fix(simplestream): move json_length to outer scope to prevent dangling pointer (#1107)

In sendJSON mode, `uint32_t json_length` was declared inside the
`if (stream.sendJSON==true)` block, but the `send_to()`/`send()` call
that reads from the scatter-gather buffer is outside that block. By the
time the UDP send executes, `json_length` has gone out of scope and its
stack memory may be reclaimed — causing the 4-byte length prefix in the
packet to contain garbage data instead of the actual JSON length.

Move `json_length` to the same scope as `send_buffer` so it remains
alive at send time. Fixes all three affected functions: `audio_stream`,
`call_start`, and `call_end`.

Fixes #1106

Author: LumenPrima

plugins/simplestream/simplestream.cc

@@ -104,6 +104,7 @@ class Simple_Stream : public Plugin_Api {
             json json_object;
             std::string json_string;
             std::vector<boost::asio::const_buffer> send_buffer;
+            uint32_t json_length = 0;
             if (stream.sendJSON==true){
               //create JSON metadata
               json_object = {
@@ -117,7 +118,7 @@ class Simple_Stream : public Plugin_Api {
                  {"event","audio"},
               };
               json_string = json_object.dump();
-              uint32_t json_length = json_string.length();  //determine length in bytes
+              json_length = json_string.length();  //determine length in bytes
               //BOOST_LOG_TRIVIAL(debug) << "json_length is " <<json_length <<" bytes";
               send_buffer.push_back(buffer(&json_length,4));  //prepend length of the json data
               send_buffer.push_back(buffer(json_string));  //prepend json data
@@ -180,6 +181,7 @@ class Simple_Stream : public Plugin_Api {
               json json_object;
               std::string json_string;
               std::vector<boost::asio::const_buffer> send_buffer;
+              uint32_t json_length = 0;
               if (stream.sendJSON==true){
                 //create JSON metadata
                 json_object = {
@@ -194,7 +196,7 @@ class Simple_Stream : public Plugin_Api {
                    {"event","call_start"},
                 };
                 json_string = json_object.dump();
-                uint32_t json_length = json_string.length();  //determine length in bytes
+                json_length = json_string.length();  //determine length in bytes
                 send_buffer.push_back(buffer(&json_length,4));  //prepend length of the json data
                 send_buffer.push_back(buffer(json_string));  //prepend json data
               }
@@ -231,6 +233,7 @@ class Simple_Stream : public Plugin_Api {
               json json_object;
               std::string json_string;
               std::vector<boost::asio::const_buffer> send_buffer;
+              uint32_t json_length = 0;
               if (stream.sendJSON==true){
                 //create JSON metadata
                 json_object = {
@@ -241,7 +244,7 @@ class Simple_Stream : public Plugin_Api {
                    {"event","call_end"},
                 };
                 json_string = json_object.dump();
-                uint32_t json_length = json_string.length();  //determine length in bytes
+                json_length = json_string.length();  //determine length in bytes
                 send_buffer.push_back(buffer(&json_length,4));  //prepend length of the json data
                 send_buffer.push_back(buffer(json_string));  //prepend json data
               }