Implement oapi-codegen/nullable

This bootstraps a new repository for the `oapi-codegen` organisation's
standards, and then implements the `Nullable` type as per [0] and [1].

Using a `map` as the underlying type allows us to take advantage of
`json.Marshal`'s inbuilt checks to determine whether to `omitempty` a
JSON value, which isn't possible with a `struct`.

We can make sure this is a multi-module project, similar to
other projects, so we can isolate test-only dependencies from the core
project, which has zero dependencies.

We can also add convenience helpers for `NewNullableWithValue` and
`NewNullNullable` as they can be useful when constructing `struct`s in
tests.

In the top-level project we can use runnable examples to indicate the
example usage and cover all the test cases we need, and then use the
`internal/test` package to perform further checks.

Co-authored-by: Sebastien Guilloux <sebastien.guilloux@elastic.co>
Co-authored-by: Ashutosh Kumar <ashutosh.kumar@elastic.co>

[0]: golang/go#64515 (comment)
[1]: https://github.com/sebgl/nullable/

.github/CODEOWNERS

@@ -0,0 +1 @@
+* @oapi-codegen/maintainers

.github/release-drafter.yml

@@ -0,0 +1 @@
+_extends: .github

.github/workflows/ci.yml

@@ -0,0 +1,24 @@
+name: Build project
+on: [ push, pull_request ]
+jobs:
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      # perform matrix testing to give us an earlier insight into issues with different versions of supported major versions of Go
+      matrix:
+        version:
+          - "1.20"
+          - "1.21"
+    steps:
+      - name: Check out source code
+        uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: ${{ matrix.version }}
+
+      - name: Test
+        run: make test

.github/workflows/lint.yml

@@ -0,0 +1,24 @@
+name: Lint project
+on: [push, pull_request]
+jobs:
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      # perform matrix testing to give us an earlier insight into issues with different versions of supported major versions of Go
+      matrix:
+        version:
+          - "1.20"
+          - "1.21"
+    steps:
+      - name: Check out source code
+        uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: ${{ matrix.version }}
+
+      - name: Run `make lint-ci`
+        run: make lint-ci

.github/workflows/release-drafter.yml

@@ -0,0 +1,25 @@
+name: Release Drafter
+
+on:
+  push:
+    branches:
+      - main
+  workflow_dispatch: {}
+
+permissions:
+  contents: read
+
+jobs:
+  update_release_draft:
+    permissions:
+      contents: write
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+      - uses: release-drafter/release-drafter@v5
+        with:
+          name: next
+          tag: next
+          version: next
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/tidy.yml

@@ -0,0 +1,27 @@
+name: Ensure `go mod tidy` has been run
+on: [ push, pull_request ]
+jobs:
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      # perform matrix testing to give us an earlier insight into issues with different versions of supported major versions of Go
+      matrix:
+        version:
+          - "1.20"
+          - "1.21"
+    steps:
+      - name: Check out source code
+        uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: ${{ matrix.version }}
+
+      - name: Install `tidied`
+        run: go install gitlab.com/jamietanna/tidied@latest
+
+      - name: Check for no untracked files
+        run: tidied -verbose

.gitignore

@@ -0,0 +1 @@
+/bin

LICENSE

@@ -0,0 +1,13 @@
+Copyright 2024 oapi-codegen
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.

Makefile

@@ -0,0 +1,28 @@
+GOBASE=$(shell pwd)
+GOBIN=$(GOBASE)/bin
+
+help:
+	@echo "This is a helper makefile for oapi-codegen"
+	@echo "Targets:"
+	@echo "    test:        run all tests"
+	@echo "    tidy         tidy go mod"
+	@echo "    lint         run linting"
+
+$(GOBIN)/golangci-lint:
+	curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s -- -b $(GOBIN) v1.55.2
+
+.PHONY: tools
+tools: $(GOBIN)/golangci-lint
+
+lint: tools
+	git ls-files go.mod '**/*go.mod' -z | xargs -0 -I{} bash -xc 'cd $$(dirname {}) && $(GOBIN)/golangci-lint run ./...'
+
+lint-ci: tools
+	git ls-files go.mod '**/*go.mod' -z | xargs -0 -I{} bash -xc 'cd $$(dirname {}) && $(GOBIN)/golangci-lint run ./... --out-format=github-actions --timeout=5m'
+
+test:
+	git ls-files go.mod '**/*go.mod' -z | xargs -0 -I{} bash -xc 'cd $$(dirname {}) && go test -cover ./...'
+
+tidy:
+	@echo "tidy..."
+	git ls-files go.mod '**/*go.mod' -z | xargs -0 -I{} bash -xc 'cd $$(dirname {}) && go mod tidy'

README.md

@@ -0,0 +1,47 @@
+# oapi-codegen/nullable
+
+> An implementation of a `Nullable` type for JSON bodies, indicating whether the field is absent, set to null, or set to a value
+
+Unlike other known implementations, this makes it possible to both marshal and unmarshal the value, as well as represent all three states:
+
+- the field is _not set_
+- the field is _explicitly set to null_
+- the field is _explicitly set to a given value_
+
+And can be embedded in structs, for instance with the following definition:
+
+```go
+obj := struct {
+		// RequiredID is a required, nullable field
+		RequiredID     nullable.Nullable[int]     `json:"id"`
+		// RequiredID is an optional, nullable field
+		OptionalString *nullable.Nullable[string] `json:"optionalString,omitempty"`
+}{}
+```
+
+## Usage
+
+> [!IMPORTANT]
+> Although this project is under the [oapi-codegen org](https://github.com/oapi-codegen) for the `oapi-codegen` OpenAPI-to-Go code generator, this is intentionally released as a separate, standalone library which can be used by other projects.
+
+First, add to your project with:
+
+```sh
+go get github.com/oapi-codegen/nullable
+```
+
+Check out the examples in [the package documentation on pkg.go.dev](https://pkg.go.dev/github.com/oapi-codegen/nullable) for more details.
+
+## Credits
+
+- [KumanekoSakura](https://github.com/KumanekoSakura), [via](https://github.com/golang/go/issues/64515#issuecomment-1841057182)
+- [Sebastien Guilloux], [via](https://github.com/sebgl/nullable/)
+
+As well as contributions from:
+
+- [Jamie Tanna](https://www.jvt.me)
+- [Ashutosh Kumar](https://github.com/sonasingh46)
+
+## License
+
+Licensed under the Apache-2.0 license.

go.mod

@@ -0,0 +1,3 @@
+module github.com/oapi-codegen/nullable
+
+go 1.20

internal/test/go.mod

@@ -0,0 +1,16 @@
+module github.com/oapi-codegen/nullable/internal/test
+
+go 1.20
+
+replace github.com/oapi-codegen/nullable => ../../
+
+require (
+	github.com/oapi-codegen/nullable v0.0.0-00010101000000-000000000000
+	github.com/stretchr/testify v1.8.4
+)
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)

internal/test/go.sum

@@ -0,0 +1,10 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/testify v1.8.4 h1:CcVxjf3Q8PM0mHUKJCdn+eZZtm5yQwehR5yeSVQQcUk=
+github.com/stretchr/testify v1.8.4/go.mod h1:sz/lmYIOXD/1dqDmKjjqLyZ2RngseejIcXlSw2iwfAo=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

internal/test/nullable_test.go

@@ -0,0 +1,88 @@
+package nullable_test
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/oapi-codegen/nullable"
+
+	"github.com/stretchr/testify/require"
+)
+
+type Obj struct {
+	Foo nullable.Nullable[string] `json:"foo,omitempty"` // note "omitempty" is important for fields that are optional
+}
+
+func TestNullable(t *testing.T) {
+	// --- parsing from json and serializing back to JSON
+
+	// -- case where there is an actual value
+	data := `{"foo":"bar"}`
+	// deserialize from json
+	myObj := parse(data, t)
+	require.Equal(t, myObj, Obj{Foo: nullable.Nullable[string]{true: "bar"}})
+	require.False(t, myObj.Foo.IsNull())
+	require.True(t, myObj.Foo.IsSpecified())
+	value, err := myObj.Foo.Get()
+	require.NoError(t, err)
+	require.Equal(t, "bar", value)
+	// serialize back to json: leads to the same data
+	require.Equal(t, data, serialize(myObj, t))
+
+	// -- case where no value is specified: parsed from JSON
+	data = `{}`
+	// deserialize from json
+	myObj = parse(data, t)
+	require.Equal(t, myObj, Obj{Foo: nil})
+	require.False(t, myObj.Foo.IsNull())
+	require.False(t, myObj.Foo.IsSpecified())
+	_, err = myObj.Foo.Get()
+	require.ErrorContains(t, err, "value is not specified")
+	// serialize back to json: leads to the same data
+	require.Equal(t, data, serialize(myObj, t))
+
+	// -- case where the specified value is explicitly null
+	data = `{"foo":null}`
+	// deserialize from json
+	myObj = parse(data, t)
+	require.Equal(t, myObj, Obj{Foo: nullable.Nullable[string]{false: ""}})
+	require.True(t, myObj.Foo.IsNull())
+	require.True(t, myObj.Foo.IsSpecified())
+	_, err = myObj.Foo.Get()
+	require.ErrorContains(t, err, "value is null")
+	// serialize back to json: leads to the same data
+	require.Equal(t, data, serialize(myObj, t))
+
+	// --- building objects from a Go client
+
+	// - case where there is an actual value
+	myObj = Obj{}
+	myObj.Foo.Set("bar")
+	require.Equal(t, `{"foo":"bar"}`, serialize(myObj, t))
+
+	// - case where the value should be unspecified
+	myObj = Obj{}
+	// do nothing: unspecified by default
+	require.Equal(t, `{}`, serialize(myObj, t))
+	// explicitly mark unspecified
+	myObj.Foo.SetUnspecified()
+	require.Equal(t, `{}`, serialize(myObj, t))
+
+	// - case where the value should be null
+	myObj = Obj{}
+	myObj.Foo.SetNull()
+	require.Equal(t, `{"foo":null}`, serialize(myObj, t))
+}
+
+func parse(data string, t *testing.T) Obj {
+	var myObj Obj
+	err := json.Unmarshal([]byte(data), &myObj)
+	require.NoError(t, err)
+	return myObj
+}
+
+func serialize(o Obj, t *testing.T) string {
+	data, err := json.Marshal(o)
+	require.NoError(t, err)
+	return string(data)
+}