predicates: add Config predicate

E.g. it could be used to let skipper instance know in which zone it is deployed:
```yaml
containers:
  - env:
      - name: TOPOLOGY_ZONE
        valueFrom:
          fieldRef:
            fieldPath: metadata.annotations['topology.kubernetes.io/zone']
    args:
      - skipper
      - -config-predicate-values=zone=$(TOPOLOGY_ZONE)
      - ...
```

such that it only enables specific route for that zone:
```
za: Path("/foo") && Config("zone", "a") -> "http://backend.zone-a.test";
zb: Path("/foo") && Config("zone", "b") -> "http://backend.zone-b.test";
zc: Path("/foo") && Config("zone", "c") -> "http://backend.zone-c.test";
```

Signed-off-by: Alexander Yastrebov <[email protected]>

config/config.go

@@ -66,6 +66,7 @@ type Config struct {
 	DataclientPlugins               *pluginFlag    `yaml:"dataclient-plugin"`
 	MultiPlugins                    *pluginFlag    `yaml:"multi-plugin"`
 	CompressEncodings               *listFlag      `yaml:"compress-encodings"`
+	ConfigPredicateValues           mapFlags       `yaml:"config-predicate-values"`
 
 	// logging, metrics, profiling, tracing:
 	EnablePrometheusMetrics             bool      `yaml:"enable-prometheus-metrics"`
@@ -348,6 +349,7 @@ func NewConfig() *Config {
 	flag.Var(cfg.DataclientPlugins, "dataclient-plugin", "set a custom dataclient plugins to load, a comma separated list of name and arguments")
 	flag.Var(cfg.MultiPlugins, "multi-plugin", "set a custom multitype plugins to load, a comma separated list of name and arguments")
 	flag.Var(cfg.CompressEncodings, "compress-encodings", "set encodings supported for compression, the order defines priority when Accept-Header has equal quality values, see RFC 7231 section 5.3.1")
+	flag.Var(&cfg.ConfigPredicateValues, "config-predicate-values", "sets name-value pairs for Config predicate")
 
 	// logging, metrics, tracing:
 	flag.BoolVar(&cfg.EnablePrometheusMetrics, "enable-prometheus-metrics", false, "*Deprecated*: use metrics-flavour. Switch to Prometheus metrics format to expose metrics")
@@ -704,6 +706,7 @@ func (c *Config) ToOptions() skipper.Options {
 		Plugins:                         c.MultiPlugins.values,
 		PluginDirs:                      []string{skipper.DefaultPluginDir},
 		CompressEncodings:               c.CompressEncodings.values,
+		ConfigPredicateValues:           c.ConfigPredicateValues.values,
 
 		// logging, metrics, profiling, tracing:
 		EnablePrometheusMetrics:             c.EnablePrometheusMetrics,

predicates/predicates.go

@@ -24,6 +24,7 @@ const (
 	TrueName                  = "True"
 	FalseName                 = "False"
 	ShutdownName              = "Shutdown"
+	ConfigName                = "Config"
 	MethodName                = "Method"
 	MethodsName               = "Methods"
 	HeaderName                = "Header"

predicates/primitive/config.go

@@ -0,0 +1,49 @@
+package primitive
+
+import (
+	"regexp"
+
+	"github.com/zalando/skipper/predicates"
+	"github.com/zalando/skipper/routing"
+)
+
+type (
+	configSpec struct {
+		values map[string]string
+	}
+)
+
+// NewConfig provides a predicate spec to create predicates
+// that evaluate to true if config value matches regular expression
+func NewConfig(values map[string]string) routing.PredicateSpec {
+	return &configSpec{values}
+}
+
+func (*configSpec) Name() string { return predicates.ConfigName }
+
+func (s *configSpec) Create(args []interface{}) (routing.Predicate, error) {
+	if len(args) != 2 {
+		return nil, predicates.ErrInvalidPredicateParameters
+	}
+
+	name, ok := args[0].(string)
+	if !ok {
+		return nil, predicates.ErrInvalidPredicateParameters
+	}
+
+	value, ok := args[1].(string)
+	if !ok {
+		return nil, predicates.ErrInvalidPredicateParameters
+	}
+
+	re, err := regexp.Compile(value)
+	if err != nil {
+		return nil, predicates.ErrInvalidPredicateParameters
+	}
+
+	if re.MatchString(s.values[name]) {
+		return &truePredicate{}, nil
+	} else {
+		return &falsePredicate{}, nil
+	}
+}

skipper.go

@@ -806,6 +806,9 @@ type Options struct {
 	// CompressEncodings, if not empty replace default compression encodings
 	CompressEncodings []string
 
+	// ConfigPredicateValues sets name-value pairs for Config predicate
+	ConfigPredicateValues map[string]string
+
 	// OIDCSecretsFile path to the file containing key to encrypt OpenID token
 	OIDCSecretsFile string
 
@@ -1798,6 +1801,7 @@ func run(o Options, sig chan os.Signal, idleConnsCH chan struct{}) error {
 		primitive.NewTrue(),
 		primitive.NewFalse(),
 		primitive.NewShutdown(),
+		primitive.NewConfig(o.ConfigPredicateValues),
 		pauth.NewJWTPayloadAllKV(),
 		pauth.NewJWTPayloadAnyKV(),
 		pauth.NewJWTPayloadAllKVRegexp(),