From 4f007d96212d3dfd11dfaaf9ed7758fd1ea37a25 Mon Sep 17 00:00:00 2001
From: Ahmed Hani <hanyahmed11811@gmail.com>
Date: Thu, 20 Feb 2025 18:49:33 +0200
Subject: [PATCH] fix: truncate single log file upon reopening (#1143)

---
 src/logging.cc          | 15 ++++++++++++++
 src/logging_unittest.cc | 43 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+)

diff --git a/src/logging.cc b/src/logging.cc
index 08b2ab3..a452cff 100644
--- a/src/logging.cc
+++ b/src/logging.cc
@@ -995,7 +995,22 @@ bool LogFileObject::CreateLogfile(const string& time_pid_string) {
   if (FLAGS_timestamp_in_logfile_name) {
     // demand that the file is unique for our timestamp (fail if it exists).
     flags = flags | O_EXCL;
+  } else {
+    // logs are written to a single file, where: a log file is created for the
+    // the first time or a file is being recreated due to exceeding max size
+
+    struct stat statbuf;
+    if (stat(filename, &statbuf) == 0) {
+      // truncate the file if it exceeds the max size
+      if ((static_cast<uint32>(statbuf.st_size) >> 20U) >= MaxLogSize()) {
+        flags |= O_TRUNC;
+      }
+
+      // update file length to sync file size
+      file_length_ = static_cast<uint32>(statbuf.st_size);
+    }
   }
+
   FileDescriptor fd{
       open(filename, flags, static_cast<mode_t>(FLAGS_logfile_mode))};
   if (!fd) return false;
diff --git a/src/logging_unittest.cc b/src/logging_unittest.cc
index 321f38f..65c28ee 100644
--- a/src/logging_unittest.cc
+++ b/src/logging_unittest.cc
@@ -110,6 +110,7 @@ static void TestLogSinkWaitTillSent();
 static void TestCHECK();
 static void TestDCHECK();
 static void TestSTREQ();
+static void TestMaxLogSizeWhenNoTimestamp();
 static void TestBasename();
 static void TestBasenameAppendWhenNoTimestamp();
 static void TestTwoProcessesWrite();
@@ -288,6 +289,7 @@ int main(int argc, char** argv) {
       MungeAndDiffTestStdout(FLAGS_test_srcdir + "/src/logging_unittest.out"));
   FLAGS_logtostdout = false;
 
+  TestMaxLogSizeWhenNoTimestamp();
   TestBasename();
   TestBasenameAppendWhenNoTimestamp();
   TestTwoProcessesWrite();
@@ -806,6 +808,47 @@ static void CheckFile(const string& name, const string& expected_string,
              << expected_string << " in " << files[0];
 }
 
+static void TestMaxLogSizeWhenNoTimestamp() {
+  fprintf(stderr, "==== Test setting max log size without timestamp\n");
+  const string dest = FLAGS_test_tmpdir + "/logging_test_max_log_size";
+  DeleteFiles(dest + "*");
+
+  auto original_max_log_size = FLAGS_max_log_size;
+  auto original_timestamp_in_logfile_name = FLAGS_timestamp_in_logfile_name;
+
+  FLAGS_max_log_size = 1;  // Set max log size to 1MB
+  FLAGS_timestamp_in_logfile_name = false;
+
+  // Set log destination
+  SetLogDestination(GLOG_INFO, dest.c_str());
+
+  // 1e4 info logs -> is about 772 KB in size
+  // 2e4 info logs -> is around 1500 KB in size -> 1.5MB
+  // If our max_log_size constraint is respected, it will truncate earlier logs
+  // and the file size will be lesser than 1MB (around 0.5MB)
+  const int num_logs = 2e4;
+  for (int i = 0; i < num_logs; i++) {
+    LOG(INFO) << "Hello world";
+  }
+  FlushLogFiles(GLOG_INFO);
+
+  // Check log file size
+  struct stat statbuf;
+  stat(dest.c_str(), &statbuf);
+
+  // Verify file size is less than the max log size limit
+  CHECK_LT(static_cast<unsigned int>(statbuf.st_size),
+           FLAGS_max_log_size << 20U);
+
+  // Reset flag values to their original values
+  FLAGS_max_log_size = original_max_log_size;
+  FLAGS_timestamp_in_logfile_name = original_timestamp_in_logfile_name;
+
+  // Release file handle for the destination file to unlock the file in Windows.
+  LogToStderr();
+  DeleteFiles(dest + "*");
+}
+
 static void TestBasename() {
   fprintf(stderr, "==== Test setting log file basename\n");
   const string dest = FLAGS_test_tmpdir + "/logging_test_basename";