Add config checking for inflight limits

When API Priority and Fairness is enabled, the inflight limits must
add up to something positive.
This rejects the configuration that prompted
https://github.com/kubernetes/kubernetes/issues/102885

Update help for max inflight flags

cmd/kube-apiserver/app/server.go

@@ -466,10 +466,11 @@ func buildGenericConfig(
 		pluginInitializers...)
 	if err != nil {
 		lastErr = fmt.Errorf("failed to initialize admission: %v", err)
+		return
 	}
 
 	if utilfeature.DefaultFeatureGate.Enabled(genericfeatures.APIPriorityAndFairness) && s.GenericServerRunOptions.EnablePriorityAndFairness {
-		genericConfig.FlowControl = BuildPriorityAndFairness(s, clientgoExternalClient, versionedInformers)
+		genericConfig.FlowControl, lastErr = BuildPriorityAndFairness(s, clientgoExternalClient, versionedInformers)
 	}
 
 	return
@@ -491,13 +492,16 @@ func BuildAuthorizer(s *options.ServerRunOptions, EgressSelector *egressselector
 }
 
 // BuildPriorityAndFairness constructs the guts of the API Priority and Fairness filter
-func BuildPriorityAndFairness(s *options.ServerRunOptions, extclient clientgoclientset.Interface, versionedInformer clientgoinformers.SharedInformerFactory) utilflowcontrol.Interface {
+func BuildPriorityAndFairness(s *options.ServerRunOptions, extclient clientgoclientset.Interface, versionedInformer clientgoinformers.SharedInformerFactory) (utilflowcontrol.Interface, error) {
+	if s.GenericServerRunOptions.MaxRequestsInFlight+s.GenericServerRunOptions.MaxMutatingRequestsInFlight <= 0 {
+		return nil, fmt.Errorf("invalid configuration: MaxRequestsInFlight=%d and MaxMutatingRequestsInFlight=%d; they must add up to something positive", s.GenericServerRunOptions.MaxRequestsInFlight, s.GenericServerRunOptions.MaxMutatingRequestsInFlight)
+	}
 	return utilflowcontrol.New(
 		versionedInformer,
 		extclient.FlowcontrolV1beta1(),
 		s.GenericServerRunOptions.MaxRequestsInFlight+s.GenericServerRunOptions.MaxMutatingRequestsInFlight,
 		s.GenericServerRunOptions.RequestTimeout/4,
-	)
+	), nil
 }
 
 // completedServerRunOptions is a private wrapper that enforces a call of Complete() before Run can be invoked.

staging/src/k8s.io/apiserver/pkg/server/options/recommended.go

@@ -17,6 +17,8 @@ limitations under the License.
 package options
 
 import (
+	"fmt"
+
 	"github.com/spf13/pflag"
 
 	"k8s.io/apimachinery/pkg/runtime"
@@ -126,6 +128,10 @@ func (o *RecommendedOptions) ApplyTo(config *server.RecommendedConfig) error {
 	}
 	if feature.DefaultFeatureGate.Enabled(features.APIPriorityAndFairness) {
 		if config.ClientConfig != nil {
+			if config.MaxRequestsInFlight+config.MaxMutatingRequestsInFlight <= 0 {
+				return fmt.Errorf("invalid configuration: MaxRequestsInFlight=%d and MaxMutatingRequestsInFlight=%d; they must add up to something positive", config.MaxRequestsInFlight, config.MaxMutatingRequestsInFlight)
+
+			}
 			config.FlowControl = utilflowcontrol.New(
 				config.SharedInformerFactory,
 				kubernetes.NewForConfigOrDie(config.ClientConfig).FlowcontrolV1beta1(),

staging/src/k8s.io/apiserver/pkg/server/options/server_run_options.go

@@ -204,12 +204,16 @@ func (s *ServerRunOptions) AddUniversalFlags(fs *pflag.FlagSet) {
 		"DEPRECATED: the namespace from which the Kubernetes master services should be injected into pods.")
 
 	fs.IntVar(&s.MaxRequestsInFlight, "max-requests-inflight", s.MaxRequestsInFlight, ""+
-		"The maximum number of non-mutating requests in flight at a given time. When the server exceeds this, "+
+		"This and --max-mutating-requests-inflight are summed to determine the server's total concurrency limit "+
-		"it rejects requests. Zero for no limit.")
+		"(which must be positive) if --enable-priority-and-fairness is true. "+
+		"Otherwise, this flag limits the maximum number of non-mutating requests in flight, "+
+		"or a zero value disables the limit completely.")
 
 	fs.IntVar(&s.MaxMutatingRequestsInFlight, "max-mutating-requests-inflight", s.MaxMutatingRequestsInFlight, ""+
-		"The maximum number of mutating requests in flight at a given time. When the server exceeds this, "+
+		"This and --max-requests-inflight are summed to determine the server's total concurrency limit "+
-		"it rejects requests. Zero for no limit.")
+		"(which must be positive) if --enable-priority-and-fairness is true. "+
+		"Otherwise, this flag limits the maximum number of mutating requests in flight, "+
+		"or a zero value disables the limit completely.")
 
 	fs.DurationVar(&s.RequestTimeout, "request-timeout", s.RequestTimeout, ""+
 		"An optional field indicating the duration a handler must keep a request open before timing "+