api: image inspect: remove temporary backfill for Config fields

Commit 4dc961d0e9 (API v1.50) fixed the type
used for Config to point to the correct type, which is the Config struct
from the [Docker image spec] (which embeds the [OCI Image Specification]
type); however, those types use an omitempty, which wasn't documented as
part of the API changes, so f85394dd5d added
a temporary backfill for empty fields.

This removes that backfill for API v1.52 so that empty image config fields
are now omitted.

[OCI Image Specification]: https://github.com/opencontainers/image-spec/blob/v1.1.1/specs-go/v1/config.go#L23-L62
[Docker image spec]: https://github.com/moby/docker-image-spec/blob/v1.3.1/specs-go/v1/image.go#L19-L32

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

api/docs/CHANGELOG.md

@@ -26,6 +26,9 @@ keywords: "API, Docker, rcli, REST, documentation"
   a PortBinding with an empty HostIP and HostPort when calling `POST /containers/{id}/start`.
   This behavior is now deprecated, and a warning is returned by `POST /containers/create`.
   The next API version will drop empty `PortBindings` list altogether.
+* `GET /images/{name}/json` now omits the following `Config` fields when
+  not set, to closer align with the implementation of the [OCI Image Specification](https://github.com/opencontainers/image-spec/blob/v1.1.1/specs-go/v1/config.go#L23-L62)
+  `Cmd`, `Entrypoint`, `Env`, `Labels`, `OnBuild`, `User`, `Volumes`, and `WorkingDir`.
 
 ## v1.51 API changes
 

daemon/server/router/image/image_routes.go

@@ -372,12 +372,8 @@ func (ir *imageRouter) getImagesByName(ctx context.Context, w http.ResponseWrite
 		return err
 	}
 
-	// inspectResponse preserves fields in the response that have an
-	// "omitempty" in the OCI spec, but didn't omit such fields in
-	// legacy responses before API v1.50.
 	imageInspect := &inspectCompatResponse{
 		InspectResponse: resp,
-		legacyConfig:    legacyConfigFields["current"],
 	}
 
 	// Make sure we output empty arrays instead of nil. While Go nil slice is functionally equivalent to an empty slice,
@@ -406,8 +402,15 @@ func (ir *imageRouter) getImagesByName(ctx context.Context, w http.ResponseWrite
 	if versions.LessThan(version, "1.48") {
 		imageInspect.Descriptor = nil
 	}
-	if versions.LessThan(version, "1.50") {
-		imageInspect.legacyConfig = legacyConfigFields["v1.49"]
+	if versions.LessThan(version, "1.52") {
+		if versions.LessThan(version, "1.50") {
+			imageInspect.legacyConfig = legacyConfigFields["v1.49"]
+		} else {
+			// inspectResponse preserves fields in the response that have an
+			// "omitempty" in the OCI spec, but didn't omit such fields in
+			// legacy responses before API v1.50.
+			imageInspect.legacyConfig = legacyConfigFields["v1.50-v1.51"]
+		}
 	}
 
 	return httputils.WriteJSON(w, http.StatusOK, imageInspect)

daemon/server/router/image/inspect_response.go

@@ -31,10 +31,10 @@ var legacyConfigFields = map[string]map[string]any{
 		"Volumes":      nil,
 		"WorkingDir":   "",
 	},
-	// Legacy fields for current API versions (v1.50 and up). These fields
+	// Legacy fields for current API versions (v1.50 and v1.52). These fields
 	// did not have an "omitempty" and were always included in the response,
 	// even if not set; see https://github.com/moby/moby/issues/50134
-	"current": {
+	"v1.50-v1.51": {
 		"Cmd":        nil,
 		"Entrypoint": nil,
 		"Env":        nil,

daemon/server/router/image/inspect_response_test.go

@@ -40,12 +40,12 @@ func TestInspectResponse(t *testing.T) {
 			expected:     `{"AttachStderr":false,"AttachStdin":false,"AttachStdout":false,"Cmd":["/bin/sh"],"Domainname":"","Entrypoint":null,"Env":null,"Hostname":"","Image":"","Labels":null,"OnBuild":null,"OpenStdin":false,"StdinOnce":false,"StopSignal":"SIGQUIT","Tty":false,"User":"","Volumes":null,"WorkingDir":""}`,
 		},
 		{
-			doc: "api >= v1.50",
+			doc: "api v1.50 - v1.51",
 			cfg: &ocispec.ImageConfig{
 				Cmd:        []string{"/bin/sh"},
 				StopSignal: "SIGQUIT",
 			},
-			legacyConfig: legacyConfigFields["current"],
+			legacyConfig: legacyConfigFields["v1.50-v1.51"],
 			expected:     `{"Cmd":["/bin/sh"],"Entrypoint":null,"Env":null,"Labels":null,"OnBuild":null,"StopSignal":"SIGQUIT","User":"","Volumes":null,"WorkingDir":""}`,
 		},
 	}