func build(args []string) {
 		}
 
 		log.Infof("Create outputs:")
-		err = moby.Formats(filepath.Join(*buildDir, name), image, buildFormats, size, cacheDir)
+		err = moby.Formats(filepath.Join(*buildDir, name), image, buildFormats, size, cacheDir.String())
 		if err != nil {
 			log.Fatalf("Error writing outputs: %v", err)
 		}

src/cmd/linuxkit/cache_clean.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"flag"
+	"fmt"
 	"os"
 
 	log "github.com/sirupsen/logrus"
@@ -10,14 +11,15 @@ import (
 func cacheClean(args []string) {
 	flags := flag.NewFlagSet("clean", flag.ExitOnError)
 
-	cacheDir := flags.String("cache", defaultLinuxkitCache(), "Directory for caching and finding cached image")
+	cacheDir := flagOverEnvVarOverDefaultString{def: defaultLinuxkitCache(), envVar: envVarCacheDir}
+	flags.Var(&cacheDir, "cache", fmt.Sprintf("Directory for caching and finding cached image, overrides env var %s", envVarCacheDir))
 
 	if err := flags.Parse(args); err != nil {
 		log.Fatal("Unable to parse args")
 	}
 
-	if err := os.RemoveAll(*cacheDir); err != nil {
+	if err := os.RemoveAll(cacheDir.String()); err != nil {
-		log.Fatalf("Unable to clean cache %s: %v", *cacheDir, err)
+		log.Fatalf("Unable to clean cache %s: %v", cacheDir, err)
 	}
-	log.Infof("Cache cleaned: %s", *cacheDir)
+	log.Infof("Cache cleaned: %s", cacheDir)
 }

src/cmd/linuxkit/cache_export.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"flag"
+	"fmt"
 	"io"
 	"os"
 	"runtime"
@@ -15,7 +16,8 @@ import (
 func cacheExport(args []string) {
 	fs := flag.NewFlagSet("export", flag.ExitOnError)
 
-	cacheDir := fs.String("cache", defaultLinuxkitCache(), "Directory for caching and finding cached image")
+	cacheDir := flagOverEnvVarOverDefaultString{def: defaultLinuxkitCache(), envVar: envVarCacheDir}
+	fs.Var(&cacheDir, "cache", fmt.Sprintf("Directory for caching and finding cached image, overrides env var %s", envVarCacheDir))
 	arch := fs.String("arch", runtime.GOARCH, "Architecture to resolve an index to an image, if the provided image name is an index")
 	outfile := fs.String("outfile", "", "Path to file to save output, '-' for stdout")
 	format := fs.String("format", "oci", "export format, one of 'oci', 'filesystem'")
@@ -33,7 +35,7 @@ func cacheExport(args []string) {
 	name := names[0]
 	fullname := util.ReferenceExpand(name)
 
-	p, err := cachepkg.NewProvider(*cacheDir)
+	p, err := cachepkg.NewProvider(cacheDir.String())
 	if err != nil {
 		log.Fatalf("unable to read a local cache: %v", err)
 	}

src/cmd/linuxkit/cache_ls.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"flag"
+	"fmt"
 
 	cachepkg "github.com/linuxkit/linuxkit/src/cmd/linuxkit/cache"
 	log "github.com/sirupsen/logrus"
@@ -10,14 +11,15 @@ import (
 func cacheList(args []string) {
 	flags := flag.NewFlagSet("list", flag.ExitOnError)
 
-	cacheDir := flags.String("cache", defaultLinuxkitCache(), "Directory for caching and finding cached image")
+	cacheDir := flagOverEnvVarOverDefaultString{def: defaultLinuxkitCache(), envVar: envVarCacheDir}
+	flags.Var(&cacheDir, "cache", fmt.Sprintf("Directory for caching and finding cached image, overrides env var %s", envVarCacheDir))
 
 	if err := flags.Parse(args); err != nil {
 		log.Fatal("Unable to parse args")
 	}
 
 	// list all of the images and content in the cache
-	p, err := cachepkg.Get(*cacheDir)
+	p, err := cachepkg.Get(cacheDir.String())
 	if err != nil {
 		log.Fatalf("unable to read a local cache: %v", err)
 	}

src/cmd/linuxkit/pkg_build.go

@@ -13,6 +13,7 @@ import (
 
 const (
 	buildersEnvVar = "LINUXKIT_BUILDERS"
+	envVarCacheDir = "LINUXKIT_CACHE"
 	// this is the most recent manifest pointed to by moby/buildkit:master as of 2022-07-22, so it includes
 	// our required commit. Once there is a normal semver tag later than this, we should switch to it.
 	defaultBuilderImage = "moby/buildkit@sha256:19c4637f8809f21af01dedf65f7f0d64636165d8191381ec9d5150fccedbae48"
@@ -44,7 +45,8 @@ func pkgBuildPush(args []string, withPush bool) {
 	builders := flags.String("builders", "", "Which builders to use for which platforms, e.g. linux/arm64=docker-context-arm64, overrides defaults and environment variables, see https://github.com/linuxkit/linuxkit/blob/master/docs/packages.md#Providing-native-builder-nodes")
 	builderImage := flags.String("builder-image", defaultBuilderImage, "buildkit builder container image to use")
 	builderRestart := flags.Bool("builder-restart", false, "force restarting builder, even if container with correct name and image exists")
-	buildCacheDir := flags.String("cache", defaultLinuxkitCache(), "Directory for storing built image, incompatible with --docker")
+	cacheDir := flagOverEnvVarOverDefaultString{def: defaultLinuxkitCache(), envVar: envVarCacheDir}
+	flags.Var(&cacheDir, "cache", fmt.Sprintf("Directory for caching and finding cached image, overrides env var %s", envVarCacheDir))
 
 	// some logic clarification:
 	// pkg build                   - always builds unless is in cache
@@ -84,7 +86,8 @@ func pkgBuildPush(args []string, withPush bool) {
 	if *ignoreCache {
 		opts = append(opts, pkglib.WithBuildIgnoreCache())
 	}
-	opts = append(opts, pkglib.WithBuildCacheDir(*buildCacheDir))
+
+	opts = append(opts, pkglib.WithBuildCacheDir(cacheDir.String()))
 
 	if withPush {
 		opts = append(opts, pkglib.WithBuildPush())

src/cmd/linuxkit/util.go

@@ -114,6 +114,31 @@ func stringToIntArray(l string, sep string) ([]int, error) {
 	return i, nil
 }
 
+// handle string with built-in default, overridden by env var, overridden by CLI flag
+type flagOverEnvVarOverDefaultString struct {
+	value  string
+	def    string
+	envVar string
+}
+
+func (f *flagOverEnvVarOverDefaultString) String() string {
+	val := f.def
+	if f.envVar != "" {
+		if e := os.Getenv(f.envVar); e != "" {
+			val = e
+		}
+	}
+	if f.value != "" {
+		val = f.value
+	}
+	return val
+}
+
+func (f *flagOverEnvVarOverDefaultString) Set(value string) error {
+	f.value = value
+	return nil
+}
+
 // Convert a multi-line string into an array of strings
 func splitLines(in string) []string {
 	res := []string{}