folly/json: serialize \r and \n as \r and \n

Summary:
Background: we want to use folly::json but not change the way data is represented.
Since we need to store \r and \n in strings and the library we're currently using does this,
let's do it in folly too.

Test Plan: unit tests pass

Reviewed By: delong.j@fb.com

FB internal diff: D584960

folly/json.cpp

@@ -145,23 +145,27 @@ void escapeString(StringPiece input,
       out.push_back(hexDigit((v >> 8) & 0x0f));
       out.push_back(hexDigit((v >> 4) & 0x0f));
       out.push_back(hexDigit(v & 0x0f));
-      continue;
-    }
-    if (*p == '\\' || *p == '\"') {
+    } else if (*p == '\\' || *p == '\"') {
       out.push_back('\\');
       out.push_back(*p++);
-      continue;
-    }
-    if (*p <= 0x1f) {
-      // note that this if condition captures both control characters
-      // and extended ascii characters
-      out.append("\\u00");
-      out.push_back(hexDigit((*p & 0xf0) >> 4));
-      out.push_back(hexDigit(*p & 0xf));
-      p++;
-      continue;
+    } else if (*p <= 0x1f) {
+      switch (*p) {
+      case '\b': out.append("\\b"); p++; break;
+      case '\f': out.append("\\f"); p++; break;
+      case '\n': out.append("\\n"); p++; break;
+      case '\r': out.append("\\r"); p++; break;
+      case '\t': out.append("\\t"); p++; break;
+      default:
+        // note that this if condition captures both control characters
+        // and extended ascii characters
+        out.append("\\u00");
+        out.push_back(hexDigit((*p & 0xf0) >> 4));
+        out.push_back(hexDigit(*p & 0xf));
+        p++;
+      }
+    } else {
+      out.push_back(*p++);
     }
-    out.push_back(*p++);
   }
 
   out.push_back('\"');

folly/test/JsonTest.cpp

@@ -173,6 +173,13 @@ TEST(Json, Produce) {
   EXPECT_TRUE(caught);
 }
 
+TEST(Json, JsonEscape) {
+  folly::json::serialization_opts opts;
+  EXPECT_EQ(
+    folly::json::serialize("\b\f\n\r\x01\t\\\"/\v\a", opts),
+    R"("\b\f\n\r\u0001\t\\\"/\u000b\u0007")");
+}
+
 TEST(Json, JsonNonAsciiEncoding) {
   folly::json::serialization_opts opts;
   opts.encode_non_ascii = true;