builtins.validateJsonSchema

- Add https://github.com/pboettch/json-schema-validator
  as a dependency

  `json-schema-validator` is a library that uses `nlohmann` internally,
  which is being used in other components of Nix already

  `json-schema-validator` is MIT licensed,
  complies with the latest draft of JSON schema
  and is the second one with most stars on GitHub
- Add a new builtin for validating data with a JSON schema.

  This builtin receives two arguments: `data` and `schema`,
  both in Nix format
- Demo:
  ```nix
  let
    schema = {
      title = "A person";
      properties = {
        age = {
          description = "Age of the person";
          type = "number";
          minimum = 1;
          maximum = 200;
        };
        name = {
          description = "Complete Name for the person";
          first.type = "string";
          last.type = "string";
          required = [ "first" "last" ];
          type = "object";
        };
      };
      required = [ "name" "age" ];
      type = "object";
    };
  in
  builtins.map (builtins.validateJsonSchema schema) [
    { }
    { age = 24; name.first = "Jane"; }
    { age = 24; name.first = "Jane"; name.last = "Doe"; }
  ]
  ```

  ```bash
  $ nix-instantiate --eval --strict test.nix
  [
    { success = false;
      value = "At '/', required property 'name' not found in object"; }
    { success = false;
      value = "At '/name', required property 'last' not found in object"; }
    { success = true;
      value = { age = 24; name = { first = "Jane"; last = "Doe"; }; }; }
  ]
  ```

- Pending work:
  - Add docs
  - Add tests

Mentions NixOS#5392

src/libexpr/primops/validateJsonSchema.cc

@@ -0,0 +1,74 @@
+#include "eval-inline.hh"
+#include "primops.hh"
+#include "value-to-json.hh"
+
+#include <iostream>
+#include <sstream>
+
+#include <nlohmann/json-patch.cpp>
+#include <nlohmann/json-schema.hpp>
+#include <nlohmann/json-uri.cpp>
+#include <nlohmann/json-validator.cpp>
+#include <nlohmann/json.hpp>
+
+class custom_error_handler : public error_handler
+{
+    void error(const json::json_pointer &ptr, const json &instance, const std::string &message) override
+    {
+        std::string pos = ptr.to_string();
+
+        if (pos == "")
+            pos = "/";
+
+        throw std::invalid_argument("At '" + pos + "', " + message);
+    }
+};
+
+namespace nix
+{
+
+static void prim_validateJsonSchema(EvalState &state, const Pos &pos, Value **args, Value &v)
+{
+    state.forceValue(*args[0], pos);
+    state.forceValue(*args[1], pos);
+
+    PathSet context;
+    std::ostringstream dataStr;
+    std::ostringstream schemaStr;
+    printValueAsJSON(state, true, *args[0], schemaStr, context);
+    printValueAsJSON(state, true, *args[1], dataStr, context);
+
+    nlohmann::json dataJson = nlohmann::json::parse(dataStr.str());
+    nlohmann::json schemaJson = nlohmann::json::parse(schemaStr.str());
+
+    nlohmann::json_schema::json_validator validator;
+    custom_error_handler validator_error_handler;
+
+    state.mkAttrs(v, 2);
+    try
+    {
+        validator.set_root_schema(schemaJson);
+        validator.validate(dataJson, validator_error_handler);
+        v.attrs->push_back(Attr(state.sValue, args[1]));
+        mkBool(*state.allocAttr(v, state.symbols.create("success")), true);
+    }
+    catch (const std::exception &e)
+    {
+        Value *error = state.allocValue();
+        mkString(*error, e.what());
+        v.attrs->push_back(Attr(state.sValue, error));
+        mkBool(*state.allocAttr(v, state.symbols.create("success")), false);
+    }
+    v.attrs->sort();
+};
+
+static RegisterPrimOp r_validateJsonSchema({
+    .name = "validateJsonSchema",
+    .args = {"schema", "data"},
+    .doc = R"(
+        Validate data with the provided JSON schema.
+    )",
+    .fun = prim_validateJsonSchema,
+});
+
+} // namespace nix

src/nlohmann/json-patch.cpp

@@ -0,0 +1,121 @@
+/*
+ * JSON schema validator for JSON for modern C++
+ *
+ * Copyright (c) 2016-2019 Patrick Boettcher <p@yai.se>.
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ */
+#include "json-patch.hpp"
+
+#include <nlohmann/json-schema.hpp>
+
+namespace
+{
+
+// originally from http://jsonpatch.com/, http://json.schemastore.org/json-patch
+// with fixes
+const nlohmann::json patch_schema = R"patch({
+    "title": "JSON schema for JSONPatch files",
+    "$schema": "http://json-schema.org/draft-04/schema#",
+    "type": "array",
+
+    "items": {
+        "oneOf": [
+            {
+                "additionalProperties": false,
+                "required": [ "value", "op", "path"],
+                "properties": {
+                    "path" : { "$ref": "#/definitions/path" },
+                    "op": {
+                        "description": "The operation to perform.",
+                        "type": "string",
+                        "enum": [ "add", "replace", "test" ]
+                    },
+                    "value": {
+                        "description": "The value to add, replace or test."
+                    }
+                }
+            },
+            {
+                "additionalProperties": false,
+                "required": [ "op", "path"],
+                "properties": {
+                    "path" : { "$ref": "#/definitions/path" },
+                    "op": {
+                        "description": "The operation to perform.",
+                        "type": "string",
+                        "enum": [ "remove" ]
+                    }
+                }
+            },
+            {
+                "additionalProperties": false,
+                "required": [ "from", "op", "path" ],
+                "properties": {
+                    "path" : { "$ref": "#/definitions/path" },
+                    "op": {
+                        "description": "The operation to perform.",
+                        "type": "string",
+                        "enum": [ "move", "copy" ]
+                    },
+                    "from": {
+                        "$ref": "#/definitions/path",
+                        "description": "A JSON Pointer path pointing to the location to move/copy from."
+                    }
+                }
+            }
+        ]
+    },
+    "definitions": {
+        "path": {
+            "description": "A JSON Pointer path.",
+            "type": "string"
+        }
+    }
+})patch"_json;
+} // namespace
+
+namespace nlohmann
+{
+
+json_patch::json_patch(json &&patch) : j_(std::move(patch))
+{
+    validateJsonPatch(j_);
+}
+
+json_patch::json_patch(const json &patch) : j_(std::move(patch))
+{
+    validateJsonPatch(j_);
+}
+
+json_patch &json_patch::add(const json::json_pointer &ptr, json value)
+{
+    j_.push_back(json{{"op", "add"}, {"path", ptr}, {"value", std::move(value)}});
+    return *this;
+}
+
+json_patch &json_patch::replace(const json::json_pointer &ptr, json value)
+{
+    j_.push_back(json{{"op", "replace"}, {"path", ptr}, {"value", std::move(value)}});
+    return *this;
+}
+
+json_patch &json_patch::remove(const json::json_pointer &ptr)
+{
+    j_.push_back(json{{"op", "remove"}, {"path", ptr}});
+    return *this;
+}
+
+void json_patch::validateJsonPatch(json const &patch)
+{
+    // static put here to have it created at the first usage of validateJsonPatch
+    static nlohmann::json_schema::json_validator patch_validator(patch_schema);
+
+    patch_validator.validate(patch);
+
+    for (auto const &op : patch)
+        json::json_pointer(op["path"].get<std::string>());
+}
+
+} // namespace nlohmann

src/nlohmann/json-patch.hpp

@@ -0,0 +1,53 @@
+/*
+ * JSON schema validator for JSON for modern C++
+ *
+ * Copyright (c) 2016-2019 Patrick Boettcher <p@yai.se>.
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ */
+#pragma once
+
+#include <nlohmann/json.hpp>
+#include <string>
+
+namespace nlohmann
+{
+class JsonPatchFormatException : public std::exception
+{
+  public:
+    explicit JsonPatchFormatException(std::string msg) : ex_{std::move(msg)}
+    {
+    }
+
+    inline const char *what() const noexcept override final
+    {
+        return ex_.c_str();
+    }
+
+  private:
+    std::string ex_;
+};
+
+class json_patch
+{
+  public:
+    json_patch() = default;
+    json_patch(json &&patch);
+    json_patch(const json &patch);
+
+    json_patch &add(const json::json_pointer &, json value);
+    json_patch &replace(const json::json_pointer &, json value);
+    json_patch &remove(const json::json_pointer &);
+
+    operator json() const
+    {
+        return j_;
+    }
+
+  private:
+    json j_;
+
+    static void validateJsonPatch(json const &patch);
+};
+} // namespace nlohmann