Fixed issue #36 regarding negative timestamps

Previously, the decompressor would output negative timestamps when it
encountered a log message with a timestamp earlier than the conversion
time marked in the log file's checkpoint. This patch fixes this issue
by checking for this case and adjusting the output as necessary.

runtime/Log.cc

@@ -1,4 +1,4 @@
-/* Copyright (c) 2017-2018 Stanford University
+/* Copyright (c) 2017-2020 Stanford University
  *
  * Permission to use, copy, modify, and distribute this software for any
  * purpose with or without fee is hereby granted, provided that the above
@@ -1352,6 +1352,14 @@ Log::Decoder::BufferFragment::decompressNextLogStatement(FILE *outputFd,
         int64_t wholeSeconds = static_cast<int64_t>(secondsSinceCheckpoint);
         nanos = 1.0e9 * (secondsSinceCheckpoint
                                 - static_cast<double>(wholeSeconds));
+
+        // If the timestamp occurred before the checkpoint, we may have to
+        // adjust the times so that nanos remains positive.
+        if (nanos < 0.0) {
+            wholeSeconds--;
+            nanos += 1.0e9;
+        }
+
         std::time_t absTime = wholeSeconds + checkpoint.unixTime;
         std::tm *tm = localtime(&absTime);
         strftime(timeString, sizeof(timeString), "%Y-%m-%d %H:%M:%S", tm);

runtime/LogTest.cc

@@ -1,4 +1,4 @@
-/* Copyright (c) 2016-2018 Stanford University
+/* Copyright (c) 2016-2020 Stanford University
  *
  * Permission to use, copy, modify, and distribute this software for any
  * purpose with or without fee is hereby granted, provided that the above
@@ -1918,7 +1918,7 @@ TEST_F(LogTest, Decoder_decompressNextLogStatement_timeTravel) {
     LogMessage logMsg;
 
     UncompressedEntry* ue = reinterpret_cast<UncompressedEntry*>(inputBuffer);
-    ue->timestamp = 10e9;
+    ue->timestamp = 10e9 + 1;
     ue->fmtId = noParamsId;
     ue->entrySize = sizeof(UncompressedEntry);
 
@@ -1959,7 +1959,7 @@ TEST_F(LogTest, Decoder_decompressNextLogStatement_timeTravel) {
     ASSERT_TRUE(iFile.good());
 
     const char* expectedLines[] = {
-        "1969-12-31 16:00:20.000000000 testHelper/client.cc:20 NOTICE[1]: Simple log message with 0 parameters\r"
+        "1969-12-31 16:00:20.000000001 testHelper/client.cc:20 NOTICE[1]: Simple log message with 0 parameters\r"
     };
 
     std::string iLine;