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/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));
+}
+
+}