[Tracing] Minor bug fixes related to trace serialization

src/libplatform/tracing/trace-writer.cc

Index: src/libplatform/tracing/trace-writer.cc
diff --git a/src/libplatform/tracing/trace-writer.cc b/src/libplatform/tracing/trace-writer.cc
index 6d94350cfd9d165a5c632cb8088a928a598afa96..ec95527d5f6a892e4cc9437b5f526af471219830 100644
--- a/src/libplatform/tracing/trace-writer.cc
+++ b/src/libplatform/tracing/trace-writer.cc
@@ -13,17 +13,45 @@ namespace v8 {
 namespace platform {
 namespace tracing {
 
-// Currently we do not support JSON-escaping strings in trace arguments.
-// Thus we perform an IsJSONString() check before writing any string argument.
-// In particular, this means strings cannot have control characters or \.
-V8_INLINE static bool IsJSONString(const char* str) {
+// Writes the given string to a stream, taking care to escape characters
+// when necessary.
+V8_INLINE static void WriteJSONStringToStream(const char* str,
+                                              std::ostream& stream) {
   size_t len = strlen(str);
+  stream << "\"";
   for (size_t i = 0; i < len; ++i) {
-    if (iscntrl(str[i]) || str[i] == '\\') {
-      return false;
+    // All of the permitted escape sequences in JSON strings, as per
+    // https://mathiasbynens.be/notes/javascript-escapes
+    switch (str[i]) {
+      case '\b':
+        stream << "\\b";
+        break;
+      case '\f':
+        stream << "\\f";
+        break;
+      case '\n':
+        stream << "\\n";
+        break;
+      case '\r':
+        stream << "\\r";
+        break;
+      case '\t':
+        stream << "\\t";
+        break;
+      case '\"':
+        stream << "\\\"";
+        break;
+      case '\\':
+        stream << "\\\\";
+        break;
+      // Note that because we use double quotes for JSON strings,
+      // we don't need to escape single quotes.
+      default:
+        stream << str[i];
+        break;
     }
   }
-  return true;
+  stream << "\"";
 }
 
 void JSONTraceWriter::AppendArgValue(uint8_t type,
@@ -72,10 +100,11 @@ void JSONTraceWriter::AppendArgValue(uint8_t type,
       break;
     case TRACE_VALUE_TYPE_STRING:
     case TRACE_VALUE_TYPE_COPY_STRING:
-      // Strings are currently not JSON-escaped, so we need to perform a check
-      // to see if they are valid JSON strings.
-      CHECK(value.as_string != nullptr && IsJSONString(value.as_string));
-      stream_ << (value.as_string ? value.as_string : "NULL");
+      if (value.as_string == nullptr) {
+        stream_ << "\"NULL\"";
+      } else {
+        WriteJSONStringToStream(value.as_string, stream_);
+      }
       break;
     default:
       UNREACHABLE();