json: replace literal string to constexpr and force Streamer to use s…

…imple output abstraction (#35892)

Continuous work of #35891. This replace the literal string to constexpr
string view to make it's more friendly for dev tools. And this PR also
create a simple wrapper to the Buffer::Instance to force the Streamer
only access the limited abstraction (only provides an add()) to
streaming it's output.

Risk Level: low.
Testing: n/a.
Docs Changes: n/a.
Release Notes: n/a.
Platform Specific Features: n/a.

---------

Signed-off-by: wangbaiping <[email protected]>

source/common/json/BUILD

@@ -50,9 +50,18 @@ envoy_cc_library(
     name = "json_streamer_lib",
     hdrs = ["json_streamer.h"],
     deps = [
+        ":constants_lib",
         ":json_sanitizer_lib",
         "//envoy/buffer:buffer_interface",
         "//source/common/buffer:buffer_util_lib",
         "//source/common/common:assert_lib",
     ],
 )
+
+envoy_cc_library(
+    name = "constants_lib",
+    hdrs = ["constants.h"],
+    external_deps = [
+        "abseil_strings",
+    ],
+)

source/common/json/constants.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include "absl/strings/string_view.h"
+
+namespace Envoy {
+namespace Json {
+
+class Constants {
+public:
+  // Constants for common JSON values.
+  static constexpr absl::string_view True = "true";
+  static constexpr absl::string_view False = "false";
+  static constexpr absl::string_view Null = "null";
+};
+
+} // namespace Json
+} // namespace Envoy

source/common/json/json_streamer.h

@@ -9,6 +9,7 @@
 #include "envoy/buffer/buffer.h"
 
 #include "source/common/buffer/buffer_util.h"
+#include "source/common/json/constants.h"
 #include "source/common/json/json_sanitizer.h"
 
 #include "absl/strings/string_view.h"
@@ -38,6 +39,18 @@ namespace Json {
 #define ASSERT_LEVELS_EMPTY ASSERT(this->levels_.empty())
 #endif
 
+// Simple abstraction that provide a output buffer for streaming JSON output.
+class BufferOutput {
+public:
+  void add(absl::string_view a) { buffer_.addFragments({a}); }
+  void add(absl::string_view a, absl::string_view b, absl::string_view c) {
+    buffer_.addFragments({a, b, c});
+  }
+
+  explicit BufferOutput(Buffer::Instance& output) : buffer_(output) {}
+  Buffer::Instance& buffer_;
+};
+
 /**
  * Provides an API for streaming JSON output, as an alternative to populating a
  * JSON structure with an image of what you want to serialize, or using a
@@ -143,7 +156,7 @@ class Streamer {
     void addString(absl::string_view str) {
       ASSERT_THIS_IS_TOP_LEVEL;
       nextField();
-      streamer_.addSanitized("\"", str, "\"");
+      streamer_.addString(str);
     }
 
     /**
@@ -243,7 +256,7 @@ class Streamer {
       ASSERT_THIS_IS_TOP_LEVEL;
       ASSERT(!expecting_value_);
       nextField();
-      this->streamer_.addSanitized(R"(")", key, R"(":)");
+      this->streamer_.addSanitized("\"", key, "\":");
       expecting_value_ = true;
     }
 
@@ -331,32 +344,39 @@ class Streamer {
    */
   void addSanitized(absl::string_view prefix, absl::string_view token, absl::string_view suffix) {
     absl::string_view sanitized = Json::sanitize(sanitize_buffer_, token);
-    response_.addFragments({prefix, sanitized, suffix});
+    response_.add(prefix, sanitized, suffix);
   }
 
+  /**
+   * Serializes a string to the output stream. The input string value will be sanitized and
+   * surrounded by quotes.
+   * @param str the string to be serialized.
+   */
+  void addString(absl::string_view str) { addSanitized("\"", str, "\""); }
+
   /**
    * Serializes a number.
    */
   void addNumber(double d) {
     if (std::isnan(d)) {
-      response_.addFragments({"null"});
+      response_.add(Constants::Null);
     } else {
-      Buffer::Util::serializeDouble(d, response_);
+      Buffer::Util::serializeDouble(d, response_.buffer_);
     }
   }
-  void addNumber(uint64_t u) { response_.addFragments({absl::StrCat(u)}); }
-  void addNumber(int64_t i) { response_.addFragments({absl::StrCat(i)}); }
+  void addNumber(uint64_t u) { response_.add(absl::StrCat(u)); }
+  void addNumber(int64_t i) { response_.add(absl::StrCat(i)); }
 
   /**
    * Serializes a bool to the output stream.
    */
-  void addBool(bool b) { response_.addFragments({b ? "true" : "false"}); }
+  void addBool(bool b) { response_.add(b ? Constants::True : Constants::False); }
 
   /**
    * Adds a constant string to the output stream. The string must outlive the
    * Streamer object, and is intended for literal strings such as punctuation.
    */
-  void addConstantString(absl::string_view str) { response_.addFragments({str}); }
+  void addConstantString(absl::string_view str) { response_.add(str); }
 
 #ifndef NDEBUG
   /**
@@ -379,7 +399,7 @@ class Streamer {
 
 #endif
 
-  Buffer::Instance& response_;
+  BufferOutput response_;
   std::string sanitize_buffer_;
 
 #ifndef NDEBUG