fix: Prevent panic when parsing empty documents (#389)

The ParseDocument functions were returning the first element of the content slice unconditionally, which panicked if the given document bytes were empty. Check if the slice has some elements and return an error if it's empty instead.

discovery/discovery_test.go

@@ -46,3 +46,28 @@ func TestParseDocument(t *testing.T) {
 		t.Errorf("unexpected value for Title: %s (expected %s)", d.Title, title)
 	}
 }
+
+func TestParseDocument_Empty(t *testing.T) {
+	for _, test := range []struct {
+		name string
+		data []byte
+	}{
+		{"nil", nil},
+		{"zero_bytes", []byte{}},
+		{"whitespace", []byte("   ")},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			d, err := ParseDocument(test.data)
+
+			t.Log(err)
+			if err == nil {
+				t.Error("expected error")
+			} else if want, got := "document has no content", err.Error(); want != got {
+				t.Errorf("unexpected error: %q (expected %q)", got, want)
+			}
+			if d != nil {
+				t.Error("expected document to be nil")
+			}
+		})
+	}
+}

discovery/document.go

@@ -15,6 +15,8 @@
 package discovery_v1
 
 import (
+	"errors"
+
 	"github.com/google/gnostic/compiler"
 )
 
@@ -29,6 +31,11 @@ func ParseDocument(b []byte) (*Document, error) {
 	if err != nil {
 		return nil, err
 	}
+
+	if len(info.Content) < 1 {
+		return nil, errors.New("document has no content")
+	}
+
 	root := info.Content[0]
 	return NewDocument(root, compiler.NewContext("$root", root, nil))
 }

openapiv2/document.go

@@ -15,6 +15,8 @@
 package openapi_v2
 
 import (
+	"errors"
+
 	"gopkg.in/yaml.v3"
 
 	"github.com/google/gnostic/compiler"
@@ -26,6 +28,11 @@ func ParseDocument(b []byte) (*Document, error) {
 	if err != nil {
 		return nil, err
 	}
+
+	if len(info.Content) < 1 {
+		return nil, errors.New("document has no content")
+	}
+
 	root := info.Content[0]
 	return NewDocument(root, compiler.NewContextWithExtensions("$root", root, nil, nil))
 }

openapiv2/document_test.go

@@ -0,0 +1,42 @@
+// Copyright 2023 Google LLC. All Rights Reserved.
+//
+// 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.
+
+package openapi_v2
+
+import "testing"
+
+func TestParseDocument_Empty(t *testing.T) {
+	for _, test := range []struct {
+		name string
+		data []byte
+	}{
+		{"nil", nil},
+		{"zero_bytes", []byte{}},
+		{"whitespace", []byte("   ")},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			d, err := ParseDocument(test.data)
+
+			t.Log(err)
+			if err == nil {
+				t.Error("expected error")
+			} else if want, got := "document has no content", err.Error(); want != got {
+				t.Errorf("unexpected error: %q (expected %q)", got, want)
+			}
+			if d != nil {
+				t.Error("expected document to be nil")
+			}
+		})
+	}
+}

openapiv3/document.go

@@ -15,6 +15,8 @@
 package openapi_v3
 
 import (
+	"errors"
+
 	"gopkg.in/yaml.v3"
 
 	"github.com/google/gnostic/compiler"
@@ -26,6 +28,11 @@ func ParseDocument(b []byte) (*Document, error) {
 	if err != nil {
 		return nil, err
 	}
+
+	if len(info.Content) < 1 {
+		return nil, errors.New("document has no content")
+	}
+
 	root := info.Content[0]
 	return NewDocument(root, compiler.NewContextWithExtensions("$root", root, nil, nil))
 }

openapiv3/openapiv3_test.go

@@ -36,3 +36,28 @@ func TestParseDocument(t *testing.T) {
 		t.Errorf("unexpected value for Title: %s (expected %s)", d.Info.Title, title)
 	}
 }
+
+func TestParseDocument_Empty(t *testing.T) {
+	for _, test := range []struct {
+		name string
+		data []byte
+	}{
+		{"nil", nil},
+		{"zero_bytes", []byte{}},
+		{"whitespace", []byte("   ")},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			d, err := ParseDocument(test.data)
+
+			t.Log(err)
+			if err == nil {
+				t.Error("expected error")
+			} else if want, got := "document has no content", err.Error(); want != got {
+				t.Errorf("unexpected error: %q (expected %q)", got, want)
+			}
+			if d != nil {
+				t.Error("expected document to be nil")
+			}
+		})
+	}
+}