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.

1 files changed, 8 insertions, 6 deletions

diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index ef1135601..c49be5e79 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -1207,12 +1207,14 @@ void DerivationGoal::handleChildOutput(int fd, std::string_view data)
     if (hook && fd == hook->fromHook.readSide.get()) {
         for (auto c : data)
             if (c == '\n') {
-                auto s = handleJSONLogMessage(currentHookLine, 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"];
-                        (*logSink)((f.size() > 0 ? f.at(0).get<std::string>() : "") + "\n");
+                auto json = parseJSONMessage(currentHookLine);
+                if (json) {
+                    auto s = handleJSONLogMessage(*json, worker.act, hook->activities, true);
+                    if (s && !isWrittenToLog && logSink) {
+                        if ((*json)["type"] == resBuildLogLine) {
+                            auto f = (*json)["fields"];
+                            (*logSink)((f.size() > 0 ? f.at(0).get<std::string>() : "") + "\n");
+                        }
                     }
                 }
                 currentHookLine.clear();