DRA API: update some comments

Mentioning potential future API extensions help with understanding why the API
is defined the way it is when reading just the types.go. Those stand-alone
comments appear neither in the Go doc nor OpenAPI.

This corresponds to kubernetes/enhancements#4808.

Author: pohly

pkg/apis/resource/types.go

@@ -362,6 +362,12 @@ type DeviceClaim struct {
 	// +optional
 	// +listType=atomic
 	Config []DeviceClaimConfiguration
+
+	// Potential future extension, ignored by older schedulers. This is
+	// fine because scoring allows users to define a preference, without
+	// making it a hard requirement.
+	//
+	// Score *SomeScoringStruct
 }
 
 const (
@@ -552,6 +558,16 @@ type DeviceConstraint struct {
 	// +optional
 	// +oneOf=ConstraintType
 	MatchAttribute *FullyQualifiedName
+
+	// Potential future extension, not part of the current design:
+	// A CEL expression which compares different devices and returns
+	// true if they match.
+	//
+	// Because it would be part of a one-of, old schedulers will not
+	// accidentally ignore this additional, for them unknown match
+	// criteria.
+	//
+	// MatchExpression string
 }
 
 // DeviceClaimConfiguration is used for configuration parameters in DeviceClaim.

staging/src/k8s.io/api/resource/v1alpha3/types.go

@@ -368,6 +368,12 @@ type DeviceClaim struct {
 	// +optional
 	// +listType=atomic
 	Config []DeviceClaimConfiguration `json:"config,omitempty" protobuf:"bytes,3,opt,name=config"`
+
+	// Potential future extension, ignored by older schedulers. This is
+	// fine because scoring allows users to define a preference, without
+	// making it a hard requirement.
+	//
+	// Score *SomeScoringStruct
 }
 
 const (
@@ -558,6 +564,16 @@ type DeviceConstraint struct {
 	// +optional
 	// +oneOf=ConstraintType
 	MatchAttribute *FullyQualifiedName `json:"matchAttribute,omitempty" protobuf:"bytes,2,opt,name=matchAttribute"`
+
+	// Potential future extension, not part of the current design:
+	// A CEL expression which compares different devices and returns
+	// true if they match.
+	//
+	// Because it would be part of a one-of, old schedulers will not
+	// accidentally ignore this additional, for them unknown match
+	// criteria.
+	//
+	// MatchExpression string
 }
 
 // DeviceClaimConfiguration is used for configuration parameters in DeviceClaim.

staging/src/k8s.io/kubelet/pkg/apis/dra/v1alpha4/api.pb.go

@@ -67,7 +67,7 @@ message NodePrepareResourceResponse {
     // may have zero or more devices.
     repeated Device devices = 1;
     // If non-empty, preparing the ResourceClaim failed.
-    // cdi_devices is ignored in that case.
+    // Devices are ignored in that case.
     string error = 2;
 }