allow controlling detected platforms cache timeout

Because detecting emulator changes can be relatively
expensive, avoid doing it very frequently. A new config
parameter allows controlling if users prefer more or
less frequent updates or want to disable detecting
changes completely and only rely on emultor configuration
at boot time.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

cmd/buildkitd/config/config.go

@@ -38,6 +38,14 @@ type Config struct {
 		Dockerfile DockerfileFrontendConfig `toml:"dockerfile.v0"`
 		Gateway    GatewayFrontendConfig    `toml:"gateway.v0"`
 	} `toml:"frontend"`
+
+	System *SystemConfig `toml:"system"`
+}
+
+type SystemConfig struct {
+	// PlatformCacheMaxAge controls how often supported platforms
+	// are refreshed by rescanning the system.
+	PlatformsCacheMaxAge *Duration `toml:"platformsCacheMaxAge"`
 }
 
 type LogConfig struct {

cmd/buildkitd/main.go

@@ -262,6 +262,12 @@ func main() {
 			logrus.SetLevel(logrus.TraceLevel)
 		}
 
+		if sc := cfg.System; sc != nil {
+			if v := sc.PlatformsCacheMaxAge; v != nil {
+				archutil.CacheMaxAge = v.Duration
+			}
+		}
+
 		if cfg.GRPC.DebugAddress != "" {
 			if err := setupDebugHandlers(cfg.GRPC.DebugAddress); err != nil {
 				return err

util/archutil/detect.go

@@ -4,21 +4,28 @@ import (
 	"sort"
 	"strings"
 	"sync"
+	"time"
 
 	"github.com/containerd/containerd/platforms"
 	"github.com/moby/buildkit/util/bklog"
 	ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
-var mu sync.Mutex
-var arr []ocispecs.Platform
+var CacheMaxAge = 20 * time.Second
+
+var (
+	mu          sync.Mutex
+	arr         []ocispecs.Platform
+	lastRefresh time.Time
+)
 
 func SupportedPlatforms(noCache bool) []ocispecs.Platform {
 	mu.Lock()
 	defer mu.Unlock()
-	if !noCache && arr != nil {
+	if arr != nil && (!noCache || CacheMaxAge < 0 || time.Since(lastRefresh) < CacheMaxAge) {
 		return arr
 	}
+	defer func() { lastRefresh = time.Now() }()
 	def := nativePlatform()
 	arr = append([]ocispecs.Platform{}, def)
 

worker/base/worker.go

@@ -245,10 +245,14 @@ func (w *Worker) Labels() map[string]string {
 
 func (w *Worker) Platforms(noCache bool) []ocispecs.Platform {
 	if noCache {
+		matchers := make([]platforms.MatchComparer, len(w.WorkerOpt.Platforms))
+		for i, p := range w.WorkerOpt.Platforms {
+			matchers[i] = platforms.Only(p)
+		}
 		for _, p := range archutil.SupportedPlatforms(noCache) {
 			exists := false
-			for _, pp := range w.WorkerOpt.Platforms {
-				if platforms.Only(pp).Match(p) {
+			for _, m := range matchers {
+				if m.Match(p) {
 					exists = true
 					break
 				}