Simplify extractJSON: remove startChar param, add tests

Drop the startChar parameter — the helper now finds the first line
starting with '{' or '[' automatically. Use json.Decoder to extract
the complete JSON value, which correctly handles trailing log lines
that might contain braces. Keep null handling in ListImages where
it's semantically relevant.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: stbenjam

pkg/test/extensions/binary.go

@@ -335,15 +335,15 @@ var extensionBinaries = []TestBinary{
 	},
 }
 
-// extractJSON finds the first JSON value in output that starts with startChar,
-// skipping any non-JSON log lines that precede it. This is necessary because some extension binaries
-// output warnings or debug logging to stdout before the JSON payload.
-func extractJSON(output []byte, startChar byte) ([]byte, error) {
+// extractJSON finds the first JSON object or array in output, skipping any non-JSON log lines
+// that precede it. This is necessary because some extension binaries output warnings or debug
+// logging to stdout before the JSON payload.
+func extractJSON(output []byte) ([]byte, error) {
 	jsonBegins := -1
 	lines := bytes.Split(output, []byte("\n"))
 	for i, line := range lines {
 		trimmed := bytes.TrimSpace(line)
-		if len(trimmed) > 0 && trimmed[0] == startChar {
+		if len(trimmed) > 0 && (trimmed[0] == '{' || trimmed[0] == '[') {
 			jsonBegins = 0
 			for j := 0; j < i; j++ {
 				jsonBegins += len(lines[j]) + 1 // +1 for the newline character
@@ -362,9 +362,6 @@ func extractJSON(output []byte, startChar byte) ([]byte, error) {
 	if err := dec.Decode(&raw); err != nil {
 		return nil, fmt.Errorf("no valid JSON found in output: %w", err)
 	}
-	if len(raw) == 0 || raw[0] != startChar {
-		return nil, fmt.Errorf("no valid JSON found in output: %s", string(output))
-	}
 	return raw, nil
 }
 
@@ -385,7 +382,7 @@ func (b *TestBinary) Info(ctx context.Context) (*Extension, error) {
 		logrus.Errorf("Command output for %s: %s", binName, string(infoJson))
 		return nil, fmt.Errorf("failed running '%s info': %w\nOutput: %s", b.binaryPath, err, infoJson)
 	}
-	jsonData, err := extractJSON(infoJson, '{')
+	jsonData, err := extractJSON(infoJson)
 	if err != nil {
 		logrus.Errorf("No valid JSON found in output from %s info command", binName)
 		logrus.Errorf("Raw output from %s: %s", binName, string(infoJson))
@@ -576,7 +573,7 @@ func (b *TestBinary) ListImages(ctx context.Context) (ImageSet, error) {
 		return nil, fmt.Errorf("failed running '%s images': %w\nOutput: %s", b.binaryPath, err, output)
 	}
 
-	jsonData, err := extractJSON(output, '[')
+	jsonData, err := extractJSON(output)
 	if err != nil {
 		// Extensions that have no images may output "null" instead of an array
 		if bytes.Contains(output, []byte("null")) {

pkg/test/extensions/extract_json_test.go

@@ -0,0 +1,70 @@
+package extensions
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestExtractJSON(t *testing.T) {
+	tests := []struct {
+		name    string
+		input   string
+		want    string
+		wantErr bool
+	}{
+		{
+			name:  "clean JSON object",
+			input: `{"key": "value"}`,
+			want:  `{"key": "value"}`,
+		},
+		{
+			name:  "clean JSON array",
+			input: `[{"index": 1}]`,
+			want:  `[{"index": 1}]`,
+		},
+		{
+			name:  "warning lines before JSON object",
+			input: "W0414 08:58:39.856273 46367 controller.go:47] some warning\nW0414 08:58:39.865859 46367 feature_gate.go:352] another warning\n{\"key\": \"value\"}\n",
+			want:  `{"key": "value"}`,
+		},
+		{
+			name:  "warning lines before JSON array",
+			input: "W0414 08:58:39.856273 46367 controller.go:47] some warning\n[{\"index\": 1}]\n",
+			want:  `[{"index": 1}]`,
+		},
+		{
+			name:  "log lines after JSON are ignored",
+			input: "{\"key\": \"value\"}\nW0414 trailing log with } brace\n",
+			want:  `{"key": "value"}`,
+		},
+		{
+			name:    "no JSON in output",
+			input:   "W0414 just warnings\nI0414 and info lines\n",
+			wantErr: true,
+		},
+		{
+			name:    "empty output",
+			input:   "",
+			wantErr: true,
+		},
+		{
+			name:    "null is not a JSON object or array",
+			input:   "W0414 warning\nnull\n",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := extractJSON([]byte(tt.input))
+			if tt.wantErr {
+				assert.Error(t, err)
+				return
+			}
+			require.NoError(t, err)
+			assert.JSONEq(t, tt.want, string(got))
+		})
+	}
+}