Fix crash on untrusted XMLRPC connections

Root cause: network.rpc.use_xmlrpc and network.rpc.use_jsonrpc were not
marked as untrusted-safe, but RpcManager::process() calls them before
dispatching to the protocol handler. When an untrusted request arrived,
call_command() threw untrusted_error for these gatekeepers, which escaped
the callback_interrupt_pollling callback and crashed rtorrent.

Fix: Mark network.rpc.use_xmlrpc/jsonrpc as safe (CMD2_VAR_BOOL_U).

Also harden exception safety:
- SCGI callback catch-all now sends a generic error response instead of
  re-throwing, since the callback infrastructure may not support
  exception propagation.
- xmlrpc_c.cc now has catch(std::exception&) and catch(...) safety nets
  after the specific exception handlers.

src/command_network.cc

@@ -248,8 +248,8 @@ initialize_command_network() {
   CMD2_ANY_U         ("network.xmlrpc.size_limit",     [](const auto&, const auto&)     { return rpc::rpc.size_limit(); });
   CMD2_ANY_VALUE_V_U ("network.xmlrpc.size_limit.set", [](const auto&, const auto& arg) { return rpc::rpc.set_size_limit(arg); });
 
-  CMD2_VAR_BOOL    ("network.rpc.use_xmlrpc",        true);
-  CMD2_VAR_BOOL    ("network.rpc.use_jsonrpc",       true);
+  CMD2_VAR_BOOL_U  ("network.rpc.use_xmlrpc",        true);
+  CMD2_VAR_BOOL_U  ("network.rpc.use_jsonrpc",       true);
 
   CMD2_ANY         ("network.block.ipv4",            [nw_config](auto, auto)        { return nw_config->is_block_ipv4(); });
   CMD2_ANY_VALUE_V ("network.block.ipv4.set",        [nw_config](auto, auto& value) { return nw_config->set_block_ipv4(value); });

src/rpc/scgi_task.cc

@@ -3,6 +3,7 @@
 #include "rpc/scgi_task.h"
 
 #include <cstdio>
+#include <cstring>
 #include <unistd.h>
 #include <vector>
 #include <sys/types.h>
@@ -315,7 +316,17 @@ SCgiTask::receive_call(const char* buffer, uint32_t length) {
                     });
       } catch (...) {
         rpc::RpcManager::set_trusted(true);
-        throw;
+
+        // Send a generic error response instead of re-throwing, as the
+        // callback infrastructure may not support exception propagation.
+        const char* err_xml = "<?xml version=\"1.0\"?><methodResponse><fault><value><struct>"
+                              "<member><name>faultCode</name><value><i8>-500</i8></value></member>"
+                              "<member><name>faultString</name><value><string>Internal error</string></value></member>"
+                              "</struct></value></fault></methodResponse>";
+        const char* err_json = "{\"jsonrpc\":\"2.0\",\"error\":{\"code\":-32603,\"message\":\"Internal error\"},\"id\":null}";
+        const char* err = (rpc_type == RpcManager::RPCType::JSON) ? err_json : err_xml;
+        result_callback(err, std::strlen(err));
+        return;
       }
 
       rpc::RpcManager::set_trusted(true);

src/rpc/xmlrpc_c.cc

@@ -396,6 +396,14 @@ xmlrpc_call_command(xmlrpc_env* env, xmlrpc_value* args, void* voidServerInfo) {
   } catch (torrent::local_error& e) {
     xmlrpc_env_set_fault(env, XMLRPC_PARSE_ERROR, e.what());
     return NULL;
+
+  } catch (std::exception& e) {
+    xmlrpc_env_set_fault(env, XMLRPC_PARSE_ERROR, e.what());
+    return NULL;
+
+  } catch (...) {
+    xmlrpc_env_set_fault(env, XMLRPC_PARSE_ERROR, "Unknown exception in command execution.");
+    return NULL;
   }
 }