Answer an oversized RPC response with a fault

Cap XML-RPC and JSON-RPC output at the SCGI response limit before handing it over.

src/rpc/jsonrpc.cc

@@ -13,6 +13,7 @@
 #include "rpc/command_map.h"
 #include "rpc/nlohmann/json.h"
 #include "rpc/parse_commands.h"
+#include "rpc/scgi_task.h"
 #include "torrent/exceptions.h"
 #include "torrent/object.h"
 #include "utils/functional.h"
@@ -264,6 +265,13 @@ JsonRpc::process(const char* in_buffer, uint32_t length, slot_write callback) {
 
     std::string response_str = response.dump();
 
+    if (response_str.size() > SCgiTask::max_response_size) {
+      const auto& id = response.is_object() && response.contains("id") ? response["id"] : json(nullptr);
+      auto err_str = json_error(JSONRPC_INTERNAL_ERROR, "response size exceeds maximum RPC limit", id).dump();
+
+      return callback(err_str.c_str(), err_str.size());
+    }
+
     return callback(response_str.c_str(), response_str.size());
 
   } catch (json::exception& e) {

src/rpc/scgi_task.cc

@@ -405,7 +405,7 @@ void
 SCgiTask::receive_write(const char* buffer, uint32_t length) {
   assert(torrent::this_thread::thread() == torrent::main_thread::thread());
 
-  if (buffer == nullptr || length > (100 << 20))
+  if (buffer == nullptr || length > max_response_size)
     throw torrent::internal_error("SCgiTask::receive_write(...) received bad input.");
 
   // Main thread callback already locked this mutex.

src/rpc/scgi_task.h

@@ -2,6 +2,7 @@
 #define RTORRENT_RPC_SCGI_TASK_H
 
 #include <chrono>
+#include <cstdint>
 #include <memory>
 #include <mutex>
 #include <vector>
@@ -18,6 +19,8 @@ public:
   static constexpr int max_header_size     = 2000;
   static constexpr int max_content_size    = (1 << 26);
 
+  static constexpr uint32_t max_response_size = (100 << 20);
+
   static constexpr auto timeout_request = std::chrono::seconds(60);
 
   enum ContentType { XML, JSON };

src/rpc/xmlrpc_tinyxml2.cc

@@ -425,6 +425,14 @@ XmlRpc::process(const char* inBuffer, uint32_t length, slot_write slotWrite) {
     // remains.
     tinyxml2::XMLPrinter printer(nullptr, true, 0);
     process_document(&doc, &printer);
+
+    if (printer.CStrSize() - 1 > static_cast<int>(SCgiTask::max_response_size)) {
+      tinyxml2::XMLPrinter fault_printer(nullptr, true, 0);
+      print_xmlrpc_fault(XMLRPC_LIMIT_EXCEEDED_ERROR, "Response size exceeds maximum XML-RPC limit", &fault_printer);
+
+      return slotWrite(fault_printer.CStr(), fault_printer.CStrSize() - 1);
+    }
+
     return slotWrite(printer.CStr(), printer.CStrSize() - 1);
   } catch (rpc_error& e) {
     tinyxml2::XMLPrinter printer(nullptr, true, 0);