Add json-log-path setting

This setting specifies a path (which can be a regular file or Unix
domain socket) that receives a copy of all Nix log messages (in JSON
format).

src/libutil/logging.cc

@@ -7,6 +7,7 @@
 #include "nix/util/source-path.hh"
 #include "nix/util/position.hh"
 #include "nix/util/sync.hh"
+#include "nix/util/unix-domain-socket.hh"
 
 #include <atomic>
 #include <sstream>
@@ -182,8 +183,12 @@ void to_json(nlohmann::json & json, std::shared_ptr<Pos> pos)
 
 struct JSONLogger : Logger {
     Descriptor fd;
+    bool includeNixPrefix;
 
-    JSONLogger(Descriptor fd) : fd(fd) { }
+    JSONLogger(Descriptor fd, bool includeNixPrefix)
+        : fd(fd)
+        , includeNixPrefix(includeNixPrefix)
+    { }
 
     bool isVerbose() override {
         return true;
@@ -212,7 +217,7 @@ struct JSONLogger : Logger {
     void write(const nlohmann::json & json)
     {
         auto line =
-            "@nix " +
+            (includeNixPrefix ? "@nix " : "") +
             json.dump(-1, ' ', false, nlohmann::json::error_handler_t::replace);
 
         /* Acquire a lock to prevent log messages from clobbering each
@@ -300,9 +305,49 @@ struct JSONLogger : Logger {
     }
 };
 
-std::unique_ptr<Logger> makeJSONLogger(Descriptor fd)
+std::unique_ptr<Logger> makeJSONLogger(Descriptor fd, bool includeNixPrefix)
 {
-    return std::make_unique<JSONLogger>(fd);
+    return std::make_unique<JSONLogger>(fd, includeNixPrefix);
+}
+
+std::unique_ptr<Logger> makeJSONLogger(const std::filesystem::path & path, bool includeNixPrefix)
+{
+    struct JSONFileLogger : JSONLogger {
+        AutoCloseFD fd;
+
+        JSONFileLogger(AutoCloseFD && fd, bool includeNixPrefix)
+            : JSONLogger(fd.get(), includeNixPrefix)
+            , fd(std::move(fd))
+        { }
+    };
+
+    AutoCloseFD fd =
+        std::filesystem::is_socket(path)
+        ? connect(path)
+        : toDescriptor(open(path.c_str(), O_CREAT | O_APPEND | O_WRONLY, 0644));
+    if (!fd)
+        throw SysError("opening log file %1%", path);
+
+    return std::make_unique<JSONFileLogger>(std::move(fd), includeNixPrefix);
+}
+
+void applyJSONLogger()
+{
+    if (!loggerSettings.jsonLogPath.get().empty()) {
+        try {
+            std::vector<std::unique_ptr<Logger>> loggers;
+            loggers.push_back(makeJSONLogger(std::filesystem::path(loggerSettings.jsonLogPath.get()), false));
+            try {
+                logger = makeTeeLogger(std::move(logger), std::move(loggers));
+            } catch (...) {
+                // `logger` is now gone so give up.
+                abort();
+            }
+        } catch (...) {
+            ignoreExceptionExceptInterrupt();
+        }
+
+    }
 }
 
 static Logger::Fields getFields(nlohmann::json & json)