Fix runaway allocation on /v2/_catalog

Introduced a Catalog entry in the configuration struct. With it,
it's possible to control the maximum amount of entries returned
by /v2/catalog (`GetCatalog` in registry/handlers/catalog.go).

It's set to a default value of 1000.

`GetCatalog` returns 100 entries by default if no `n` is
provided. When provided it will be validated to be between `0`
and `MaxEntries` defined in Configuration. When `n` is outside
the aforementioned boundary, ErrorCodePaginationNumberInvalid is
returned.

`GetCatalog` now handles `n=0` gracefully with an empty response
as well.

Signed-off-by: José D. Gómez R. <1josegomezr@gmail.com>
Co-authored-by: Cory Snider <corhere@gmail.com>

configuration/configuration.go

@@ -193,7 +193,8 @@ type Configuration struct {
 		} `yaml:"pool,omitempty"`
 	} `yaml:"redis,omitempty"`
 
-	Health Health `yaml:"health,omitempty"`
+	Health  Health  `yaml:"health,omitempty"`
+	Catalog Catalog `yaml:"catalog,omitempty"`
 
 	Proxy Proxy `yaml:"proxy,omitempty"`
 
@@ -244,6 +245,16 @@ type Configuration struct {
 	} `yaml:"policy,omitempty"`
 }
 
+// Catalog is composed of MaxEntries.
+// Catalog endpoint (/v2/_catalog) configuration, it provides the configuration
+// options to control the maximum number of entries returned by the catalog endpoint.
+type Catalog struct {
+	// Max number of entries returned by the catalog endpoint. Requesting n entries
+	// to the catalog endpoint will return at most MaxEntries entries.
+	// An empty or a negative value will set a default of 1000 maximum entries by default.
+	MaxEntries int `yaml:"maxentries,omitempty"`
+}
+
 // LogHook is composed of hook Level and Type.
 // After hooks configuration, it can execute the next handling automatically,
 // when defined levels of log message emitted.
@@ -670,6 +681,11 @@ func Parse(rd io.Reader) (*Configuration, error) {
 					if v0_1.Loglevel != Loglevel("") {
 						v0_1.Loglevel = Loglevel("")
 					}
+
+					if v0_1.Catalog.MaxEntries <= 0 {
+						v0_1.Catalog.MaxEntries = 1000
+					}
+
 					if v0_1.Storage.Type() == "" {
 						return nil, errors.New("no storage configuration provided")
 					}

configuration/configuration_test.go

@@ -71,6 +71,9 @@ var configStruct = Configuration{
 			},
 		},
 	},
+	Catalog: Catalog{
+		MaxEntries: 1000,
+	},
 	HTTP: struct {
 		Addr         string        `yaml:"addr,omitempty"`
 		Net          string        `yaml:"net,omitempty"`
@@ -524,6 +527,7 @@ func copyConfig(config Configuration) *Configuration {
 	configCopy.Version = MajorMinorVersion(config.Version.Major(), config.Version.Minor())
 	configCopy.Loglevel = config.Loglevel
 	configCopy.Log = config.Log
+	configCopy.Catalog = config.Catalog
 	configCopy.Log.Fields = make(map[string]interface{}, len(config.Log.Fields))
 	for k, v := range config.Log.Fields {
 		configCopy.Log.Fields[k] = v