Add log_year_in_prefix flag

The flag allows suppressing the year in the log line prefix. At the same
time, the flag allows to generate log output compatible to glog releases
prior to version 0.5.

src/glog/logging.h.in

@@ -430,6 +430,9 @@ DECLARE_int32(stderrthreshold);
 // Set whether the log prefix should be prepended to each line of output.
 DECLARE_bool(log_prefix);
 
+// Set whether the year should be included in the log prefix.
+DECLARE_bool(log_year_in_prefix);
+
 // Log messages at a level <= this flag are buffered.
 // Log messages at a higher level are flushed immediately.
 DECLARE_int32(logbuflevel);

src/logging.cc

@@ -144,6 +144,8 @@ GLOG_DEFINE_string(alsologtoemail, "",
                    "in addition to logfiles");
 GLOG_DEFINE_bool(log_prefix, true,
                  "Prepend the log prefix to the start of each log line");
+GLOG_DEFINE_bool(log_year_in_prefix, true,
+                 "Include the year in the log prefix");
 GLOG_DEFINE_int32(minloglevel, 0, "Messages logged at a lower level than this don't "
                   "actually get logged anywhere");
 GLOG_DEFINE_int32(logbuflevel, 0,
@@ -1224,10 +1226,12 @@ void LogFileObject::Write(bool force_flush,
     if(!g_application_fingerprint.empty()) {
       file_header_stream << "Application fingerprint: " << g_application_fingerprint << '\n';
     }
-
+    const char* const date_time_format = FLAGS_log_year_in_prefix
+                                             ? "yyyymmdd hh:mm:ss.uuuuuu"
+                                             : "mmdd hh:mm:ss.uuuuuu";
     file_header_stream << "Running duration (h:mm:ss): "
                        << PrettyDuration(static_cast<int>(WallTime_Now() - start_time_)) << '\n'
-                       << "Log line format: [IWEF]yyyymmdd hh:mm:ss.uuuuuu "
+                       << "Log line format: [IWEF]" << date_time_format << " "
                        << "threadid file:line] msg" << '\n';
     const string& file_header_string = file_header_stream.str();
 
@@ -1620,9 +1624,11 @@ void LogMessage::Init(const char* file,
     #ifdef GLOG_CUSTOM_PREFIX_SUPPORT
       if (custom_prefix_callback == NULL) {
     #endif
-          stream() << LogSeverityNames[severity][0]
-                   << setw(4) << 1900 + logmsgtime_.year()
-                   << setw(2) << 1 + logmsgtime_.month()
+          stream() << LogSeverityNames[severity][0];
+          if (FLAGS_log_year_in_prefix) {
+            stream() << setw(4) << 1900 + logmsgtime_.year();
+          }
+          stream() << setw(2) << 1 + logmsgtime_.month()
                    << setw(2) << logmsgtime_.day()
                    << ' '
                    << setw(2) << logmsgtime_.hour() << ':'