Add a function for formatting log messages

src/common/logging.cpp

@@ -1,6 +1,9 @@
 #include "logging.h"
 
+#include <chrono>
+#include <ctime>
 #include <fstream>
+#include <iomanip>
 #include <iostream>
 
 #ifdef __WINE__
@@ -52,3 +55,28 @@ Logger Logger::create_from_environment(std::string prefix) {
         return Logger(std::move(std::cerr), verbosity_level, prefix);
     }
 }
+
+void Logger::log(const std::string& message) {
+    const auto current_time = std::chrono::system_clock::now();
+    const std::time_t timestamp =
+        std::chrono::system_clock::to_time_t(current_time);
+
+    // How did C++ manage to get time formatting libraries without a way to
+    // actually get a timestamp in a threadsafe way? `localtime_r` in C++ is not
+    // portable but luckily we only have to support GCC anyway.
+    std::tm tm;
+    localtime_r(&timestamp, &tm);
+
+    std::ostringstream formatted_message;
+    formatted_message << "[" << std::put_time(&tm, "%T") << "]";
+    formatted_message << prefix;
+    formatted_message << message;
+    // Flushing a stringstream doesn't do anything, but we need to put a
+    // linefeed in this string stream rather writing it sprightly to the output
+    // stream to prevent two messages from being put on the same row
+    formatted_message << std::endl;
+
+    // No flush. Should technically be necessary, but it decreases throughput by
+    // a lot and it seems to work fine without.
+    stream << formatted_message.str();
+}

src/common/logging.h

@@ -76,11 +76,30 @@ class Logger {
      */
     static Logger create_from_environment(std::string prefix = "");
 
+    /**
+     * Write a message to the log, prefixing it with a timestamp and this
+     * logger's prefix string.
+     *
+     * @param message The message to write.
+     */
+    void log(const std::string& message);
+
     // TODO: Add dedicated logging functions for events and the Wine process's
     //       STDOUT and STDERR
 
    private:
+    /**
+     * The output stream to write the log messages to. Typically either STDERR
+     * or a file stream.
+     */
     std::ostream& stream;
+    /**
+     * The verbosity level of this logger instance. Based on this certain
+     * messages may or may not be shown.
+     */
     Verbosity verbosity;
+    /**
+     * A prefix that gets prepended before every message.
+     */
     std::string prefix;
 };