test: added get headers builtin tests (#434)

* test: added get headers builtin tests

* refactor: comments+indent

Author: fredmaggiowski

custom_builtins/get_header.go

@@ -40,15 +40,17 @@ var GetHeaderFunction = rego.Function2(
 		Name: GetHeaderDecl.Name,
 		Decl: GetHeaderDecl.Decl,
 	},
-	func(_ rego.BuiltinContext, a, b *ast.Term) (*ast.Term, error) {
-		var headerKey string
-		var headers http.Header
-		if err := ast.As(a.Value, &headerKey); err != nil {
-			return nil, err
-		}
-		if err := ast.As(b.Value, &headers); err != nil {
-			return nil, err
-		}
-		return ast.StringTerm(headers.Get(headerKey)), nil
-	},
+	getHeaderDefinition,
 )
+
+func getHeaderDefinition(_ rego.BuiltinContext, a, b *ast.Term) (*ast.Term, error) {
+	var headerKey string
+	var headers http.Header
+	if err := ast.As(a.Value, &headerKey); err != nil {
+		return nil, err
+	}
+	if err := ast.As(b.Value, &headers); err != nil {
+		return nil, err
+	}
+	return ast.StringTerm(headers.Get(headerKey)), nil
+}

custom_builtins/get_header_test.go

@@ -0,0 +1,91 @@
+// Copyright 2025 Mia srl
+//
+// 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 custom_builtins
+
+import (
+	"encoding/json"
+	"net/textproto"
+	"testing"
+
+	"github.com/open-policy-agent/opa/ast"
+	"github.com/open-policy-agent/opa/rego"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetHeader(t *testing.T) {
+
+	buildMapTerm := func(t *testing.T, data map[string][]string) *ast.Term {
+		t.Helper()
+
+		terms := make([][2]*ast.Term, 0, len(data))
+
+		for key, values := range data {
+
+			valueTermArray := make([]*ast.Term, len(values))
+			for i, val := range values {
+				valueTermArray[i] = ast.StringTerm(val)
+			}
+
+			terms = append(terms, [2]*ast.Term{
+				// NOTE: textproto.CanonicalMIMEHeaderKey is used to canonicalize
+				// the key in the same way the net/http package does.
+				ast.StringTerm(textproto.CanonicalMIMEHeaderKey(key)),
+				{Value: ast.NewArray(valueTermArray...)},
+			})
+		}
+
+		return &ast.Term{Value: ast.NewObject(terms...)}
+	}
+
+	t.Run("GetHeader", func(t *testing.T) {
+		t.Run("returns correct value when header exists", func(t *testing.T) {
+			foundTerm, err := getHeaderDefinition(
+				rego.BuiltinContext{},
+				ast.StringTerm("X-My-Header"), // search with canonicalized key
+				buildMapTerm(t, map[string][]string{"X-My-Header": {"value"}}),
+			)
+			require.NoError(t, err)
+			require.Equal(t, ast.StringTerm("value"), foundTerm)
+		})
+
+		t.Run("returns correct value when header exists case-insensitive", func(t *testing.T) {
+			foundTerm, err := getHeaderDefinition(
+				rego.BuiltinContext{},
+				ast.StringTerm("x-my-header"), // search with lower case header key
+				buildMapTerm(t, map[string][]string{"X-My-Header": {"value"}}),
+			)
+			require.NoError(t, err)
+			require.Equal(t, ast.StringTerm("value"), foundTerm)
+		})
+
+		t.Run("returns error on invalid key", func(t *testing.T) {
+			_, err := getHeaderDefinition(
+				rego.BuiltinContext{},
+				ast.NumberTerm(json.Number("42")),
+				buildMapTerm(t, map[string][]string{}),
+			)
+			require.Error(t, err)
+		})
+
+		t.Run("returns error on invalid headers map", func(t *testing.T) {
+			_, err := getHeaderDefinition(
+				rego.BuiltinContext{},
+				ast.StringTerm("x-my-header"),
+				ast.BooleanTerm(false),
+			)
+			require.Error(t, err)
+		})
+	})
+}