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>

registry/api/v2/descriptors.go

@@ -134,6 +134,19 @@ var (
 		},
 	}
 
+	invalidPaginationResponseDescriptor = ResponseDescriptor{
+		Name:        "Invalid pagination number",
+		Description: "The received parameter n was invalid in some way, as described by the error code. The client should resolve the issue and retry the request.",
+		StatusCode:  http.StatusBadRequest,
+		Body: BodyDescriptor{
+			ContentType: "application/json",
+			Format:      errorsBody,
+		},
+		ErrorCodes: []errcode.ErrorCode{
+			ErrorCodePaginationNumberInvalid,
+		},
+	}
+
 	repositoryNotFoundResponseDescriptor = ResponseDescriptor{
 		Name:        "No Such Repository Error",
 		StatusCode:  http.StatusNotFound,
@@ -490,6 +503,7 @@ var routeDescriptors = []RouteDescriptor{
 							},
 						},
 						Failures: []ResponseDescriptor{
+							invalidPaginationResponseDescriptor,
 							unauthorizedResponseDescriptor,
 							repositoryNotFoundResponseDescriptor,
 							deniedResponseDescriptor,
@@ -1578,6 +1592,9 @@ var routeDescriptors = []RouteDescriptor{
 								},
 							},
 						},
+						Failures: []ResponseDescriptor{
+							invalidPaginationResponseDescriptor,
+						},
 					},
 				},
 			},

registry/api/v2/errors.go

@@ -133,4 +133,13 @@ var (
 		longer proceed.`,
 		HTTPStatusCode: http.StatusNotFound,
 	})
+
+	ErrorCodePaginationNumberInvalid = errcode.Register(errGroup, errcode.ErrorDescriptor{
+		Value:   "PAGINATION_NUMBER_INVALID",
+		Message: "invalid number of results requested",
+		Description: `Returned when the "n" parameter (number of results
+		to return) is not an integer, "n" is negative or "n" is bigger than
+		the maximum allowed.`,
+		HTTPStatusCode: http.StatusBadRequest,
+	})
 )