Print a file-local message of UTC time of file rotation

src/hotspot/share/logging/logFileOutput.cpp

@@ -26,6 +26,7 @@
 #include "logging/logAsyncWriter.hpp"
 #include "logging/logConfiguration.hpp"
 #include "logging/logFileOutput.hpp"
+#include "logging/logDecorations.hpp"
 #include "memory/allocation.inline.hpp"
 #include "runtime/arguments.hpp"
 #include "runtime/os.hpp"
@@ -370,6 +371,20 @@ void LogFileOutput::rotate() {
     return;
   }
 
+  { // Produce a file-unique rotation message.
+    LogTagSet& tagset = LogTagSetMapping<LOG_TAGS(logging)>::tagset();
+    LogLevelType level = tagset.level_for(this);
+    if (level >= LogLevelType::Info) {
+      LogDecorations decorations(LogLevel::Info, tagset, tagset.decorators());
+
+      stringStream st(os::iso8601_timestamp_size);
+      char buf[os::iso8601_timestamp_size];
+      char* result = os::iso8601_time(os::javaTimeMillis(), buf, os::iso8601_timestamp_size, true);
+      st.print("Rotated file at %s", result);
+      this->write_internal(decorations, st.freeze());
+    }
+  }
+
   // Reset accumulated size, increase current file counter, and check for file count wrap-around.
   _current_size = 0;
   increment_file_count();

src/hotspot/share/logging/logTagSet.hpp

@@ -116,6 +116,10 @@ class LogTagSet {
     _output_list.set_output_level(output, level);
   }
 
+  const LogDecorators& decorators() {
+    return _decorators;
+  }
+
   // Refresh the decorators for this tagset to contain the decorators for all
   // of its current outputs combined with the given decorators.
   void update_decorators(const LogDecorators& decorator = LogDecorators::None);