json-proxy: Add GetEngineConfig to advertise retry policy

common/pkg/json-proxy/handler.go

@@ -10,10 +10,12 @@ import (
 	"os"
 	"sync"
 	"syscall"
+	"time"
 
 	"github.com/opencontainers/go-digest"
 	imgspecv1 "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/sirupsen/logrus"
+	"go.podman.io/common/pkg/config"
 	"go.podman.io/image/v5/image"
 	"go.podman.io/image/v5/manifest"
 	"go.podman.io/image/v5/pkg/blobinfocache"
@@ -106,6 +108,54 @@ func (h *handler) Initialize(ctx context.Context, args []any) (replyBuf, error)
 	return r, nil
 }
 
+// GetEngineConfig returns a curated subset of the host containers.conf
+// [engine] settings (new in 0.2.9). Today this is the retry policy; clients
+// use it to drive retries with the same defaults as `podman pull` without
+// having to read containers.conf themselves.
+//
+// Note the proxy only advertises the configuration; it does not perform
+// retries itself, since a partially streamed blob cannot be transparently
+// resumed.
+func (h *handler) GetEngineConfig(_ context.Context, args []any) (replyBuf, error) {
+	h.lock.Lock()
+	defer h.lock.Unlock()
+
+	var ret replyBuf
+
+	if h.sysctx == nil {
+		return ret, errors.New("client error: must invoke Initialize")
+	}
+	if len(args) != 0 {
+		return ret, errors.New("invalid request, expecting zero arguments")
+	}
+
+	// config.Default() memoizes internally, so this is cheap to call here.
+	cfg, err := config.Default()
+	if err != nil {
+		return ret, fmt.Errorf("loading containers.conf: %w", err)
+	}
+
+	ec := engineConfig{
+		Retry: cfg.Engine.Retry,
+	}
+	// An empty retry_delay means "use exponential backoff"; only advertise a
+	// concrete value when one is configured. We normalize to milliseconds so
+	// clients don't have to parse Go's duration syntax.
+	if cfg.Engine.RetryDelay != "" {
+		d, err := time.ParseDuration(cfg.Engine.RetryDelay)
+		if err != nil {
+			return ret, fmt.Errorf("parsing containers.conf retry_delay %q: %w", cfg.Engine.RetryDelay, err)
+		}
+		// A negative delay is meaningless; treat it as unset (exponential
+		// backoff) rather than advertising a nonsensical value to clients.
+		if ms := d.Milliseconds(); ms > 0 {
+			ec.RetryDelayMS = &ms
+		}
+	}
+
+	return replyBuf{value: ec}, nil
+}
+
 // OpenImage accepts a string image reference i.e. TRANSPORT:REF - like `skopeo copy`.
 // The return value is an opaque integer handle.
 func (h *handler) OpenImage(ctx context.Context, args []any) (replyBuf, error) {
@@ -706,6 +756,8 @@ func (h *handler) processRequest(ctx context.Context, readBytes []byte) (rb repl
 	switch req.Method {
 	case "Initialize":
 		rb, err = h.Initialize(ctx, req.Args)
+	case "GetEngineConfig":
+		rb, err = h.GetEngineConfig(ctx, req.Args)
 	case "OpenImage":
 		rb, err = h.OpenImage(ctx, req.Args)
 	case "OpenImageOptional":

common/pkg/json-proxy/proxy_test.go

@@ -506,6 +506,32 @@ func TestProxyGetBlob(t *testing.T) {
 	assert.NoError(t, err)
 }
 
+func TestProxyGetEngineConfig(t *testing.T) {
+	p := newProxy(t)
+
+	v, err := p.callNoFd("GetEngineConfig", nil)
+	require.NoError(t, err)
+
+	// The reply value is a JSON object; it arrives as a map here.
+	m, ok := v.(map[string]any)
+	require.True(t, ok, "GetEngineConfig return value is %T", v)
+
+	// retry is always present. We don't assert a specific number since it
+	// comes from the host containers.conf, but it must be a non-negative
+	// integer (default 3).
+	retry, ok := m["retry"].(float64)
+	require.True(t, ok, "retry is %T", m["retry"])
+	assert.GreaterOrEqual(t, retry, float64(0))
+
+	// retry_delay_ms is optional (omitted when unset => exponential backoff).
+	// When present it must be a non-negative integer count of milliseconds.
+	if raw, present := m["retry_delay_ms"]; present {
+		delay, ok := raw.(float64)
+		require.True(t, ok, "retry_delay_ms is %T", raw)
+		assert.GreaterOrEqual(t, delay, float64(0))
+	}
+}
+
 func TestProxyPolicyVerification(t *testing.T) {
 	for _, tc := range []struct {
 		name      string

common/pkg/json-proxy/types.go

@@ -17,7 +17,11 @@ import (
 // departure from the original code which used HTTP.
 //
 // When bumping this, please also update the man page.
-const protocolVersion = "0.2.8"
+//
+// 0.2.9: Added the GetEngineConfig method, which exposes the host
+// containers.conf engine settings (e.g. retry policy) so clients can apply
+// them without re-reading the config themselves.
+const protocolVersion = "0.2.9"
 
 // maxMsgSize is the current limit on a packet size.
 // Note that all non-metadata (i.e. payload data) is sent over a pipe.
@@ -71,6 +75,23 @@ type reply struct {
 	Error string `json:"error"`
 }
 
+// engineConfig is the value returned by GetEngineConfig (new in 0.2.9). It
+// exposes a curated subset of the host containers.conf [engine] settings that
+// are relevant to a client driving image pulls through the proxy.
+//
+// This is intentionally extensible: fields are additive and optional, so
+// clients using serde(default)-style decoding remain forward compatible as new
+// settings are surfaced here.
+type engineConfig struct {
+	// Retry is the number of times a failed pull should be retried
+	// (containers.conf [engine] retry; default 3).
+	Retry uint `json:"retry"`
+	// RetryDelayMS is the fixed delay between retries in milliseconds
+	// (containers.conf [engine] retry_delay). It is omitted when unset, in
+	// which case the client should use exponential backoff.
+	RetryDelayMS *int64 `json:"retry_delay_ms,omitempty"`
+}
+
 // replyBuf is our internal deserialization of reply plus optional fd.
 type replyBuf struct {
 	// value will be converted to a reply Value.