libstore/derivation-goal: avoid double-parsing of JSON messages

To avoid that JSON messages are parsed twice in case of
remote builds with `ssh-ng://`, I split up the original
`handleJSONLogMessage` into three parts:

* `parseJSONMessage(const std::string&)` checks if it's a message in the
  form of `@nix {...}` and tries to parse it (and prints an error if the
  parsing fails).
* `handleJSONLogMessage(nlohmann::json&, ...)` reads the fields from the
  message and passes them to the logger.
* `handleJSONLogMessage(const std::string&, ...)` behaves as before, but
  uses the two functions mentioned above as implementation.

In case of `ssh-ng://`-logs the first two methods are invoked manually.

src/libstore/build/derivation-goal.cc

@@ -1207,14 +1207,16 @@ void DerivationGoal::handleChildOutput(int fd, const string & data)
     if (hook && fd == hook->fromHook.readSide.get()) {
         for (auto c : data)
             if (c == '\n') {
-                auto s = handleJSONLogMessage(currentHookLine, worker.act, hook->activities, true);
+                auto json = parseJSONMessage(currentHookLine);
+                if (json) {
+                    auto s = handleJSONLogMessage(*json, worker.act, hook->activities, true);
                     if (s && !isWrittenToLog && logSink) {
-                    auto json = nlohmann::json::parse(std::string(currentHookLine, 5));
-                    if (json["type"] == resBuildLogLine) {
-                        auto f = json["fields"];
+                        if ((*json)["type"] == resBuildLogLine) {
+                            auto f = (*json)["fields"];
                             (*logSink)((f.size() > 0 ? f.at(0).get<std::string>() : "") + "\n");
                         }
                     }
+                }
                 currentHookLine.clear();
             } else
                 currentHookLine += c;

src/libutil/logging.cc

@@ -266,14 +266,21 @@ static Logger::Fields getFields(nlohmann::json & json)
     return fields;
 }
 
-bool handleJSONLogMessage(const std::string & msg,
-    const Activity & act, std::map<ActivityId, Activity> & activities, bool trusted)
+std::optional<nlohmann::json> parseJSONMessage(const std::string & msg)
 {
-    if (!hasPrefix(msg, "@nix ")) return false;
-
+    if (!hasPrefix(msg, "@nix ")) return std::nullopt;
     try {
-        auto json = nlohmann::json::parse(std::string(msg, 5));
+        return nlohmann::json::parse(std::string(msg, 5));
+    } catch (std::exception & e) {
+        printError("bad JSON log message from builder: %s", e.what());
+    }
+    return std::nullopt;
+}
 
+bool handleJSONLogMessage(nlohmann::json & json,
+    const Activity & act, std::map<ActivityId, Activity> & activities,
+    bool trusted)
+{
     std::string action = json["action"];
 
     if (action == "start") {
@@ -304,13 +311,18 @@ bool handleJSONLogMessage(const std::string & msg,
         logger->log((Verbosity) json["level"], msg);
     }
 
-    } catch (std::exception & e) {
-        printError("bad JSON log message from builder: %s", e.what());
-    }
-
     return true;
 }
 
+bool handleJSONLogMessage(const std::string & msg,
+    const Activity & act, std::map<ActivityId, Activity> & activities, bool trusted)
+{
+    auto json = parseJSONMessage(msg);
+    if (!json) return false;
+
+    return handleJSONLogMessage(*json, act, activities, trusted);
+}
+
 Activity::~Activity()
 {
     try {

src/libutil/logging.hh

@@ -4,6 +4,8 @@
 #include "error.hh"
 #include "config.hh"
 
+#include <nlohmann/json.hpp>
+
 namespace nix {
 
 typedef enum {
@@ -166,6 +168,12 @@ Logger * makeSimpleLogger(bool printBuildLogs = true);
 
 Logger * makeJSONLogger(Logger & prevLogger);
 
+std::optional<nlohmann::json> parseJSONMessage(const std::string & msg);
+
+bool handleJSONLogMessage(nlohmann::json & json,
+    const Activity & act, std::map<ActivityId, Activity> & activities,
+    bool trusted);
+
 bool handleJSONLogMessage(const std::string & msg,
     const Activity & act, std::map<ActivityId, Activity> & activities,
     bool trusted);