Fix optional request bodies

pkg/codegen/codegen_test.go

@@ -15,6 +15,7 @@ import (
 	"go/format"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -743,3 +744,33 @@ func TestCollectWebhookDefinitions(t *testing.T) {
 		}
 	})
 }
+
+// An operation whose requestBody is optional must accept a request that carries
+// none: the decoder reports an empty body, and the handler leaves opts.Body nil
+// so the request reaches the service. A required body keeps failing to decode.
+func TestOptionalRequestBodyToleratesAnEmptyBody(t *testing.T) {
+	cfg := Configuration{
+		PackageName: "api",
+		Output:      &Output{UseSingleFile: true},
+		Generate: &GenerateOptions{
+			Handler: &HandlerOptions{Kind: "chi"},
+		},
+	}
+
+	codes, err := Generate([]byte(readTestdata(t, "optional-request-body.yml")), cfg)
+	require.NoError(t, err)
+
+	code := codes.GetCombined()
+
+	assert.Contains(t, code, "case errors.Is(err, runtime.ErrRequestBodyEmpty):",
+		"the optional body admits a request that carries none")
+	assert.Equal(t, 1, strings.Count(code, "runtime.ErrRequestBodyEmpty"),
+		"only the optional operation admits it")
+
+	// The required body keeps the shape it had before, so no consumer's
+	// generated output shifts for an operation this does not change.
+	assert.Contains(t, code, "if err := a.jsonBodyDecoder(r.Body, &body); err != nil {")
+
+	_, err = format.Source([]byte(code))
+	require.NoError(t, err, "Generated code should compile without syntax errors")
+}

pkg/codegen/templates/handler/adapter.tmpl

@@ -13,6 +13,17 @@ 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.
 */}}
+{{- define "decode-error" -}}
+{{- if .HasTypedError }}
+        a.errHandler.HandleError(w, r, {{ .Op.Response.Error.StatusCode }}, New{{ .ErrorTypeName }}(err.Error()))
+{{- else }}
+        a.errHandler.HandleError(w, r, http.StatusBadRequest, OapiHandlerError{
+            Kind:        OapiErrorKindDecode,
+            OperationID: "{{ .Op.ID }}",
+            Message:     err.Error(),
+        })
+{{- end }}
+{{- end -}}
 {{- $config := .Config -}}
 {{- $operations := .Operations -}}
 {{- $serviceName := $config.Generate.Handler.Name -}}
@@ -332,19 +343,25 @@ func (a *HTTPAdapter) {{ $op.ID | ucFirst }}(w http.ResponseWriter, r *http.Requ
     defer r.Body.Close()
     {{- if or (eq $op.Body.ContentType "application/json") (hasSuffix $op.Body.ContentType "+json") }}
     var body {{ $op.Body.Name }}
+    {{- if $op.Body.Required }}
     if err := a.jsonBodyDecoder(r.Body, &body); err != nil {
-        {{- if $hasTypedError }}
-        a.errHandler.HandleError(w, r, {{ $op.Response.Error.StatusCode }}, New{{ $errorTypeName }}(err.Error()))
-        {{- else }}
-        a.errHandler.HandleError(w, r, http.StatusBadRequest, OapiHandlerError{
-            Kind:        OapiErrorKindDecode,
-            OperationID: "{{ $op.ID }}",
-            Message:     err.Error(),
-        })
-        {{- end }}
+        {{- template "decode-error" dict "Op" $op "HasTypedError" $hasTypedError "ErrorTypeName" $errorTypeName }}
         return
     }
     opts.Body = &body
+    {{- else }}
+    switch err := a.jsonBodyDecoder(r.Body, &body); {
+    case errors.Is(err, runtime.ErrRequestBodyEmpty):
+        // requestBody is optional, so a request carrying none leaves opts.Body
+        // nil rather than failing. Decoding into the zero value instead would
+        // hand the validator a body nobody sent, and fail on its required fields.
+    case err != nil:
+        {{- template "decode-error" dict "Op" $op "HasTypedError" $hasTypedError "ErrorTypeName" $errorTypeName }}
+        return
+    default:
+        opts.Body = &body
+    }
+    {{- end }}
     {{- else if eq $op.Body.ContentType "application/x-www-form-urlencoded" }}
     var body {{ $op.Body.Name }}
     formBytes, err := io.ReadAll(r.Body)

pkg/codegen/testdata/optional-request-body.yml

@@ -0,0 +1,57 @@
+openapi: 3.0.3
+info:
+  title: Optional request body
+  version: 1.0.0
+paths:
+  /things/{id}/settle:
+    post:
+      operationId: settleThing
+      parameters:
+        - name: id
+          in: path
+          required: true
+          schema:
+            type: string
+      requestBody:
+        required: false
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/SettleRequest'
+      responses:
+        '200':
+          description: Settled.
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Thing'
+  /things:
+    post:
+      operationId: createThing
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/Thing'
+      responses:
+        '201':
+          description: Created.
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Thing'
+components:
+  schemas:
+    Thing:
+      type: object
+      required: [name]
+      properties:
+        name:
+          type: string
+    SettleRequest:
+      type: object
+      required: [amount]
+      properties:
+        amount:
+          type: integer