Parse multicall commands once per call

Keep the change to d, f, p and t multicalls. Parse each column lazily on first use and evaluate it through rpc::call_object, preserving per-target argument expansion.

Compared unpatched and patched builds on macOS: output was byte-identical across the test harness, including an erase-mid-call check.

src/rpc/parse_commands.cc

@@ -123,6 +123,39 @@ parse_command(target_type target, const char* first, const char* last) {
   return std::make_pair(commands.call_command(key, args, target), first);
 }
 
+torrent::Object
+parse_command_object(const char* first, const char* last) {
+  first = std::find_if(first, last, [&](char c) { return !command_map_is_space(c); });
+
+  if (first == last || *first == '#')
+    return torrent::Object();
+
+  char key[128];
+
+  first = parse_command_name(first, last, key, key + 128);
+  first = std::find_if(first, last, [&](char c) { return !command_map_is_space(c); });
+
+  if (first == last || *first != '=')
+    throw torrent::input_error("Could not find '=' in command '" + std::string(key) + "'.");
+
+  torrent::Object result = torrent::Object::create_dict_key();
+
+  result.as_dict_key() = key;
+
+  first = parse_whole_list(first + 1, last, &result.as_dict_obj(), &parse_is_delim_command);
+
+  // Find the last character that is part of this command, skipping
+  // the whitespace at the end.
+  first = std::find_if(first, last, [&](char c) { return !command_map_is_space(c); });
+
+  // This helper accepts exactly one command and cannot return where a next
+  // command begins, so reject every non-whitespace suffix, including ';'.
+  if (first != last && *first != '\0')
+    throw torrent::input_error("Junk at end of input.");
+
+  return result;
+}
+
 torrent::Object
 parse_command_multiple(target_type target, const char* first, const char* last) {
   parse_command_type result;

src/rpc/parse_commands.h

@@ -37,6 +37,9 @@
 
 #include <string>
 #include <cstring>
+#include <functional>
+#include <utility>
+#include <vector>
 
 #include "xmlrpc.h"
 #include "rpc_manager.h"
@@ -67,6 +70,34 @@ parse_command_single(target_type target, const std::string& cmd) {
   return parse_command(target, cmd.c_str(), cmd.c_str() + cmd.size()).first;
 }
 
+// Parse one RPC command without executing it. Repeated evaluations can use
+// call_object on the result, which handles per-target argument expansion.
+torrent::Object        parse_command_object(const char* first, const char* last);
+
+inline torrent::Object parse_command_object(const std::string& cmd) {
+  return parse_command_object(cmd.c_str(), cmd.c_str() + cmd.size());
+}
+
+// Prepare a multicall's commands once, on the first target that uses them. This
+// keeps empty target lists from parsing commands that would never be evaluated.
+struct preparsed_commands : public std::vector<torrent::Object> {
+  explicit preparsed_commands(std::function<void(preparsed_commands&)> prepare)
+    : m_prepare(std::move(prepare)) {}
+
+  void prepare_if_needed() {
+    if (m_prepare) {
+      // Clear before invoking: the callback may inspect this vector, and a
+      // throwing callback must not be run again against partially added items.
+      auto prepare = std::move(m_prepare);
+      m_prepare = {};
+      prepare(*this);
+    }
+  }
+
+private:
+  std::function<void(preparsed_commands&)> m_prepare;
+};
+
 inline torrent::Object
 parse_command_multiple_std(const std::string& cmd, target_type target = rpc::make_target()) {
   return parse_command_multiple(target, cmd.c_str(), cmd.c_str() + cmd.size());