JSONLogger: Log to a file descriptor instead of another Logger

Logging to another Logger was kind of nonsensical - it was really just
an easy way to get it to write its output to stderr, but that only
works if the underlying logger writes to stderr.

This change is needed to make it easy to log JSON output somewhere
else (like a file or socket).

src/build-remote/build-remote.cc

@@ -51,7 +51,7 @@ static bool allSupportedLocally(Store & store, const std::set<std::string>& requ
 static int main_build_remote(int argc, char * * argv)
 {
     {
-        logger = makeJSONLogger(*logger);
+        logger = makeJSONLogger(STDERR_FILENO);
 
         /* Ensure we don't get any SSH passphrase or host key popups. */
         unsetenv("DISPLAY");

src/libmain/loggers.cc

@@ -27,7 +27,7 @@ Logger * makeDefaultLogger() {
     case LogFormat::rawWithLogs:
         return makeSimpleLogger(true);
     case LogFormat::internalJSON:
-        return makeJSONLogger(*makeSimpleLogger(true));
+        return makeJSONLogger(STDERR_FILENO);
     case LogFormat::bar:
         return makeProgressBar();
     case LogFormat::barWithLogs: {

src/libstore/unix/build/local-derivation-goal.cc

@@ -2225,7 +2225,7 @@ void LocalDerivationGoal::runChild()
         /* Execute the program.  This should not return. */
         if (drv->isBuiltin()) {
             try {
-                logger = makeJSONLogger(*logger);
+                logger = makeJSONLogger(STDERR_FILENO);
 
                 std::map<std::string, Path> outputs;
                 for (auto & e : drv->outputs)

src/libutil/logging.cc

@@ -167,9 +167,9 @@ void to_json(nlohmann::json & json, std::shared_ptr<Pos> pos)
 }
 
 struct JSONLogger : Logger {
-    Logger & prevLogger;
+    Descriptor fd;
 
-    JSONLogger(Logger & prevLogger) : prevLogger(prevLogger) { }
+    JSONLogger(Descriptor fd) : fd(fd) { }
 
     bool isVerbose() override {
         return true;
@@ -190,7 +190,7 @@ struct JSONLogger : Logger {
 
     void write(const nlohmann::json & json)
     {
-        prevLogger.log(lvlError, "@nix " + json.dump(-1, ' ', false, nlohmann::json::error_handler_t::replace));
+        writeLine(fd, "@nix " + json.dump(-1, ' ', false, nlohmann::json::error_handler_t::replace));
     }
 
     void log(Verbosity lvl, std::string_view s) override
@@ -262,9 +262,9 @@ struct JSONLogger : Logger {
     }
 };
 
-Logger * makeJSONLogger(Logger & prevLogger)
+Logger * makeJSONLogger(Descriptor fd)
 {
-    return new JSONLogger(prevLogger);
+    return new JSONLogger(fd);
 }
 
 static Logger::Fields getFields(nlohmann::json & json)

src/libutil/logging.hh

@@ -3,6 +3,7 @@
 
 #include "error.hh"
 #include "config.hh"
+#include "file-descriptor.hh"
 
 #include <nlohmann/json_fwd.hpp>
 
@@ -183,7 +184,7 @@ extern Logger * logger;
 
 Logger * makeSimpleLogger(bool printBuildLogs = true);
 
-Logger * makeJSONLogger(Logger & prevLogger);
+Logger * makeJSONLogger(Descriptor fd);
 
 /**
  * @param source A noun phrase describing the source of the message, e.g. "the builder".