Allow separate JSON logging

If the NIX_LOG_FILE environment variable is set, Nix will write JSON
log messages to that file in addition to the regular logger (e.g. the
progress bar).

src/libutil/logging.cc

@@ -267,6 +267,24 @@ Logger * makeJSONLogger(Descriptor fd)
     return new JSONLogger(fd);
 }
 
+Logger * makeJSONLogger(const std::filesystem::path & path)
+{
+    struct JSONFileLogger : JSONLogger {
+        AutoCloseFD fd;
+
+        JSONFileLogger(AutoCloseFD && fd)
+            : JSONLogger(fd.get())
+            , fd(std::move(fd))
+        { }
+    };
+
+    auto fd{toDescriptor(open(path.c_str(), O_CREAT | O_APPEND | O_WRONLY, 0644))};
+    if (!fd)
+        throw SysError("opening log file '%1%'", path);
+
+    return new JSONFileLogger(std::move(fd));
+}
+
 static Logger::Fields getFields(nlohmann::json & json)
 {
     Logger::Fields fields;

src/libutil/logging.hh

@@ -5,6 +5,8 @@
 #include "config.hh"
 #include "file-descriptor.hh"
 
+#include <filesystem>
+
 #include <nlohmann/json_fwd.hpp>
 
 namespace nix {
@@ -185,8 +187,12 @@ extern Logger * logger;
 
 Logger * makeSimpleLogger(bool printBuildLogs = true);
 
+Logger * makeTeeLogger(std::vector<Logger *> loggers);
+
 Logger * makeJSONLogger(Descriptor fd);
 
+Logger * makeJSONLogger(const std::filesystem::path & path);
+
 /**
  * @param source A noun phrase describing the source of the message, e.g. "the builder".
  */

src/libutil/meson.build

@@ -158,6 +158,7 @@ sources = files(
   'strings.cc',
   'suggestions.cc',
   'tarfile.cc',
+  'tee-logger.cc',
   'terminal.cc',
   'thread-pool.cc',
   'unix-domain-socket.cc',

src/libutil/tee-logger.cc

@@ -0,0 +1,102 @@
+#include "logging.hh"
+
+namespace nix {
+
+struct TeeLogger : Logger
+{
+    std::vector<Logger *> loggers;
+
+    TeeLogger(std::vector<Logger *> loggers)
+        : loggers(std::move(loggers))
+    {
+    }
+
+    void stop() override
+    {
+        for (auto & logger : loggers)
+            logger->stop();
+    };
+
+    void pause() override
+    {
+        for (auto & logger : loggers)
+            logger->pause();
+    };
+
+    void resume() override
+    {
+        for (auto & logger : loggers)
+            logger->resume();
+    };
+
+    void log(Verbosity lvl, std::string_view s) override
+    {
+        for (auto & logger : loggers)
+            logger->log(lvl, s);
+    }
+
+    void logEI(const ErrorInfo & ei) override
+    {
+        for (auto & logger : loggers)
+            logger->logEI(ei);
+    }
+
+    void startActivity(
+        ActivityId act,
+        Verbosity lvl,
+        ActivityType type,
+        const std::string & s,
+        const Fields & fields,
+        ActivityId parent) override
+    {
+        for (auto & logger : loggers)
+            logger->startActivity(act, lvl, type, s, fields, parent);
+    }
+
+    void stopActivity(ActivityId act) override
+    {
+        for (auto & logger : loggers)
+            logger->stopActivity(act);
+    }
+
+    void result(ActivityId act, ResultType type, const Fields & fields) override
+    {
+        for (auto & logger : loggers)
+            logger->result(act, type, fields);
+    }
+
+    void writeToStdout(std::string_view s) override
+    {
+        for (auto & logger : loggers) {
+            /* Let only the first logger write to stdout to avoid
+               duplication. This means that the first logger needs to
+               be the one managing stdout/stderr
+               (e.g. `ProgressBar`). */
+            logger->writeToStdout(s);
+            break;
+        }
+    }
+
+    std::optional<char> ask(std::string_view s) override
+    {
+        for (auto & logger : loggers) {
+            auto c = logger->ask(s);
+            if (c)
+                return c;
+        }
+        return std::nullopt;
+    }
+
+    void setPrintBuildLogs(bool printBuildLogs) override
+    {
+        for (auto & logger : loggers)
+            logger->setPrintBuildLogs(printBuildLogs);
+    }
+};
+
+Logger * makeTeeLogger(std::vector<Logger *> loggers)
+{
+    return new TeeLogger(std::move(loggers));
+}
+
+}

src/nix/main.cc

@@ -485,6 +485,10 @@ void mainWrapped(int argc, char * * argv)
         if (!args.helpRequested && !args.completions) throw;
     }
 
+    if (auto logFile = getEnv("NIX_LOG_FILE")) {
+        logger = makeTeeLogger({logger, makeJSONLogger(*logFile)});
+    }
+
     if (args.helpRequested) {
         std::vector<std::string> subcommand;
         MultiCommand * command = &args;