nix daemon: Respect json-log-path and re-open for every connection

We don't want to inherit the parent's JSON logger since then messages
from different daemon processes may clobber each other.

src/libstore/daemon.cc

@@ -15,6 +15,7 @@
 #include "derivations.hh"
 #include "args.hh"
 #include "git.hh"
+#include "logging.hh"
 
 #ifndef _WIN32 // TODO need graceful async exit support on Windows?
 # include "monitor-fd.hh"
@@ -1044,9 +1045,18 @@ void processConnection(
     auto tunnelLogger = new TunnelLogger(conn.to, protoVersion);
     auto prevLogger = nix::logger;
     // FIXME
-    if (!recursive)
+    if (!recursive) {
         logger = tunnelLogger;
 
+        if (!loggerSettings.jsonLogPath.get().empty()) {
+            try {
+                logger = makeTeeLogger({logger, makeJSONLogger(std::filesystem::path(loggerSettings.jsonLogPath.get()), false)});
+            } catch (...) {
+                ignoreExceptionExceptInterrupt();
+            }
+        }
+    }
+
     unsigned int opCount = 0;
 
     Finally finally([&]() {

src/libutil/logging.hh

@@ -52,6 +52,14 @@ struct LoggerSettings : Config
           Whether Nix should print out a stack trace in case of Nix
           expression evaluation errors.
         )"};
+
+    Setting<Path> jsonLogPath{
+        this, "", "json-log-path",
+        R"(
+          A path to which JSON records of Nix's log output will be
+          written, in the same format as `--log-format internal-json`
+          (without the `@nix ` prefixes on each line).
+        )"};
 };
 
 extern LoggerSettings loggerSettings;

src/nix/main.cc

@@ -348,21 +348,6 @@ struct CmdHelpStores : Command
 
 static auto rCmdHelpStores = registerCommand<CmdHelpStores>("help-stores");
 
-struct ExtLoggerSettings : Config
-{
-    Setting<Path> jsonLogPath{
-        this, "", "json-log-path",
-        R"(
-          A path to which JSON records of Nix's log output will be
-          written, in the same format as `--log-format internal-json`
-          (without the `@nix ` prefixes on each line).
-        )"};
-};
-
-static ExtLoggerSettings extLoggerSettings;
-
-static GlobalConfig::Register rExtLoggerSettings(&extLoggerSettings);
-
 void mainWrapped(int argc, char * * argv)
 {
     savedArgv = argv;
@@ -501,9 +486,9 @@ void mainWrapped(int argc, char * * argv)
         if (!args.helpRequested && !args.completions) throw;
     }
 
-    if (!extLoggerSettings.jsonLogPath.get().empty()) {
+    if (!loggerSettings.jsonLogPath.get().empty()) {
         try {
-            logger = makeTeeLogger({logger, makeJSONLogger(std::filesystem::path(extLoggerSettings.jsonLogPath.get()), false)});
+            logger = makeTeeLogger({logger, makeJSONLogger(std::filesystem::path(loggerSettings.jsonLogPath.get()), false)});
         } catch (...) {
             ignoreExceptionExceptInterrupt();
         }