Reject extra documents in parameter files

params/load.go

@@ -3,7 +3,9 @@ package params
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
+	"io"
 	"maps"
 	"math"
 	"os"
@@ -71,6 +73,15 @@ func LoadParamFile(path string) (map[string]string, error) {
 		if err := dec.Decode(&raw); err != nil {
 			return nil, fmt.Errorf("parse param file as JSON: %w", err)
 		}
+		if raw == nil {
+			return nil, fmt.Errorf("parse param file as JSON: top-level value must be a mapping")
+		}
+		if err := dec.Decode(new(any)); !errors.Is(err, io.EOF) {
+			if err != nil {
+				return nil, fmt.Errorf("parse param file as JSON: %w", err)
+			}
+			return nil, fmt.Errorf("parse param file as JSON: multiple top-level values")
+		}
 	default:
 		return loadParamYAMLFile(b)
 	}
@@ -87,9 +98,19 @@ func LoadParamFile(path string) (map[string]string, error) {
 
 func loadParamYAMLFile(b []byte) (map[string]string, error) {
 	var raw map[string]yaml.RawMessage
-	if err := yaml.Unmarshal(b, &raw); err != nil {
+	dec := yaml.NewDecoder(bytes.NewReader(b))
+	if err := dec.Decode(&raw); err != nil {
+		if errors.Is(err, io.EOF) {
+			return map[string]string{}, nil
+		}
 		return nil, fmt.Errorf("parse param file as YAML: %w", err)
 	}
+	if err := dec.Decode(new(any)); !errors.Is(err, io.EOF) {
+		if err != nil {
+			return nil, fmt.Errorf("parse param file as YAML: %w", err)
+		}
+		return nil, fmt.Errorf("parse param file as YAML: multiple documents are not supported")
+	}
 	out := make(map[string]string, len(raw))
 	for k, msg := range raw {
 		s, err := paramFileYAMLValueToString(msg)

params/load_test.go

@@ -109,6 +109,116 @@ func TestLoadParamFile(t *testing.T) {
 	}
 }
 
+func TestLoadParamFileAcceptsSingleMappingDocument(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		filename string
+		content  string
+		want     map[string]string
+	}{
+		{
+			name:     "JSON with trailing whitespace",
+			filename: "params.json",
+			content:  "{}  \n\t",
+			want:     map[string]string{},
+		},
+		{
+			name:     "YAML empty mapping",
+			filename: "params.yaml",
+			content:  "{}\n",
+			want:     map[string]string{},
+		},
+		{
+			name:     "YAML comments only",
+			filename: "params.yaml",
+			content:  "# no parameters yet\n",
+			want:     map[string]string{},
+		},
+		{
+			name:     "YAML empty document marker",
+			filename: "params.yaml",
+			content:  "---\n",
+			want:     map[string]string{},
+		},
+		{
+			name:     "YAML trailing empty document marker",
+			filename: "params.yaml",
+			content:  "x: INT64\n---\n",
+			want:     map[string]string{"x": "INT64"},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			path := filepath.Join(t.TempDir(), tc.filename)
+			if err := os.WriteFile(path, []byte(tc.content), 0o644); err != nil {
+				t.Fatal(err)
+			}
+			got, err := LoadParamFile(path)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if diff := cmp.Diff(tc.want, got); diff != "" {
+				t.Fatalf("LoadParamFile() mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
+func TestLoadParamFileRejectsInvalidDocuments(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		filename string
+		content  string
+	}{
+		{
+			name:     "JSON trailing non-whitespace content",
+			filename: "params.json",
+			content:  `{"x":1} trailing`,
+		},
+		{
+			name:     "JSON second value",
+			filename: "params.json",
+			content:  `{"x":1} {"y":2}`,
+		},
+		{
+			name:     "JSON top-level null",
+			filename: "params.json",
+			content:  `null`,
+		},
+		{
+			name:     "YAML second document",
+			filename: "params.yaml",
+			content:  "x: 1\n---\ny: 2\n",
+		},
+		{
+			name:     "YAML top-level null",
+			filename: "params.yaml",
+			content:  "null\n",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			path := filepath.Join(t.TempDir(), tc.filename)
+			if err := os.WriteFile(path, []byte(tc.content), 0o644); err != nil {
+				t.Fatal(err)
+			}
+			if _, err := LoadParamFile(path); err == nil {
+				t.Fatal("expected an error")
+			}
+		})
+	}
+}
+
 func TestFormatParamFloat(t *testing.T) {
 	t.Parallel()